Switchboard Heat Load Calculation and Choosing a Cooling Solution
Switchboard heat load calculation: Calculating internal power losses, the heat an enclosure can reject and how to select the right cooling solution.
ContentsA switchboard heat load calculation is a simple but frequently skipped exercise that shows whether the losses generated inside can actually be rejected by the enclosure. When it is skipped the outcome is predictable: thermal relays tripping in summer, contactor coil failures, shortened life of electronic boards and accelerated oxidation at busbar joints. Heat is the invisible but most common root cause of switchboard failures.
Adding up the losses
The calculation starts by summing the power loss of every component installed. Manufacturer catalogues give the loss of breakers and contactors at rated current, and where the actual load is lower the loss falls with the square of the current. Variable frequency drives and soft starters are the dominant sources, generating heat equal to roughly 2 to 4 per cent of their rated power. Transformers, power supplies, resistors and busbar losses must be added as well. For outdoor enclosures, solar gain enters the calculation as a separate item.
How much heat can an enclosure reject?
The heat a sealed enclosure can reject depends on its effective surface area multiplied by the difference between internal and external temperature. For sheet steel enclosures the heat transfer coefficient is typically around 5.5 W/m²K. The consequence is striking: an enclosure with 2 m² of effective surface can only reject about 165 W at a 15 K temperature difference. A modern switchboard generates far more than that, so in a wall mounted, single sided, IP54 enclosure active cooling is usually unavoidable.
Cooling options
| Solution | Achievable condition | IP compatibility | Notes |
|---|---|---|---|
| Natural ventilation (louvres) | Internal temperature stays above ambient | IP31-IP43 | Most economical; open to dust ingress |
| Filter fan | Internal temperature approaches ambient | Up to IP54 | Efficiency drops quickly if filter maintenance is neglected |
| Air to air heat exchanger | Internal temperature slightly above ambient | IP55-IP66 | Closed loop; suitable for dusty environments |
| Cooling unit (air conditioner) | Internal temperature can go below ambient | IP54-IP66 | Condensate drainage and energy consumption must be planned |
| Water to air exchanger | Highest capacity for large losses | IP55-IP66 | Requires a chilled water infrastructure |
An approach that works in practice
- Reduce the losses first: correct cross-sections, correct device selection and avoiding an unnecessarily high IP rating all cut the cooling demand
- Place heat generating devices in the upper part of the board and temperature sensitive electronics lower down
- Group variable frequency drives in a separate compartment or a separate enclosure
- Position air inlet and outlet diagonally so that no dead zone forms
- Apply the altitude correction when selecting fans and cooling units; capacity falls above 1000 m
- Measure and record the internal temperature on the hottest day after commissioning
Key takeaways: switchboard heat load calculation
- Adding up the losses — The calculation starts by summing the power loss of every component installed.
- How much heat can an enclosure reject — The heat a sealed enclosure can reject depends on its effective surface area multiplied by the difference between internal and external temperature.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is fitting a fan always the answer?
No. At best a filter fan brings the internal temperature close to ambient; if the room is at 45 °C no fan will bring the board below 45 °C. When the ambient itself is high, a heat exchanger or a cooling unit is required.
Does lowering the IP rating solve the thermal problem?
Partly, but it increases dust and moisture ingress. In a dusty plant, lowering IP improves temperature in the short term and reliability in the long term gets worse. The correct route is closed loop cooling. The subject is directly linked to IP rating selection.
Related articles
- Busbar Trunking or Cable? Choosing the Right Distribution Method
- Cable Zone and Cable Entry Design: Common Switchboard Mistakes
- Cable Sizing and Voltage Drop Calculation
- All technical articles
At Devpan we produce a loss breakdown for every project, calculate the heat the enclosure can reject and size the cooling solution accordingly. Share your device list and we can tell you in advance what internal temperature the board will run at in summer.
