When a Thailand factory finds moisture on a control panel door or intermittent PLC faults at morning startup, the first question is often “How many watts should the heater be?” For control panel heater selection, that question comes too early. Condensation can form when a surface exposed to humid air falls to or below that air’s dew point. The coldest door skin, terminal, mounting plate or component matters more than a single average relative-humidity reading. A heater can help keep surfaces above the dew point, but it cannot repair a leaking cable entry, a damaged door seal or a blocked cooling-unit drain.
This guide helps engineering, maintenance and procurement teams specify a new panel or a retrofit. It covers site measurements, heater capacity, thermostat and hygrostat control, equipment placement, quotation requirements and acceptance tests. There is no universal wattage or humidity setpoint: enclosure geometry, operating states, equipment ratings and the site environment determine them. The practical objective is to give every bidder the same inputs and then verify the chosen solution under defined conditions.
Start with dew point and the coldest important surface
The US National Weather Service defines dew point as the temperature at which air cooled at constant pressure becomes saturated. A displayed relative humidity of 60% does not mean every surface inside a panel is at the same temperature. A steel door can cool overnight; a mounting plate near a chilled structure can be cooler than panel air; a cooling outlet may create a local cold spot. Compare the air’s dew point with the temperature of a surface that moisture can actually reach, especially near terminals and electronics.
Heating air lowers its relative humidity when its water-vapour content stays the same, and heating cold surfaces increases the margin above dew point. It does not remove incoming water. Photograph where the moisture occurs before cleaning. Separate surface condensation from rain or wash-down ingress, a leaking gland, or a cooling-unit drain fault. A larger heater is a poor remedy for a continuing water leak.
Rittal describes enclosure heaters as a way to regulate relative humidity and prevent components from dropping below dew point. Schneider Electric notes that the heat generated by electrical equipment may be insufficient during unloaded, standby or de-energised periods, so another heat source may be needed. The important procurement implication is to evaluate the panel when the machine is off, not only when drives and power supplies are operating.
Measure four operating states in the actual location
Log normal running, planned shutdown, overnight or holiday shutdown, and startup or restoration of power. Record inside and outside air temperature and humidity at the same time, surface temperature at selected cold points, the cooling unit’s state, door opening and HVAC operation. Location matters: an air-conditioned electronics room, a loading dock, a wash-down line and a semi-outdoor conveyor have different exposure even within one factory. Do not substitute a generic “Thai rainy season” figure for site data.
| Operating state | Evidence to capture | Why it matters |
|---|---|---|
| Normal running | Panel and ambient conditions, equipment load, cooler state | Establish heat generated by the equipment |
| Immediately after stop | Stop time, falling losses, surface and air temperatures | Reveal different cooling rates |
| Overnight and holidays | HVAC schedule, humid-air exposure, heater supply | Check protection when production power is off |
| Startup and restoration | Door opening, controls, dew point, alarm history | Find the cold-surface and humid-air encounter |
Attach sensor location, instrument type and calibration status, sampling interval and machine mode to each record. A sensor directly above the heater can falsely suggest that the whole cabinet is dry. Select points at the door, terminal area, upper and lower sections, near cooling outlets and sensitive equipment as appropriate. Capture short changes around door opening and HVAC switching as well as long trends. Surface-temperature measurement near live equipment needs a safe test method defined by the electrical team.
Inspect moisture entry and drainage before ordering a heater
Check gasket compression, cable glands, unused holes, panel joints, vents, rain direction, wash-down spray and cooling-unit drainage. The protection rating on an enclosure does not automatically describe a panel after site drilling, installation and years of gasket wear. A closed panel can still contain moist air from previous openings. Conversely, more ventilation is not always drying: when the outside air is humid, uncontrolled ventilation can bring in more vapour.
Map where air, heat and water move in every operating state. If droplets are concentrated below a cooling unit, investigate drainage. If water runs in from the door bottom, check cleaning and rain exposure. A broad film on cold metal near terminals suggests checking dew point and surface temperature. More than one mechanism may be present. Correct ingress, sealing and drain defects, then reassess the residual condensation risk that heating must address.
Size heating from a documented heat balance, not a fixed W per volume
Enclosure volume alone omits material, exposed surface area, mounting, minimum ambient temperature, shutdown heat loss and the temperature margin needed at cold surfaces. Pfannenberg describes sizing to maintain a minimum enclosure temperature using environmental and material factors and manufacturer software. A defensible calculation identifies effective heat-transfer area, temperature difference, enclosure and component losses, ventilation or leakage, mounting and any relevant external influences. It then checks that the resulting heating cannot drive nearby equipment above its permitted operating temperature.
