Magnetic Contactor Selection | AC-3 Rating, Coordination, Overload Relays and RFP for Thai Factories

Introduction: When a Contactor Chosen by Amps Alone Stops the Factory

For motor starting circuits on conveyors and pumps, do you think of magnetic contactor selection as simply looking up the model that matches the motor’s kW and amps in a table? Stories like the following are often heard at Japanese-owned factories in Thailand, too.

• The contactor was replaced with one of the same ampere rating, yet some time later its contacts welded.
• The thermal overload relay trips at every start and the line stops.
• After a short-circuit fault, not only the circuit breaker but also the contactor and thermal overload relay had to be replaced, and recovery took a long time.
• On dry-season afternoons the inside of the panel got hot and trips increased.
• A design for the Japanese 200 V class was brought over as-is, and the column that should be read in the rating table had changed.

Each of these can be traced, at least in part, to selecting without first deciding “which column to read” in the model table. In this article, we take the view that magnetic contactor selection is not settled just by pulling a model out of a selection table; deciding which column to read is the selection itself. Specifically, we organize the decision into the following seven points.

1. Utilization category (AC-1, AC-3, AC-4) and the voltage column
2. The full-load current of the motor actually delivered, how often it starts and stops, and electrical durability
3. Trip class and setting of the overload relay (thermal overload relay)
4. Coordination with the short-circuit protective device (Type 1 and Type 2)
5. Coil (AC/DC, operating voltage range, power consumption, surge) and auxiliary contacts
6. Derating for ambient temperature and mounting density
7. Mirror contacts and mechanical interlocks when combined with safety circuits

Our position in this article is that the buyer should write these into the RFP and, at FAT/SAT, confirm the applicable row of the combination table, the setting values, the coil voltage and the temperature inside the panel.

A few disclaimers up front. This article is not a recommendation or ranking of specific products. Manufacturer ratings and functions are only introduced as stated in the catalogs and technical documents we obtained, together with the model and conditions. The cost calculation in the second half uses hypothetical figures prepared for this article. Also, we have not reviewed the text of the IEC 60947-4-1 standard itself. What we write about the standard comes through the explanations in manufacturer documents.

Why Thai Factories Should Review Magnetic Contactor Selection Now

There are three triggers for a review: the movement in capital investment, updates to standards and products, and differences in voltage.

First, investment. According to the Thailand Board of Investment (BOI) announcement (PR No.117/2569, July 23, 2026), applications for investment promotion in the first half of 2026 (January to June) numbered 1,299 projects worth about 1.47 trillion baht, up 37 % year on year. Of these, applications in “machinery, automation and robotics” numbered 82, worth about 13.1 billion baht. There were also 132 applications, worth about 17.2 billion baht, under the Smart and Sustainable Industry measure, which supports equipment upgrades and automation. Note that “approvals” in the same period were 1,300 projects worth about 1.31 trillion baht, a separate figure from applications. As new and upgraded panels increase, so do the occasions for selecting contactors.

Next, standards and products. As of October 16, 2026, the IEC Webstore lists the March 2026 corrected version (including corrigendum 1) of IEC 60947-4-1:2023 (Edition 5). There are also movements on the product side. Mitsubishi Electric had announced that it would end production of its enclosed magnetic starters and enclosed magnetic contactors (MS-(2X)□ type and S-(2X)□CZ type) at the end of September 2026 (Notice No.214, March 2025). The scope is the enclosed types; the open-type MSO and S-T types are not covered. Schneider Electric explains in an FAQ that TeSys Deca LC1D115 and LC1D150 are scheduled to be discontinued at the end of 2026. Change notices for older Mitsubishi Electric series have continued into 2026, so there are quite a few opportunities to review the models of maintenance spare parts.

Finally, voltage. In a service quality report from a Provincial Electricity Authority (PEA) office, the low-voltage system is written as “380/230 V,” and the reference for the 220 V system as 200 to 240 V. When a panel designed for the Japanese 200 V class is used in Thailand, the column to read in the rating table changes. For example, according to the Mitsubishi Electric MS-T/N catalog (October 2023 edition, p.39), the AC-3 class rated capacity of the S-T21 is 4 kW in the 200–220 V column and 7.5 kW in the 380–440 V column. Even for the same model, the value you read changes when the column changes. In this article, we consider that selection begins with confirming this column.

Seven Things to Decide and One Thing to Verify in Magnetic Contactor Selection

Here are the seven items from the introduction again, in the order of selection.

1. Utilization category and voltage column: For which duty (AC-1, AC-3 or AC-4), and in which voltage column, do you read the rated operational current?
2. Motor full-load current, switching frequency and electrical durability: Work from the nameplate values of the motor actually delivered, and decide the model from the number of starts and the required life.
3. Overload relay: Trip class, setting value and corrections.
4. Short-circuit protection coordination: Type 1 or Type 2, and which row of the manufacturer’s combination table is used.
5. Coil and auxiliary contacts: AC or DC, voltage, power consumption, surge suppression, and number of auxiliary contacts.
6. Ambient temperature and mounting density: Whether the panel temperature and mounting spacing satisfy the assumptions behind the rating.
7. Combination with safety circuits: Mirror contacts and mechanical interlocks.

And the one thing to verify is writing these into the RFP and confirming them at FAT/SAT.

Why isn’t the decision settled by “amps” alone? The basis lies in the definition of rated operational current. In its catalog, Mitsubishi Electric defines rated operational current as “the maximum applicable current that satisfies the making capacity, breaking capacity, switching frequency and switching durability at the rated operational voltage.” A Schneider Electric technical document (posted on a distributor’s site) also explains that the rated operational current is determined taking into account the rated operational voltage, switching frequency and duty, utilization category and ambient temperature. In other words, the ampere figure is not a standalone value; it is a value conditional on voltage, category, frequency, life and temperature.

