Sep 1, 2026
Electrical Enclosure Thermal Load Sizing: Will Your Panel Overheat?
A practical way to size an enclosure for heat: estimate internal watt loss, choose passive ventilation vs air conditioning vs heat exchanger, and avoid VFD overheating.
You sized the enclosure for the components, mounted everything, and closed the door. Two weeks later the VFD trips on over-temperature during a summer afternoon. The box was big enough to fit the gear — but not to cool it.
Enclosure sizing is really two problems: physical space, and thermal space. This short guide gives you a working method to estimate the heat your panel generates and pick the right cooling strategy before you buy the box.
The basic heat balance
Inside a sealed enclosure, every watt of electrical losses becomes heat that has nowhere to go. If the heat entering (from components and ambient) exceeds the heat leaving (through walls, vents, or active cooling), the internal temperature climbs until something derates or trips.
The target is simple: keep the internal air at or below the temperature your most sensitive component will tolerate — typically around 40–50 °C internal for many drives and power supplies, with tighter limits for some electronics.
Buyer Decision: Ask for the internal temperature rise, not just the enclosure's IP rating. A sealed IP66 box with 300 W of losses and no cooling will exceed component limits in a warm room regardless of how good the seals are.
Step-by-step: estimate the internal heat load
Before choosing cooling, add up the losses:
- VFD / variable-frequency drive — typically 2–4% of the motor rating as heat (a 22 kW VFD can dump 600–900 W).
- Transformer — 2–5% of rated VA as heat.
- PLC and I/O — usually small, 10–50 W per rack.
- Power supplies — 10–25% of output wattage lost as heat.
- Contactors, relays, resistors — check nameplate; braking resistors can be hundreds of watts intermittently.
Sum these to get total internal watt loss. This single number drives everything below.
Where the heat comes from (and why VFDs dominate)
In a motor control panel, the VFD is usually the largest heat source by far and the component most likely to trip. If your panel has a VFD, size the cooling around it, then confirm the rest fits. Our motor VFD control panel spec checklist covers the wider design points.
Choosing the cooling strategy: a sizing table
Internal load | Enclosure / ambient | Recommended approach | Notes |
|---|---|---|---|
< 50 W | Cool room (<25 °C) | Sealed, no cooling | Natural convection through walls |
50–200 W | Moderate room | Passive vents + fan (filtered) | Needs IP rating trade-off |
200–600 W | Warm or sealed (IP65/66) | Air conditioner (closed-loop) | Keeps ingress rating intact |
600 W – 1.5 kW | Harsh / sealed | Enclosure air conditioner or heat exchanger | Heat exchanger keeps IP, no outside air |
> 1.5 kW | Dense drives | Air conditioner + internal fan + layout planning | May need split cooling zones |
Engineering Note: A filtered fan lowers the IP rating of the panel because it opens it to airflow. If you need IP65/66, you generally must use a closed-loop air conditioner or heat exchanger instead of passive vents.
Quick rules of thumb
- A small fan moves roughly 1–2 W of heat per cubic meter per hour of airflow — but only if the panel is not sealed. Use it for indoor, lower-IP panels.
- Enclosure air conditioners are rated in watts of heat removed (e.g., 300 W, 500 W, 1000 W units). Size to ~1.2× your calculated loss to leave margin.
- Heat exchangers transfer internal heat to outside air without mixing — they preserve the ingress rating, which matters for washdown or outdoor duty.
- Layout matters: mount the VFD away from the top (heat rises), keep clearance around vents, and separate high-loss devices from temperature-sensitive ones.
When passive ventilation is not enough
If the site is outdoor, washdown, or must stay IP65/66, passive ventilation is off the table. You need active closed-loop cooling. The failure mode we see most is a buyer specifying a sealed enclosure with no AC "because the components fit" — then the VFD overheats on the first hot day. The guide to choosing the right electrical enclosure covers the wider selection logic.
Frequently asked questions
How do I know if my panel needs an air conditioner? Add up the internal watt losses. If they exceed what passive convection can shed for your ambient temperature and your required IP rating, you need an air conditioner or heat exchanger. As a rule, sealed IP65/66 panels above ~200 W almost always do.
My VFD keeps tripping in summer. Is the enclosure too small? Often it is a thermal sizing gap, not a physical one. Check the VFD's heat loss against the panel's cooling capacity. Adding a closed-loop air conditioner or relocating the VFD usually solves it without a bigger box.
Can I just add a fan to an IP66 enclosure? Not without compromising the IP66 rating — a fan needs an opening. For sealed duty, use an enclosure air conditioner or heat exchanger that keeps the ingress rating intact.
Engineering Takeaway
Size the enclosure for heat, not just hardware. For complete enclosure and panel cooling packages, see our electrical enclosure solutions. Total the internal watt losses (the VFD usually dominates), then match a cooling method to your IP requirement: passive vents for indoor lower-IP panels, closed-loop air conditioning or heat exchangers when the box must stay sealed. Order the cooling rated to ~1.2× your calculated loss, and you avoid the summer over-temperature trip that sends buyers back to the drawing board.
Related guides
Get a panel that stays cool
Tell us the devices going inside — especially any VFDs and their ratings — and the environment the panel will sit in, and we will size the enclosure and cooling together so it does not overheat in the field. Start on our requirements page, and we will return a matched enclosure and cooling proposal from Foshan-area suppliers with the component brands you use.