List internal heat sources separately for running, standby and shutdown. Do not count a drive’s running loss as overnight heating if its supply is switched off. Specify the coldest plausible ambient and surface condition, the humid-air condition, door-opening pattern and a justified dew-point margin. The margin must account for measurement uncertainty and variation across the cabinet; this guide does not prescribe a universal number. Transient events, such as opening a cold cabinet to humid air, also need separate consideration because a steady-state heat calculation may not describe them.
| Sizing input | Information given to bidders | Error if omitted |
|---|---|---|
| Enclosure geometry and material | Dimensions, exposed faces, wall or adjacent-panel contact | Incorrect heat-transfer area |
| Minimum and maximum ambient | Measurement location, time and operating condition | Heating shortage or overheating |
| Moisture and dew point | Simultaneous inside/outside measurements and cold surfaces | False confidence from average RH |
| Device losses | Part number, watts and operating mode | Counting unavailable shutdown heat |
| Ventilation and leakage | Fans, vents, openings and seals | Missing heat and moisture exchange |
| Heater arrangement | Quantity, location, clearances, supply and control | Hot spots or cold areas |
Read product terminology carefully. Continuous heat output, input consumption, positive-temperature-coefficient behaviour and fan airflow are different specifications. Compare candidates at their documented voltage and ambient conditions. Rittal gives a 10–150 W continuous-output range for one series of fanless enclosure heaters and recommends several low-output units for even heat distribution in larger enclosures. This is a product range, not a universal prescription for cabinets. A fan heater can distribute heat, but adds a fan failure mode, dust movement, power draw and maintenance. A nominal airflow does not guarantee flow through a crowded cabinet.
Coordinate heating and cooling
A panel may need cooling during production and heating during shutdown. Define the state logic so heater and cooler do not routinely oppose each other. Document temperature and humidity thresholds, hysteresis, cooling-unit drainage and restart behaviour. If factory HVAC stops at night, its schedule may dominate the cabinet’s dew-point exposure. Use logs to identify which system cools surfaces and which brings in humid air.
Running a heater continuously can raise local equipment temperature. Equally, a high-humidity command may turn the heater on while temperature control wants it off. Define which condition has priority, the independent overtemperature protection, and behaviour when a sensor or controller fails. Assess communication loss if a PLC or remote system participates in the control loop. This belongs in the wiring diagram and setting schedule, not only in a commissioning note.
Choose thermostat and hygrostat control deliberately
A thermostat switches on temperature; a hygrostat switches on relative humidity. Temperature-only control can miss a humid-air event, while humidity-only control can ignore excessive temperature. Rittal recommends a thermostat for precise enclosure temperature control and a hygrostat for condensation prevention. The final circuit depends on the panel’s component ratings and operating modes.
Rittal’s SK 3118.000 hygrostat specifies a 50–100% RH setting range and an approximately 4% switching differential. These are specifications of that model, not a recommended factory setpoint. Its published contact ratings also vary by load: the document states AC 5 A, 0.2 A for an inductive load at cos φ 0.6, and DC maximum 20 W. Check heater voltage, current, inrush and whether an intermediate relay or contactor is needed. Do not assume a small sensor contact can directly switch any candidate heater.
Place the sensor where it represents the protected volume, away from the heater plume, a cold-air outlet and a door gap unless one of those is the intentional measurement point. Document settings, hysteresis, fault behaviour, manual test procedure, supply loss and automatic recovery. Handover should identify who may change settings and how changes are recorded. PTC self-regulation does not replace appropriate protection or model-specific instructions. Rittal’s instructions for one specific heater, SK 3105.370, require an upstream thermostat with a setting no higher than 30°C; that limit must not be generalised to every model or panel.
| Control approach | Main use | Procurement check |
|---|---|---|
| Thermostat switching | Maintaining a minimum temperature | Moisture excursions and overtemperature limit |
| Hygrostat switching | Controlling condensation risk | Sensor position, contact rating, temperature interaction |
| Combined control and upper limit | Complex run/stop/cooling states | Circuit priority, failures, restart and test method |
Place heaters for distribution, clearance and maintainability
A low position often takes advantage of rising warm air, but the correct location depends on wiring ducts, component layout and any forced flow. A heater that warms only its own sensor can leave the door and terminal area wet. Show candidate locations on the panel layout and verify temperature differences at critical surfaces. Several smaller heaters may be better than one high-output device for a large enclosure.