Let us also confirm the approach to protection. In the protection coordination section of its catalog, Mitsubishi Electric lists the following principles.

• The combined operating characteristic of the thermal overload relay and the molded case circuit breaker should lie below the motor’s thermal characteristic and above its starting characteristic and full-load current.
• For overloads at or below the locked-rotor current, the thermal overload relay should operate first.
• For currents exceeding the breaking capacity of the magnetic starter, the molded case circuit breaker should operate.
• It is desirable for the breaker to operate at or below the overload withstand capability of the magnetic starter.
• The operating characteristic of the breaker should lie below the current-time characteristic of the wire’s allowable current.

The contactor, overload relay and short-circuit protective device work not one by one but as a combination. The design and procurement flow for the control panel as a whole is covered in Control Panel Design for Thailand: Circuit, Thermal and FAT. This article focuses on selecting components for motor starting circuits.

Figure 1: Conceptual diagram of the magnetic contactor selection flow. Decisions are made in the order MOTOR LOAD LIST, UTILIZATION CATEGORY (AC-1 / AC-3 / AC-4), STARTS & DURABILITY (number of starts and durability), OVERLOAD RELAY (trip class, setting), SHORT-CIRCUIT COORDINATION (Type 1 / Type 2), COIL & AUX CONTACTS (coil and auxiliary contacts), AMBIENT & MOUNTING (ambient temperature and mounting), and SAFETY, RFP & FAT / SAT (safety circuits, RFP and acceptance tests).

Terminology: Magnetic Contactor, Magnetic Starter, Thermal Overload Relay and MMS

Because names differ by manufacturer and language, let us sort them out first.

Mitsubishi Electric calls the version with phase-loss protection “2E” (overload and locked rotor, phase loss) and the electronic version that also covers reverse phase “3E” (overload and locked rotor, phase loss, reverse phase). When people on the shop floor say “magnet” or “magnet switch,” whether they mean the contactor alone or the magnetic starter with a thermal overload relay differs from person to person. In RFPs and quotations, distinguishing magnetic contactor from magnetic starter by model reduces misunderstandings.

There are also configurations that combine an MMS and a contactor (and an overload relay) into one circuit. In this article, we describe this as an “MMS plus contactor combination.”

We should also note where the standard stands. The standard for low-voltage contactors and motor starters is IEC 60947-4-1. As of October 16, 2026, the version listed on the IEC Webstore is IEC 60947-4-1:2023 (Edition 5, March 2026 corrected version). According to the summary on that page, the scope covers electromechanical contactors and starters with rated voltages not exceeding 1,000 V AC / 1,500 V DC. Some catalogs, like Mitsubishi Electric’s, show ratings under the Japanese standard JIS C 8201-4-1 alongside, and others, like Fuji Electric’s, list JIS ratings and IEC ratings in separate tables.

Utilization Category: AC-1 and AC-3 Rated Operational Currents Differ Even for the Same Model

What utilization categories mean

A utilization category classifies the duty under which a contactor switches. Manufacturer documents explain them as follows.

• AC-1: Mitsubishi Electric describes it as “switching duty for electric heating, resistive loads and the like, with almost no inrush current at starting,” and states that the durability test condition is making and breaking 1 time (1×) the rated operational current.
• AC-3: Mitsubishi Electric describes it as “switching duty for normal starting and stopping of motors,” and states that the durability test condition is making 6 times and breaking 1 time (1×) the rated operational current. A Schneider Electric technical document gives the general explanation that under AC-3 the contactor makes a starting current of about 5 to 7 times the motor’s rated current on closing, and breaks the rated current on opening.
• AC-4: Mitsubishi Electric explains that it is a duty of switching the starting current (durability test: making 6 times, breaking 6 times), more severe switching than AC-3 class, and that inching and plugging fall into this category.

The multiples here are test conditions and manufacturers’ general explanations. The actual starting current of a motor differs by model, so it needs to be checked against the nameplate or the motor manufacturer’s data.

Examples where the column changes for the same model

The following table lists the rated operational currents from the catalogs and data sheets we obtained, by category and voltage column. It is not a comparison between manufacturers; it is a table for seeing how values change within the same model.

In the Mitsubishi Electric rows, the values in ( ) are for the magnetic contactor alone. According to the notes in the company’s catalog, the values in ( ) for S-T21 and S-T35 apply when the contactor is used alone; for a magnetic starter with a thermal overload relay, the smaller value outside the ( ) applies. In the S-T21 AC-3 class 200–220 V column, that is 18 A for the magnetic starter and 20 A for the contactor alone.

Three things can be read from this table.

• Even for the same S-T21, values differ greatly by column: AC-1 class 32 A, AC-3 class 18(20) A, and AC-4 class (380–440 V) 13 A.
• For the Fuji Electric SC09X as well, AC-1 is 20 A while AC-3 (200–240 V) is 11 A.
• For the Schneider Electric LC1D38, the rated operational current carries an ambient temperature condition of “below 60 °C.”

If you see the large AC-1 figure and use it for a motor circuit, the contactor will be switching under a duty beyond its rating. The starting point is the distinction of reading the AC-3 column for direct-on-line motor starting, and the AC-4 column when there is a lot of inching or plugging.

How AC-3e is handled

The IEC Webstore summary of IEC 60947-4-1:2023 lists “update of Annex C including rational about AC-3e” among the changes. The Fuji Electric SC-NEXT catalog and the Schneider Electric LC1D38 data sheet list AC-3 and AC-3e side by side with the same values. This is as far as this article can go. We have not reviewed the text of the AC-3e definition.

Even in the same motor circuit, the category changes with position

In its catalog, Mitsubishi Electric states that magnetic contactors used on the input side of its inverters are selected at AC-1 class, and that their electrical durability is 500,000 operations, or 25 operations when used for emergency stops while the motor is running. On the other hand, it states that a contactor on the inverter output side that switches over to commercial-power operation is selected by the AC-3 class rated operational current against the motor’s rated current. This is an example where, even for contactors connected to the same motor, the category changes depending on where in the circuit it sits and what it switches.

