The short answer
Enclosure Heat Calculations
Everything in a panel that carries current turns some of it into heat, and the enclosure surface is the only thing carrying that heat out to the room. The temperature rise inside is the total heat divided by the product of the surface area and a heat transfer coefficient that depends on the enclosure material and the air around it. If the resulting inside temperature exceeds the rating of the most sensitive component, the panel needs a bigger enclosure, less inside it, or cooling. A variable frequency drive is the component that most often forces that decision.
Key points
- Sum the heat: drive losses, power supplies, transformers, the controller, relays, and anything with a heatsink.
- Temperature rise is heat divided by surface area times a coefficient; the enclosure manufacturer publishes the coefficient.
- Compare the inside temperature to the lowest rating in the panel, usually the drive, the controller, or the UPS battery.
- Outdoors, add the sun. A dark enclosure in direct sun gains more from the sun than from its contents.
- The remedies in order: a larger enclosure, moving the drive out, ventilation, a heat exchanger, an air conditioner.
Where the heat comes from
| Component | Heat | Where to get the number |
|---|---|---|
| Variable frequency drive | A few percent of the drive rating at full load, often the largest single source | The drive manual, as watts of loss at rated load |
| Power supplies | The difference between input and output power, ten to fifteen percent of the load on a modern supply | The datasheet efficiency at the actual load |
| Control transformer | Core and copper losses, a few percent of the rating | The transformer datasheet |
| Controller and I/O | Backplane draw times voltage, a few watts to tens of watts | The platform power calculation |
| Relays, contactors, starters | Coil power while energized, plus contact losses under load | The datasheets; count what is on at once |
| Soft starters | Bypass contactors reduce it at speed; significant during starts | The manual |
| Network switches, radios, HMIs | Their rated power, nearly all of it becomes heat | The datasheets |
| Conductors and terminals | Resistive losses, usually small unless currents are large | Estimate from current and length |
Add them for the worst case that actually occurs: everything that runs at once, at the load it runs at, on the hottest day. A duplex pump panel with both drives at full speed on a summer afternoon is the case, not a quiet night in winter.
The estimate
Temperature rise = Total heat / (Effective surface area x Heat transfer coefficient)
- Total heat is the sum of the losses, in watts
- Effective surface area is the enclosure area that can shed heat: for a wall-mounted enclosure, the front, sides, top, and bottom, but not the back against the wall
- The heat transfer coefficient depends on the material and the airflow: on the order of 5 to 7 watts per square meter per degree Celsius for painted steel in still air, lower for non-metallic enclosures, and the enclosure manufacturer publishes the figure to use
- Inside temperature is the ambient plus the rise; compare it to the rating of the most sensitive component
The enclosure heat load calculator on this site does this arithmetic and shows the working. A worked example: a wall-mounted painted steel enclosure 36 by 30 by 12 inches has roughly 2.7 square meters of effective surface. Two small drives at 150 watts of loss each, a 100 watt power supply at 85 percent efficiency, a controller at 20 watts, and relays at 30 watts come to about 365 watts. At 6 watts per square meter per degree, the rise is about 23 degrees Celsius. In a 35 degree room that is 58 degrees inside, above the 50 degree rating of most drives and controllers, and the panel needs help.
Outdoors
Direct sunlight on an enclosure adds heat that has nothing to do with the contents, on the order of several hundred watts per square meter of exposed dark surface at midday, reduced sharply by a light color, a reflective finish, or a sun shield with an air gap. An outdoor panel is calculated with the solar gain added and the shield assumed, and then built with the shield, because the shield is the cheapest cooling there is.
What to do when it is too hot
- 1
Take the drives out.
Put drives in their own enclosure, or in a ventilated section separated from the controls, or use drives with through-panel heatsinks that put the heat outside. Removing the largest source is worth more than any cooler.
- 2
Make it bigger.
Surface area is the denominator. A taller enclosure of the same footprint sheds more and costs less than a cooling unit.
- 3
Ventilate, if the rating allows.
Filtered fans through louvers turn the enclosure into a Type 12 at best, and a Type 1 in practice once the filter is neglected. Never on a Type 4 or 4X enclosure that must stay sealed.
- 4
Fit a heat exchanger.
An air-to-air exchanger moves heat through a sealed core without exchanging air, keeping the rating. It cannot cool below ambient and its capacity is stated in watts per degree of difference.
- 5
Fit an air conditioner.
The only option that cools below ambient. It draws power, needs a condensate drain and a filter, has a compressor that fails, and it is what a drive panel in a Florida pump house usually ends up with.
- 6
Derate or reconsider.
A drive derated for temperature, or a smaller drive because the pump never runs at full load, may reduce the heat enough. Check the drive derating curve.
Cold
The same panel in winter has the opposite problem. Displays, batteries, and some electronics have minimum temperatures, and condensation forms when the inside cools below the dew point. A thermostatically controlled heater of a few tens of watts keeps a small panel above freezing and dry. Size it from the heat loss at the design low temperature by the same arithmetic in reverse.
Frequently asked questions
- How do I calculate the temperature inside a control panel?
- Sum the watts of heat from everything inside at the worst case, divide by the effective surface area of the enclosure times the heat transfer coefficient the enclosure manufacturer publishes, and add the ambient. The enclosure heat load calculator on this site does it.
- How much heat does a VFD produce?
- Typically a few percent of its rated power at full load, stated in the drive manual as watts of loss. A 25 horsepower drive can dissipate several hundred watts, which is why drives dominate the heat load of the panels they sit in.
- Can I put a fan in a Type 4X enclosure?
- Not without losing the rating. A fan needs an opening, and an opening admits water and dust. Use a heat exchanger or an air conditioner rated for the enclosure type, or move the heat source out.
- What temperature can a PLC tolerate?
- Most controllers and drives are rated to 50 or 60 degrees Celsius inside the enclosure, and some to less. Check the rating of every component and design to the lowest one, with margin for the hot corner near the drive.
Related topics
- Enclosure SelectionChoosing the box: NEMA and UL 50 type ratings, materials for corrosive and outdoor sites, sizing for heat and wire bending space, and the details at the conduit entries and the door that decide whether the rating survives installation.
- Component LayoutArranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- Panel OverheatingA panel that runs hot: drives tripping on temperature in the afternoon, controllers faulting in summer, components discolored, a fan that never stops. The causes from an undersized enclosure to a clogged filter, measuring the rise, and the fixes.
- UL 508A Industrial Control PanelsWhat the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
- UL 698AIndustrial control panels for hazardous locations: what UL 698A adds to UL 508A, a panel in a classified area or a safe-area panel with intrinsically safe circuits, barriers and isolators, the control drawing, and where utilities meet classified areas.
Direct contact
Have a controls question?
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