Follow the selected model’s installation instructions for spacing from cable ducts, polymer components and flammable materials. Rittal’s heater instructions tell installers to keep cables away from the heater or fan body and hot-air outlet. Check permitted mounting orientation, rail or plate mounting, terminals and protective-earthing requirements by model. A retrofit should not simply displace existing wires; update the drawings and terminal schedule.
For maintenance, define how staff detect a failed element, welded controller contact, displaced sensor, fan failure, changed setting or tripped breaker. An indicator lamp alone does not prove useful heat reached the cold surface. Leave access for inspection and cleaning. If a power cut is itself a condensation scenario, remember the heater loses power too. Specify safe inspection and restart after restoration; assess a UPS separately if it is proposed.
Put comparable inputs and deliverables in the RFP
Send all bidders the same enclosure drawings, location, construction, component list, loss by mode, HVAC schedule, shutdown supply, measured temperature and humidity, moisture photographs, existing cooling and ventilation, and maintenance constraints. Mark unmeasured values as assumptions, with an owner and date for confirmation. Ask each supplier to show how changed assumptions affect heater selection and cost.
Require a heat and dew-point basis, heater and controller part numbers, wiring and protection, settings, panel layout and clearances, installation method, FAT/SAT procedures, spare parts, local service scope and updated drawings. For a retrofit, define shutdown window, isolation, supply take-off, drilling and restoration of the enclosure seal. If alarms feed a PLC or monitoring system, define I/O, communications and failure behaviour.
| RFP item | Buyer supplies | Supplier returns |
|---|---|---|
| Moisture problem | Time, location, operating state and measurements | Cause hypothesis and missing-data survey |
| Enclosure and ambient | Drawings, mounting, temperature, sealing | Heat balance and cold-surface assessment |
| Power and devices | Loss by mode, shutdown supply, voltage | Capacity, quantity, electrical protection |
| Controls | Existing cooling/HVAC and alarms | Thresholds, priority, fault responses |
| Acceptance | Site access and reproducible conditions | Measurement points, methods and pass criteria |
| Handover | Required language and maintenance model | As-built records, settings and spares |
A calculation sheet is not comparable if its inputs have no source. Identify whether a proposed temperature or humidity came from a site measurement, the HVAC design or a temporary estimate. State which conditions cannot be reproduced at FAT or SAT and when they will be checked. For the wider procurement boundary, read control panel manufacturing in Thailand and control panel design, circuit, thermal and FAT. Integrate the condensation controls into the same panel baseline.
FAT: prove installation and control before shipment
FAT must show more than “the heater gets warm.” Match installed model, rating and position to the approved drawings. Inspect supply, protective device, terminal IDs, cable clearance, bonding where applicable and alarm wiring. Simulate temperature and humidity inputs safely; record switching, hysteresis, upper limit, sensor failure, loss and restoration of supply, and interaction with the cooler. Label simulated values as such in the test record.
Do not spray humid air onto live electronics to create visible condensation for a routine FAT. Use a manufacturer-approved sensor test method or a controlled external sensor environment. Check local equipment temperature at a representative load, recording ambient and any conditions that could not be reproduced. Attach instrument calibration, setting schedules, photographs, nonconformities and retest results to the as-built revision.
| FAT area | Evidence |
|---|---|
| Equipment and installation | Nameplate, layout, clearances, protection and photographs |
| Control | Simulated inputs, switching thresholds, hysteresis and measured outputs |
| Faults | Sensor loss, control failure assumption, power loss, restart and alarms |
| Thermal | Load, panel/ambient temperatures, measurement points and stabilisation |
| Documents | As-built diagrams, settings, deviations and retests |
SAT: test real site conditions and define deferred checks
At SAT, replace FAT simulations with the actual door, HVAC schedule, load, cooling unit and wash-down or loading-dock operation. Recheck new holes, cable glands, gaskets and drains. Record air temperature and humidity inside and outside the panel together with temperatures at protected cold surfaces. A sunny one-day SAT cannot prove year-round performance. If the humid season or a long shutdown cannot be observed, record the condition as unverified and agree who will revisit it and when.