Figure 2: Rated operational current of the Mitsubishi Electric S-T21 (MS-T/N catalog, October 2023 edition, p.39). AC-1 (200–240 V) 32 A, AC-3 (200–220 V) 18 A (20 A for the magnetic contactor alone), AC-4 (200–220 V) 18 A, AC-4 (380–440 V) 13 A. Even for the same model, the rating differs depending on the column you read.

Motor Full-Load Current and Starting: Switching Frequency and Electrical Durability Are Different Values

Use the nameplate of the motor actually delivered for full-load current

In the early stages of design, the current is sometimes estimated from the motor’s kW. However, in this article we consider that contactor and overload relay selection should use the full-load current on the nameplate of the motor actually delivered. This is because, if the efficiency class differs, the current can differ even at the same kW. For example, in the application table for its TR65X thermal overload relay, Fuji Electric lists the full-load currents of standard-efficiency (IE1) and high-efficiency (IE3) motors in separate columns. Since the manufacturer itself shows them separately by efficiency class, setting the relay from an estimated value is something to avoid.

Switching frequency and electrical durability

Contactor specifications contain two values with similar-sounding names.

• Switching frequency: The number of operations that can be performed per hour (operations/hour). Mitsubishi Electric defines it as “the number of operations per hour that can be switched under conditions specified in the standard.”
• Electrical durability: A guideline for the number of operations usable in practice with current flowing in the main circuit (operations). Mitsubishi Electric defines switching durability as “a guideline (reference value) for the number of operations usable in practice under conditions based on the standard. It varies depending on the operating environment.”

In other words, switching frequency is a guideline for “how fast operations can be repeated,” and electrical durability for “how many operations it lasts in total.” Even if the switching frequency is satisfied, once the cumulative number of operations exceeds the durability, the risk of contact wear and welding rises.

Here are values from manufacturer documents, with models and conditions.

Even for the same AC-3 electrical durability, values differ by model, voltage and manufacturer. In a note in its catalog, Fuji Electric explains that electrical durability is the value at 220 V assuming JIS test conditions, that it varies with the characteristics of the motor used and the load conditions, and that a high starting current lowers durability and can cause contact welding. As mentioned, Mitsubishi Electric also defines it as a “reference value.” It is reasonable to read electrical durability not as a promised value but as a guideline the manufacturer gives under conditions.

The model changes with the required life

Fuji Electric’s catalog contains an application table showing which model to use for each required electrical durability in direct-on-line starting (AC-3) (p.27). According to that table, for a 380–440 V, 5.5 kW (13 A) motor, the SC18X is used for 1 million to 3 million operations, the SC20X(D) for 4 million, and the SC32X for 5 million. For a 200–240 V, 3.7 kW (17.4 A) motor, the SC18X is used for 1 million operations, the SC20X(D) for 2 million, the SC26X(D) for 3 million to 4 million, and the SC38X(D) for 5 million.

What this shows is that “the smallest model that matches the kW” and “the model that matches the required life” are different things. For conveying equipment that starts and stops many times a day, you need to estimate the annual number of operations and decide the model from the expected life.

Ambient temperature, ON-time ratio and mounting orientation

In its 3RT2 manual (October 2014 edition), Siemens shows that the mechanical durability and service life of the 3RT20 change with ambient temperature. For sizes S0 to S2, mechanical durability is 10 million operations with a service life of 20 years at an ambient temperature of 60 °C or below, 5 million operations and 15 years at 65 °C, and 1 million operations and 10 years at 70 °C. The company explains that these service life values assume a 100 percent ON-time ratio, and that at 50 percent the values double.

Mounting orientation also matters. Fuji Electric explains that with the horizontal mounting specification (Z109), mechanical durability, electrical durability and switching frequency become 80 percent.

Inching and high-frequency operation

Inching and plugging are AC-4 duties, and high-frequency intermittent operation that repeatedly switches the starting current comes close to the same conditions. Mitsubishi Electric states that in such applications, a thermal overload relay with a saturable reactor may avoid nuisance tripping and provide proper protection, but that detailed study is required.

Overload Relay Trip Class and Setting (Thermal Overload Relay Selection)

Trip class is a “time band”

Trip class classifies how long an overload relay takes to operate. Mitsubishi Electric and Siemens present the following table in their documents as the values specified in IEC 60947-4-1. The Mitsubishi Electric table is for an ambient temperature of 20 °C, and the Siemens table is for a balanced three-phase load from the cold state.

The Mitsubishi Electric table also shows, as specified values, no operation within 2 hours at 1.05 times the setting current and operation within 2 hours at 1.2 times. The Siemens overload relay reference manual (April 2011 edition) explains that the trip class is the range of time within which the relay must operate from the cold state at 7.2 times the current.

The point to note is that a class number is not a single time but a “band.” Class 10 and class 10A have different lower limits for the band. Furthermore, for the electronic 3RB20/3RB21, Siemens explains that with tolerance band E, class 10 is 5 to 10 seconds, a different band from the thermal type. Even for the same “class 10,” you need to check the band in the documents for the relay type and manufacturer. The larger the class number, the more the band shifts toward a longer allowable time before operation.

How to choose the class

For loads with long starting times, the overload relay is more likely to operate during starting. In this article, we consider that the order is to check whether the load’s starting time is sufficiently shorter than the lower limit of the chosen class band (4 seconds for class 10), and if not, to raise the class or consider a product with a different characteristic offered by the manufacturer. Mitsubishi Electric cites the saturable-reactor type (TH-□SR type) for fans and blowers with long starting times, and the quick-acting type (TH-□FS type) for submersible motors and compressor motors with short allowable locked-rotor times. As a selection principle, the company states that the operating characteristic curve of the thermal overload relay should lie below the motor’s thermal characteristic and above the motor’s starting characteristic curve.

The class of each model should also be checked in manufacturer documents. For example, in the Mitsubishi Electric catalog (p.149), the TH-T18, TH-T25 and TH-T50 are trip class 10A, and the TH-N120 to N400 are 10.

Setting and corrections

Mitsubishi Electric gives the following cautions about correcting setting values.

• When the thermal overload relay is mounted on its own rather than combined with a magnetic starter, divide the calculated setting value by the correction factor (for example, 1.06 for TH-T25 and 1.05 for TH-T65).
• Because the minimum operating current becomes larger when the connecting wire is thicker than standard and smaller when it is thinner, correct the setting (for example, for TH-T18 with a standard of 2 mm², 98 percent for 1.25 mm² and 103 percent for 2.5 mm²).

In addition, all models of the company’s TH-T type are fitted with ambient temperature compensation, and the operating ambient temperature is given as −10 to +40 °C (reference 20 °C, up to 55 °C inside a panel).

Phase-loss protection

In three-phase motors, phase loss is also an issue. In the operating characteristics of its TR type, Fuji Electric shows the unbalanced conditions for the version with phase-loss protection (no operation at 100 percent on 2 poles and 90 percent on 1 pole; operation within 2 hours at 115 percent on 2 poles and 0 percent on 1 pole). Mitsubishi Electric indicates that phase-loss protection is provided by the TH-□KP type with 3 elements (2E), and combined reverse-phase and phase-loss protection by the ET-□ type electronic motor protection relay (3E).

Coordination with Short-Circuit Protection: Type 1 and Type 2 (Contactor Coordination)

The difference between Type 1 and Type 2

When a short circuit occurs, the contactor and overload relay are also subjected to large currents until the circuit breaker or fuse clears the current. The type of coordination determines how much damage is permitted.

In its 3RT2 manual, Siemens explains that IEC 60947-4-1 distinguishes two types of coordination, that in both the short circuit is safely handled, and that the only difference is the degree of damage to the equipment after the short circuit.

• Type 1: Siemens explains that after the short circuit is cleared, the load feeder may become inoperable, and damage to the contactor and overload relay is permitted. A Schneider Electric technical document explains that there must be no danger to persons or installations, and that it may not be possible to resume operation without repair or replacement of parts.
• Type 2: Siemens explains that after the short circuit is cleared, there must be no damage to the overload relay or other parts, and operation can be resumed without replacement. However, it states that welding of the contactor contacts is permitted only if they can be separated easily without significant deformation. Schneider Electric likewise explains that the risk of contact welding is permissible, and in that case the manufacturer must indicate the measures to be taken for maintenance.

Type 2 does not mean “nothing happens.” It needs to be understood on the premise that light welding can occur and that it is dealt with according to the measures indicated by the manufacturer.

A Schneider Electric technical document explains that configurations without coordination are not permitted by standards such as the starter standard. The same document describes Type 1 as the solution when equipment cost is low and operational reliability is not a requirement, citing air conditioning in commercial facilities as an example. It describes Type 2 as the solution that reduces machine downtime and maintenance after a short circuit, citing escalators as an example.

Combination tables tested by manufacturers

Coordination is not determined by calculation alone. Manufacturers publish the combinations they have tested in tables.

• Fuji Electric: On p.30 of the SC-NEXT catalog, it publishes a coordination table with short-circuit protective devices based on IEC standards. It lists magnetic starter models, thermal setting ranges, MCCB and earth leakage circuit breaker models, and fuse ratings, divided into Type 1 and Type 2. For example, the row for the SW18X with a 15–18 A heater gives, for Type 1, short-circuit current r = 3 kA and a 50 A device such as the BW50SAG, and for Type 2, a 63 A fuse.
• Schneider Electric: In a technical document (posted on a distributor’s site), it lists combinations of fuse, contactor and thermal overload relay, and of circuit breaker and contactor, divided into Type 1 and Type 2. The same document explains that the company’s coordination tables are generally based on a current Iq of 50 kA or more. The LC1D38FL data sheet lists a 63 A gG fuse as Type 1 and Type 2 coordination at 690 V or less.
• Siemens: On p.310 of the 3RT2 manual, it lists, for the 3RT2535/2536, gL/gG fuse ratings of 125 A / 160 A for Type 1 and 63 A / 80 A for Type 2.

As for Mitsubishi Electric, its catalog explains that combining the MMP-T (MMS) with the S-T can expand the UL short-circuit current rating (SCCR). We have not been able to confirm IEC Type 1 or Type 2 combination tables in the body text of the catalog we obtained this time.

In the RFP, have them specify “which row of which table”

If you require Type 2, you will use a set of “short-circuit protective device (MCCB, MMS, fuse) plus contactor plus overload relay” that appears in a combination table tested by the same manufacturer. In this article, we consider the key point to be not only writing the type of coordination in the RFP, but also having the source of the combination table (document name and edition) and the applicable row (model, setting range, r or Iq) included in the delivery documents. How quickly you can recover after a short circuit can change greatly depending on this one row.

Coil and Auxiliary Contacts: Voltage Range, Power Consumption and Surge

Operating voltage range

The coil needs to operate reliably even if the control supply voltage fluctuates somewhat.

• Fuji Electric: Explains 85–110 percent for AC-operated types, and 85–110 percent (ambient temperature 55 °C) or 80–110 percent (40 °C) for DC-operated types.
• Mitsubishi Electric: Explains that the coil operates without problems at 85–110 percent of its rated voltage at an ambient temperature of 40 °C (panel temperature 55 °C). It then asks that the coil be used at an average of 95–100 percent, because continuously applying a voltage above 100 percent of the rating accelerates insulation deterioration of the coil and also lowers mechanical durability. It states that the AC-operated DC-excited coils of the T65 to T100 and similar are a low-voltage compensated type that operates without problems even if the voltage drops to 65 percent of the rating when the contacts touch (the first 1 to 2 cycles).
• Siemens: Explains that all SIRIUS contactors meet the operating range of 0.85 to 1.1 times specified by the IEC standard, and that many models operate at 0.8 to 1.1 times.