Project-specific criteria can state that, in a defined shutdown mode, the specified cold surfaces must remain above the simultaneously measured air dew point by an agreed design margin; at high running load, equipment surroundings must remain within the manufacturer’s temperature limits; and alarms and restart must follow the agreed procedure. Define the margin, instruments, period and exceptions in the project. A single average RH limit is not enough to establish that no cold surface will condense.
Maintenance handover should name inspection points, log review, setting authority, fault contacts, replacement parts and restart decision owner. If moisture recurs, log time, photographs, inside/outside temperature and humidity, surface temperature, HVAC/cooler state, door opening and washing before increasing heater power. This evidence makes the next decision more reliable.
A disciplined field sequence for finding the moisture source
When water is found, first put the equipment into a safe condition and record its location and pattern. Record more than the discovery time: when did production stop, HVAC switch, the door open, or cleaning occur? “Found in the morning” does not prove that the moisture formed in the morning. Put humidity and temperature logs, machine and power history, HVAC records and wash-down logs on one timeline. Correct clock offsets before comparing events.
Trace water from top to bottom. Streaks below a roof or conduit entry suggest rain, washing or a pipe leak. Droplets directly below a cooling unit call for a drain check. A fine film on broad steel surfaces or around terminals with no inflow path calls for simultaneous dew-point and surface-temperature measurements. Do not treat wiping a wet live terminal as a diagnostic test. Separate immediate electrical safety from the root-cause investigation.
Do not let one sensor represent the entire cabinet. The cooling-unit outlet and return, door and rear mounting plate, upper and lower sections may have different temperatures. The purpose of additional points is to explain which air reaches which cold surface, not to collect data for its own sake. Mark probes and airflow on a simple cabinet section drawing and tie each point to photographs. This allows a supplier’s proposed heater and hygrostat positions to be assessed against the surface that actually needs protection.
An intermittent fault alone does not prove condensation. Voltage dips, loose terminals, insulation damage, dust and oil mist can resemble moisture-related symptoms. Compare fault timestamps with water observations and maintenance logs. If faults disappear after heater installation, check whether the season, operating load or cleaning schedule also changed. Effectiveness should be judged over comparable operating states or with those differences recorded.
Manage uncertainty in the heating calculation
Input quality determines the value of a thermal calculation. Cabinet dimensions may be known, but heat transfer from a back panel against a wall or from an adjacent cabinet depends on installation. A “minimum ambient” taken from a daytime average does not describe a holiday shutdown or post-wash condition. Tag each input as measured, plant specification, manufacturer data or provisional assumption, with its source. Assign a person and date to close every provisional assumption before design approval.
Do not report only one calculated wattage. Vary the inputs that matter: if ambient temperature or moisture differs from the estimate, does the heater count change and how much dew-point margin remains? Avoid stacking unrelated worst values into a physically implausible scenario. Define combinations that can really occur, such as holiday shutdown, morning door opening and high-load production. Condensation protection and overheating prevention may have different governing scenarios, so test both.
When a calculated value lies close to a catalogue boundary, compare layout changes before automatically choosing the next wattage. A smaller heater near the vulnerable surface, several distributed units, an improved airflow path or reduced humid-air ingress may change both the required output and local hot spots. A manufacturer’s selection tool supplies a candidate part; the drawing and test must still show why that part works in its proposed location.
Record calculated heat demand, installed nameplate rating and expected operating duty separately. For energy estimates, multiplying nameplate watts by 24 hours every day ignores thermostat or hygrostat cycling. Conversely, cycling does not justify undersized protection. Size wiring and protection for the electrical design and startup current; update energy estimates using operating logs. If measured results disagree with the model, revisit probe location, leakage and equipment losses rather than leaving the calculation uncorrected.
Specify the heater supply and fault behaviour
Check the schematic to confirm the heater has power when condensation protection is needed. A heater downstream of a main isolator will not operate during a planned shutdown if that isolator is off. The electrical designer must coordinate supply take-off, dedicated isolation and indication, outage behaviour and maintenance procedure. If a heater remains live while the rest of the panel is isolated, clearly label and document that condition so an open cabinet is not mistaken for fully de-energised equipment. Include it in lockout procedures.