If the coil drops out due to a voltage drop at motor starting or a voltage sag, the line stops. How to measure actual voltage fluctuations is covered in Factory Power Quality Monitoring in Thailand: Sags, Harmonics and Acceptance.

Power consumption and driving from a PLC

Coil power consumption is determined by the model and coil voltage. Here are some examples.

• According to the Mitsubishi Electric catalog (p.30), the coil of the AC-operated S-T21/T25 draws 7 VA continuously and 75 VA at the instant of closing.
• The company’s DC-operated SD-T12 to T32 are given as 2.2 W with DC12 V / 24 V coils, and it explains that there is no inrush current.
• According to the Fuji Electric SC-NEXT catalog (p.22), the DC-operated standard SC09XG to 18XG (24 V) are 3.9 (stated in VA) for both closing and holding, and the low-consumption SC09XG-L to 38XG-L are 2.4. The company states that the low-consumption type is for 24 V only.
• The Schneider Electric LC1D38FL (110 V DC low-consumption coil) data sheet lists the closing and holding power as 2.4 W at 20 °C.

Whether a low-consumption coil can be used depends on the model and coil voltage. If you drive the coil directly from a PLC transistor output, check the DC coil’s power consumption and whether it has an inrush. When many contactors are run on 24 V DC, it also affects power supply capacity. For how to select the power supply, see 24V DC Power Supply Selection for Control Panels: Capacity, Protection and FAT/SAT.

Surge suppression

The surge when the coil is switched off can damage the output elements of a PLC. How manufacturers handle this differs by model.

• Fuji Electric states that neither the AC-operated nor the DC-operated SC-NEXT types have a surge absorber built into the coil, and explains that an optional unit should be used if needed. It states that output transistors of PLCs and similar devices must have a withstand voltage at or above the sum of the coil surge and the output supply voltage.
• Mitsubishi Electric states that the AC-operated coils of the T65 to N800AB types have a built-in surge absorbing function, so a coil surge absorber unit is not needed. For the T10 to T50, there are options such as manufacture with a surge absorber or the UT-SA21.
• The Schneider Electric LC1D38FL data sheet lists built-in surge suppression by a bidirectional diode.

Mitsubishi Electric cautions that combining a surge absorber delays the time from switching off the coil until the main contacts open. It also cautions that with DC coils, in a circuit that switches the negative side with a control contact, electrolytic corrosion may break the coil wire under high-humidity conditions prone to condensation, and recommends switching on the positive side. In humid Thai factories, this is an easy point to overlook.

Auxiliary contacts

The number of auxiliary contacts differs by model. According to the Mitsubishi Electric catalog (p.30), the standard auxiliary contacts are 1a for the S-T10, 1a1b for the S-T12/T20, and 2a2b for the S-T21/T25 and S-T35/T50. The auxiliary contact rating is given as 3 A at 200–240 V, AC-15 class. If they are insufficient, an add-on unit is used. Counting in advance how many points you will use for run confirmation and interlocks prevents having to reselect the model.

Ambient Temperature and Mounting Density: Rating Assumptions That Matter Inside Thai Panels

Reference temperatures differ by manufacturer

Contactor ratings assume a certain ambient temperature. That assumption differs by manufacturer.

Fuji Electric explains that the ambient temperature here is the temperature immediately around the product. You need to look not at the air temperature outside the panel but at the temperature around the contactor inside the panel. Also, because Fuji Electric’s conditions include a “24-hour average of 35 °C,” you will need to check the day-and-night average as well.

Mounting spacing

When devices are mounted side by side in close contact, each other’s heat raises the temperature.

• Fuji Electric indicates mounting spacing (arc space), states that ceiling mounting is not allowed, and that horizontal mounting requires a dedicated specification.
• Mitsubishi Electric gives the minimum spacing for IEC 35 mm rail mounting as 5 mm for the T10 to T32 and thermal relays, and 10 mm for the T65/T80. While it states that side-by-side mounting in close contact is possible, it notes that placing products with continuous current, high switching frequency or high usage rates on the same rail can shorten life due to temperature rise and shock, and that thermal overload relay characteristics change slightly due to mutual heat from the heaters, and asks that at least the minimum spacing be kept as far as possible.
• Siemens explains that up to 60 °C there are no restrictions on mounting in close contact, and that above that a spacing of 10 mm may be required.

Temperature, life and humidity

In its catalog, Mitsubishi Electric writes that it is generally said that insulation life halves for every 6 to 10 °C rise in ambient temperature (the Arrhenius law). It further states that if a voltage above the rating continues to be applied to the coil while the ambient temperature is high, life can become extremely short.

On humidity, the company states that for exports to tropical regions the effect of humidity is the greatest, and recommends putting 3 kg or more of silica gel per 1 m³ inside the packing box. It also states that anti-corrosion specification products are available for environments with corrosive gases.

For altitude, Fuji Electric gives 2,000 m or less and Siemens up to 2,000 m as standard conditions. This item is unlikely to be an issue for factories on the Thai plains, but writing it in the RFP as a condition makes things easier to organize.

Panel cooling methods and temperature rise calculations are covered in Control Panel Thermal Design & Cooling Selection in Thailand. The point of this article is to put the assumed panel temperature and the reference temperature behind the contactor rating side by side in the same table.

Combining with Safety Circuits: Mirror Contacts and Mechanical Interlocks

What mirror contacts are

In emergency stop and door interlock circuits, the main contacts of the contactor are sometimes monitored for welding. Mirror contacts are used for this.

• Fuji Electric: Explains that the SC-NEXT meets the mirror contact requirements of IEC 60947-4-1 Annex F both as a contactor alone and when used with an auxiliary contact block, and has obtained TÜV certification. It states that even if the main contacts weld, the NC auxiliary contact stays open and prevents restarting, and illustrates the NC contact gap as 0.5 mm or more.
• Mitsubishi Electric: Explains that it has obtained TÜV conformity certification for mirror contacts (applicable frames are the 10A to 32A frames), that they conform to the EN 60204-1 requirements for control functions in the event of failure, and that they can be used as contacts for interlock circuits.
• Schneider Electric: Explains in an FAQ that a mirror auxiliary contact is an NC auxiliary contact that reflects the state of the NO main contacts. For the TeSys Deca LC1D09 to LC1D95, the 21-22 NC contact on the body is a mirror contact, and because it cannot be removed, the company recommends using the body NC contact rather than a front-mounted block.
• Siemens: Explains in the 3RT2 manual that mirror contacts are NC auxiliary contacts that can never be closed at the same time as the main NO contacts. The company states that 3RT2 motor contactors with one or more NC contacts have mirror contact characteristics in combination with auxiliary switches.

A safety relay unit takes this NC contact into its external device monitoring (EDM) input and does not allow a restart if the contacts have welded. How to select the safety relay side is covered in Selecting a Safety Relay: E-stop, Guard, EDM and Reset as One Safety Function.

Mirror contacts alone do not determine safety

Using a contactor with mirror contacts does not by itself determine the level a safety circuit achieves. Our note in this article is that the category or performance level achieved is determined by the risk assessment, design and verification of the circuit as a whole. For how to proceed with validation when using a safety PLC, see Safety PLC Implementation: RFP, Validation and FAT/SAT Evidence.

In its catalog, Mitsubishi Electric cautions that the contacts of magnetic starters may weld and fail to open due to switching excessive current, abnormal contact wear, chattering of the command contact, aging or end of life, and that safety should be ensured assuming that closing or opening may become impossible. The idea is to build the circuit on the premise that welding can occur.

Mechanical interlocks for forward-reverse circuits

If the forward and reverse contactors close at the same time, a phase-to-phase short circuit results. There are configurations that use a mechanical interlock unit in addition to the electrical interlock. Fuji Electric lists a Mechanical Interlock Unit, and Mitsubishi Electric lists the UT-ML20 (T10 to T20) and UN-ML21 in their catalogs. In this article, we recommend writing even the part number of this unit into the RFP for forward-reverse circuits.

Relationship with Inverters and Soft Starters

For motors whose speed is controlled by an inverter, the role of the contactor differs from that in direct-on-line starting. As mentioned earlier, Mitsubishi Electric selects magnetic contactors on the input side of its own inverters at AC-1 class, and gives their electrical durability as 500,000 operations, or 25 operations when used for emergency stops while the motor is running. It states that a contactor that switches over to commercial-power operation on the output side is selected at AC-3 class against the motor’s rated current.

What can be said from this is that a usage in which the contactor on the inverter input side is switched at every run and stop can fall outside the duty and number of operations the company assumes. It is advisable to confirm on the drawings a division of roles in which running and stopping are done by inverter commands and the input-side contactor is used for switching the power on and off and for emergencies.

Updating and selecting the inverter itself is covered in Inverter Control: A Practical VFD Retrofit and Selection Guide, and choosing between servo and inverter in Servo Control vs Inverter Control 2026 | 4 Boundaries That Decide the Choice. Soft starter selection is outside the scope of this article.

Public Information from Major Manufacturers and How to Check Prices

How to compare public information

The following table organizes which information appeared where in the documents we obtained for this article. It does not indicate the superiority of products or make recommendations. Manufacturers are in alphabetical order.

This table is a clue to “which part of which document to look at to confirm.” When ordering, please confirm with the latest catalog or data sheet for each model.

How to check prices

The meaning of a price differs depending on the type of document. Please note that these are not prices that can be lined up and compared.

• Manufacturer’s standard price: The Mitsubishi Electric MS-T/N catalog (October 2023 edition, p.81) lists standard prices (catalog listing, October 2023 edition) of 13,970 yen for the S-T21 magnetic contactor and 22,700 yen for the MSO-T21 magnetic starter. These are catalog values for the Japanese domestic market, not actual selling prices.
• Online store selling price: On MonotaRO’s sales pages (viewed on October 16, 2026), for example, the Fuji Electric SC09XA had a selling price (excluding tax) of 4,698 yen, 5,168 yen including tax, and the Mitsubishi Electric MSO-T10 (with TH-T18 thermal overload relay, AC200 V coil) had a selling price (excluding tax) of 6,698 yen, 7,368 yen including tax. We have not been able to confirm on the pages what the suffix characters of the part numbers mean.

A contactor alone and a magnetic starter differ in whether a thermal overload relay is included, and prices also change with the coil voltage and auxiliary contact configuration. Standard prices and online store prices are also different in nature. For these reasons, this article does not line up prices for comparison. We could not confirm publicly available prices in Thailand in this research. For procurement in Thailand, the realistic approach is to align the model, coil voltage, auxiliary contacts and presence of a thermal overload relay, and then get quotations from distributors.

RFP Requirements and FAT/SAT Acceptance Criteria

Requirements to write in the RFP

In this article, we consider that even when the selection of contactors and overload relays is left to the panel builder, having the buyer write the following items into the RFP and receive a selection rationale document and the source of the coordination table reduces rework.

As Fuji Electric lists JIS ratings and IEC ratings in separate tables, the notation of rated capacity can differ by standard even for the same model. In this article, we consider that for panels destined for Thailand, having ratings written as IEC 60947-4-1 compliant ratings makes comparison and checking easier.