Match protective devices and cable to operating and startup current, voltage, frequency, run length, ambient temperature and the number of heaters operating together. Whether a controller contact switches the heater directly or through a relay/contactor depends on its rating and the load. Distinguish the sensing circuit from the heater power circuit on drawings. A valid sensor reading does not rule out a welded power contact. Where required, use independent upper-temperature protection and test it safely at FAT. Select the exact protection scheme against the panel design and applicable requirements.
There is no single “safe” default for a broken humidity sensor. Heater-off can increase condensation; heater-on can cause overheating. Decide the response from the equipment and production risk: alarm, independent high-temperature cutout, maintenance intervention and restart rule. A PLC output saying ON is not proof of useful heating. For high-availability applications, consider current monitoring or scheduled function tests to detect an open heater element.
Include follow-up monitoring and change control in the scope
SAT completion is not the end of observation. If the first long shutdown or humid season occurs later, record the due date, owner, logs required and responsibility for corrective action in the contract or handover record. Condensation can be intermittent; one dry visual inspection cannot establish year-round performance. Save initial inside/outside temperature and humidity, critical surface temperatures, heater duty and alarms as a baseline. Decide whether permanent sensors are justified by the cost of failure and of inspection.
When maintenance changes a threshold, record the previous value, reason, person, time, panel ID, expected effect and retest. Lowering a humidity threshold can lengthen heater operation and affect temperature and energy use. Raising it to save power can remove the dew-point margin at the door. Check surface and equipment temperatures as well as RH after a setting change. A change to the cooling unit or room HVAC also warrants re-evaluation of the heating logic.
Adding a power supply or communication device later changes heat generation, airflow, clearances and sometimes door-opening frequency. Its running heat does not automatically replace a heater if it is off overnight. Update the as-built drawing, mode-specific loss list, settings and test record to one revision after every material change. That evidence helps future maintenance distinguish the original selection basis from current operating conditions.
Frequently asked questions
Is a heater unnecessary in hot Thailand?
No universal answer follows from outdoor climate. Equipment may cool during shutdown, HVAC may change state, and a cold metal surface may meet humid air at startup. Measure the actual installation. Some panels need a heater; others first need a repaired gland, gasket or drain.
What wattage should I buy?
Use enclosure geometry and mounting, minimum ambient, moisture and dew point, remaining internal heat during shutdown, leakage and component temperature limits in the selection. Check the manufacturer’s tool and data sheet, then verify cold surfaces and local hot spots at FAT/SAT. A product family’s wattage range is not a sizing rule.
Thermostat or hygrostat?
Use temperature control when minimum temperature is the principal requirement and humidity control when condensation risk drives heating. Complex systems may need both plus an independent upper limit. Specify sensor positions, ratings, contact loads and failures before setting thresholds.
Does a heater replace sealing or an enclosure rating?
No. It cannot stop rain, wash water or a drain leak. Inspect the completed panel, including retrofitted openings and glands, and correct the source of water.
How can I make quotations and acceptance comparable?
Provide common drawings, loss by operating mode, shutdown supply, site measurements, moisture history and existing cooling. Require the calculation basis, layout, settings, fault behaviour, FAT/SAT records and as-built drawings from every bidder. Mark assumptions and schedule field verification.
Conclusion: buy a verified condensation-control function
Begin with the moisture source and the operating state that brings a cold surface near dew point. Size heat from the shutdown heat balance, coordinate heater, hygrostat, thermostat and cooler controls, and show safe placement on the drawings. Use the same design assumptions in the quotation and FAT/SAT criteria. That turns a comparison of watts and unit prices into a comparison of demonstrable protection.
If a Thailand plant needs to investigate moisture, retrofit an enclosure heater or coordinate heating with a cooling unit, contact TOMAS TECH. We can help structure the site survey, panel modification scope, RFP and FAT/SAT acceptance evidence.
References
- US National Weather Service, Dew Point vs Humidity — dew-point definition.
- Rittal, Enclosure Heater without fan SK 3105320 — purpose, product range and distribution guidance.
- Rittal, Hygrostat SK 3118.000 — settings, switching differential and contact ratings.
- Rittal, Enclosure heater assembly and operating instructions — model-specific installation and control requirements.
- Pfannenberg, Control cabinet heaters — heating-power calculation and selection tools.
- STEGO, DCR 030 Enclosure heater with fan datasheet — fan-heater product example.
- Schneider Electric, Moisture Contamination Avoidance and Mitigation — risks during low-load and standby periods.
*Sources reviewed on 27 October 2026. Confirm the exact model and current revision of its specifications before procurement.*