For how to define the scope of outsourced electrical design, see Electrical Design Outsourcing for Thailand Factories: RFP to FAT/SAT, and for handover practices in panel manufacturing in Thailand, see Control Panel Manufacturing Thailand: A Practical Guide to Specification, FAT and Handover.

What to confirm at FAT (factory acceptance test)

In this article, we propose confirming the following points at the panel builder’s factory.

• Whether the overload relay setting values match the nameplate values in the load list and the approach to corrections.
• Whether the models of the installed short-circuit protective device, contactor and overload relay match the applicable row of the combination table.
• Whether the measured coil voltage is within the rating and operating voltage range.
• After continuous operation, how the temperature around the contactors inside the panel compares with the manufacturer’s assumed temperature.
• Restart prevention by mirror contacts. Simulate a state in which the main contacts remain closed and confirm that the safety relay stops a restart (the procedure is decided in advance with the safety designer).

What to confirm at SAT (site acceptance test)

On site, confirm the following points with the actual load and power supply.

• Measure the starting current and starting time under actual load, and confirm that the overload relay does not trip and that the relationship between the setting value and the actual load current is as expected.
• Confirm that the coil operates reliably at the site supply voltage, including the voltage drop during motor starting. The fact that Mitsubishi Electric asks for use at an average of 95–100 percent is also a basis for judgment.
• The panel temperature after continued operation.
• That the recovery procedure in the event of a trip or welding has been handed over to the maintenance staff.

Maintenance: Contact Wear, Life and When to Replace (Magnet Switch Maintenance)

Contactors are consumables. However, this article does not give general values such as “replace after so many years” or “replace after so many operations.” As mentioned, the electrical durability shown by manufacturers is a conditional reference value. Mitsubishi Electric defines switching durability as a guideline that “varies depending on the operating environment,” and Siemens shows that service life values change with the ON-time ratio and ambient temperature.

What helps in maintenance is having the figures for your own equipment.

• Know the number of operations. If you count run commands in the PLC, you can compare them with the manufacturer’s electrical durability guideline.
• Keep a record of trips. Record which circuit stopped, when, and from what cause (overload, welding, short circuit).
• Look at coil deterioration factors. Mitsubishi Electric cautions that coil life is affected by ambient temperature and applied voltage, and that DC coils can suffer electrolytic corrosion depending on humidity and which side is switched.

There is also a method of continuously monitoring motor current to catch signs of abnormality from changes in starting current or load. This is covered in detail in Motor Current Signature Analysis Implementation: RFP, FAT and SAT.

Think also about how to hold spare parts. Manufacturers’ product updates and discontinuation notices continue. When choosing replacement parts, do not choose a successor based only on the ampere rating of the original model; check the utilization category, coil voltage, auxiliary contacts, mounting dimensions and combination with the overload relay. The same checks are needed even when extending the life of an old model with refurbished parts.

Model Calculation of Costs and Benefits (Hypothetical Figures)

From here, we estimate the costs and benefits of a review using hypothetical figures prepared for this article. These are not values from a real factory.

Scenario: A Japanese-owned electronic components factory in Thailand. In a utility panel with motor starting circuits (direct-on-line) for 12 motors, including conveyors, pumps and fans, stoppages caused by welding of contactor contacts and by overload relay trips at starting keep occurring. The panel builder is asked to provide a load list and selection rationale for a review, the contactors and overload relays are replaced from the standpoint of utilization category, trip class and coordination, and setting and an FAT-equivalent confirmation run are carried out.

The only cost item counted is the loss from stoppages caused by the contactors and overload relays in this panel. Effects on quality and other stoppages are not counted.

The breakdown of the 160,000 THB initial cost is 40,000 THB for preparing the load list and selection rationale document and checking against coordination tables, 72,000 THB for parts and replacement of contactors and overload relays for 12 circuits, and 48,000 THB for setting and the FAT-equivalent confirmation run.

Figure 3: Annual downtime loss compared for the current state (360,000 THB), after the base case (60,000 THB) and after the sensitivity case (180,000 THB). The reduction is 300,000 THB/year in the base case (payback about 6.4 months) and 180,000 THB/year in the sensitivity case (payback about 10.7 months). These are hypothetical figures prepared for this article.

How to read it: The payback period changes greatly depending on “how far stoppages are reduced” and “the loss per hour of downtime.” If stoppages fall to once a month, it is about 6.4 months; if they fall only to 3 times a month, about 10.7 months. If changing the combination to include the short-circuit protective devices or adding panel cooling is needed, the initial cost will be larger than in this calculation. Note also that replacing parts does not necessarily eliminate stoppages. Before implementation, we recommend recording the number and duration of stoppages, the devices that tripped and the causes (welding, overload, short circuit), and checking them against the load list. In this article, we consider that requesting a selection rationale document and the source of the coordination in the RFP at the time of new installation makes this kind of rework easier to reduce.

Frequently Asked Questions

What is the difference between a magnetic contactor and a magnetic starter?

In Mitsubishi Electric’s glossary, a magnetic contactor (contactor) is a contactor that opens and closes its main contacts by the force of an electromagnet, and a magnetic starter (magnet switch) is a combination of a magnetic contactor and a thermal overload relay. In orders and quotations, distinguishing which one is meant by model reduces misunderstandings.

How do AC-3 and AC-1 ratings differ?

AC-1 is switching of resistive loads and the like with almost no inrush current, and AC-3 is switching for normal starting and stopping of motors. The rated operational current differs even for the same model; for the Mitsubishi Electric S-T21 it is AC-1 class 32 A and AC-3 class (200–220 V) 18(20) A (the value in ( ) is for the magnetic contactor alone). For direct-on-line motor starting, read the AC-3 column.

How should the trip class of a thermal overload relay be chosen?

The trip class is the “band” of time until operation at 7.2 times the setting current. In Mitsubishi Electric and Siemens documents, the bands are 2 to 10 seconds for class 10A, 4 to 10 seconds for class 10, 6 to 20 seconds for class 20, and 9 to 30 seconds for class 30 (for the thermal type). Think in terms of whether the load’s starting time is sufficiently shorter than the lower limit of that band, and for long starts, raise the class or consider a product with a different characteristic offered by the manufacturer.

What is Type 2 coordination?

It is the type of coordination that requires operation to be resumable without replacement after a short circuit is cleared. However, according to Schneider Electric and Siemens documents, light contact welding that can be separated easily is permitted, and in that case the maintenance measures are to be indicated by the manufacturer. If you require Type 2, specify the applicable row of a combination table tested by the manufacturer.

Should the coil voltage be AC or DC?

There is no single answer. If you want to drive the coil directly from a PLC transistor output, check the DC coil’s power consumption, whether it has an inrush, surge suppression and the withstand voltage of the output element. For an AC coil, check the voltage fluctuation of the control supply and the operating voltage range (for many models, such as 85–110 percent of the rating; it differs by model and ambient temperature). In either case, please confirm in the manufacturer documents for each model.

How should ambient temperature be considered in a Thai factory?

The ambient temperature assumed by the rating differs by manufacturer. Fuji Electric gives −10 to +55 °C with a 24-hour average of 35 °C or below, Mitsubishi Electric a panel temperature of 55 °C, and Siemens up to 60 °C as standard. We recommend assuming not the air temperature outside the panel but the panel temperature immediately around the contactor, and measuring it at FAT and SAT to compare with the assumption.

What does a magnetic contactor cost?

The meaning differs by type of document. In the Mitsubishi Electric MS-T/N catalog (October 2023 edition), the standard price (catalog listing) of the S-T21 is 13,970 yen. At MonotaRO (viewed on October 16, 2026), the Fuji Electric SC09XA had a selling price (excluding tax) of 4,698 yen. Because the price changes depending on whether it is a contactor alone or a magnetic starter, the coil voltage and the auxiliary contacts, please do not compare them side by side; align the conditions and get quotations.

Summary

• Magnetic contactor selection is not settled just by pulling out a model that matches the ampere rating. Which column (utilization category, voltage, ambient temperature) you read is the selection itself.
• In motor circuits, check the AC-3 rated operational current (AC-4 if there is a lot of inching or plugging) against the full-load current on the nameplate of the motor actually delivered.
• Switching frequency (operations/hour) and electrical durability (operations) are different values. Electrical durability is a guideline the manufacturer gives under conditions, and the model can change depending on the required life.
• The trip class of an overload relay is a time band. Check it against the starting time; the setting has corrections for stand-alone mounting and wire size.
• In short-circuit coordination, Type 1 and Type 2 differ in the range of permissible damage. Specify the applicable row of a combination table tested by the manufacturer.
• The coil (operating voltage range, power consumption, surge), ambient temperature and mounting spacing, and mirror contacts and mechanical interlocks are also conditions of selection.
• In this article, we consider that writing these into the RFP and confirming at FAT/SAT the applicable row of the combination table, the setting values, the coil voltage and the panel temperature is the shortest route to reducing rework.

TOMAS TECH can help even from the stages of organizing the load list for motor starting circuits, reviewing selection rationale documents and coordination tables received from panel builders, and organizing RFP requirements. Please contact us here.

References

• IEC 60947-4-1:2023 Low-voltage switchgear and controlgear – Part 4-1 (Edition 5.0, March 2026 corrected version) — IEC Webstore (published 2023, checked October 16, 2026)
• Manual SIRIUS Innovations 3RT2 Contactors/Contactor assemblies — Siemens (October 2014)
• Reference Manual Protection Equipment – Overload Relays — Siemens (April 2011)
• Mitsubishi Magnetic Starters MS-T/N Series Catalog — Mitsubishi Electric (October 2023)
• Mitsubishi Magnetic Starters MS-T Series Technical Data Collection — Mitsubishi Electric (December 2021)
• Notice of End of Production for Enclosed Magnetic Starters/Magnetic Contactors (No.214) — Mitsubishi Electric (March 2025)
• Notice of Change in Structural Screw Specifications for Magnetic Starters and Magnetic Contactors (Yama-1685) — Mitsubishi Electric (June 2026)
• SC-NEXT series Magnetic Contactors and Starters catalog (62C1-E-0307) — Fuji Electric (November 2024 Ed10)
• New Magnetic Contactor and Magnetic Switches SC-NEXT series Notice of release (A23005) — Fuji Electric (November 2023)
• TeSys Complementary technical information, Chapter A6 Coordination and standards — Schneider Electric technical document (posted on a distributor’s site, 2020)
• FAQ FA105974 Type 1 / Type 2 coordination — Schneider Electric (published 2003, updated April 2026)
• FAQ FA142116 Mirror contacts on TeSys Deca / TeSys K — Schneider Electric (checked October 16, 2026)
• Product data sheet LC1D38FL — Schneider Electric data sheet (posted on a distributor site, generated May 2026)
• Product data sheet LC1D32AF7 — Schneider Electric data sheet (posted on a distributor site, generated April 2026)
• Magnetic Contactors, Fuji Electric (SC09XA, SC12XA product page) — MonotaRO (viewed October 16, 2026)
• TeSys Contactor D Series, Schneider Electric (product page) — MonotaRO (viewed October 16, 2026)
• Open-Type Magnetic Starters, AC-Operated, MSO-T Series, Mitsubishi Electric (product page) — MonotaRO (viewed October 16, 2026)
• Thailand Secures 1H 2026 Investment Surge (PR No.117/2569) — Thailand Board of Investment (July 23, 2026)
• PEA Thung Song Office Service Quality Standard Report — Provincial Electricity Authority (PEA), posted on the OIC Information Center (checked October 16, 2026)