Every watt that a drive, power supply or PLC loses ends up as heat inside your control panel. This article shows how to add up those losses, how much the enclosure skin removes and how big the fan or cooling unit must be.
A panel heat load calculation tells you how many watts must leave the enclosure to keep the electronics inside at a safe temperature. We cover the loss of each device, the surface area method, the fan formula and a solved example.
What Is Panel Heat Load Calculation?
Panel heat load calculation is the method of adding up the power lost as heat by every device inside an enclosure and comparing it with the heat the enclosure can shed to the surrounding air. The difference decides whether natural cooling is enough or a fan, heat exchanger or cooling unit is required.
- Internal heat Qv = sum of losses of VFDs, power supplies, PLC, relays and transformers
- Surface heat Qs = A × k × ΔT, with k = 5.5 W/m²K for painted sheet steel
- Fan airflow V = f × (Qv minus Qs) ÷ ΔT in m³/h, with f = 3.1 near sea level
- If ambient is hotter than the set point, size a cooling unit for Qv plus the heat gain through the walls
The input power of a device is not its heat load, only the part that is lost is. A 22 kW drive does not put 22 kW of heat into the panel, it puts only its own losses, and this simple point is the base of every correct panel heat load calculation.
EXAIR states that most electrical components are designed to operate at about 35 °C. Many plant rooms in India cross that value for several months, so a fan alone is often not enough there.
How to Calculate Heat Load of Control Panel Components
The first job is a device list with the power loss of each item, taken from the datasheet. Manufacturers usually give this as power loss, heat dissipation or watt loss, and the value at full load is the one to use.
Usually the biggest source. Pfannenberg estimates that about 3 % of the drive energy becomes heat, roughly 22 W per HP.
Loss = output power × (1 ÷ η minus 1). A 240 W unit at 90 % efficiency loses about 27 W.
CPU, I/O cards, switches and gateways each lose a few watts. Use the module datasheet values.
Control transformers and line reactors lose iron and copper loss continuously.
Coils and contact resistance add a few watts each, which adds up in large panels.
240 W × (1 ÷ 0.90 minus 1)
≈ 26.7 W
VFD loss ≈ 3 % of rated power
AutomationDirect gives the example of its GS3 2020 AC drive, which has a watt loss of 750 W, or about 2,559 BTU/hr. Always prefer such a published value over a thumb rule when you do a panel heat load calculation, and read our VFD sizing and selection guide to know which drive rating you actually have.
Switch mode supplies also age, and their efficiency drops as capacitors dry out, as explained in power supply aging in automation. If you use two supplies with a diode module, as in a redundant power supply for PLC, count the losses of both supplies and the module.
Add 10 to 20 % margin to the device total for future I/O, extra relays and the heat of cable ducts. It is far cheaper than replacing a fan with a cooling unit after commissioning.
Panel Heat Load Calculation Formula With the Surface Area Method
The enclosure walls themselves move heat to the outside air when the inside is warmer. Rittal writes this as Qs = A × k × ΔT, where A is the effective surface area per IEC 60890 and k is the heat transfer coefficient.
A = effective area in m²
k = 5.5 W/m²K (painted sheet steel)
ΔT = Ti minus Tu in K
IEC 60890 does not count all six faces equally, because a face against a wall or the floor sheds little heat. For a single enclosure standing free on all sides, the effective area is shown below, with W, H and D in metres.
| Installation | Effective area A (m²) |
|---|---|
| Single, free standing | 1.8 × H × (W + D) + 1.4 × W × D |
| Single, wall mounted | 1.4 × W × (H + D) + 1.8 × D × H |
The heat transfer coefficient depends on the material. AutomationDirect lists 5.5 W/m²K for painted steel, 3.7 for stainless steel, 12 for aluminium and 3.5 for plastic or insulated stainless steel.
The degree of protection does not change k, but a sealed IP rating like IP 55 or IP 66 rules out simple louvres.
How to Calculate Fan Airflow for Control Panel Cooling
When the walls cannot remove all the heat and the ambient air is cooler than the set point, a filter fan pushes cool air through. Rittal gives the required volume flow as V = f × Qv ÷ ΔT, where f is an altitude factor.
V in m³/h, Q in W, ΔT in K
f = 3.1 m³K/Wh at 0 to 100 m
| Altitude above sea level | Factor f (m³K/Wh) |
|---|---|
| 0 to 100 m | 3.1 |
| 100 to 250 m | 3.2 |
| 250 to 500 m | 3.3 |
| 500 to 750 m | 3.4 |
| 750 to 1000 m | 3.5 |
Thin air carries less heat, so a panel in Pune at about 560 m needs f = 3.4, nearly 10 % more air than the same panel in Mumbai. In imperial units, Pfannenberg uses CFM = 1.82 × W ÷ ΔT in °C, and AutomationDirect uses 3.17 × W ÷ ΔT in °F.
Filters create back pressure, so check fan curves as shown in electronic cooling fans and thermal efficiency.
Pfannenberg gives a quick rule of about 4 CFM per HP of drive to hold the enclosure 10 °C above ambient. It is a useful sanity check, but it never replaces a proper heat balance.
Solved Example of Panel Heat Load Calculation
Take a free standing painted steel panel 0.8 m wide, 2.0 m high and 0.6 m deep with 600 W of internal losses. The maximum inside temperature is 40 °C, the plant room ambient is 30 °C and the site is near sea level.
A = 5.04 + 0.672 = 5.712 m²
Qs = 5.712 × 5.5 × 10 = 314.2 W
Net heat = 600 minus 314.2 = 285.8 W
V = 3.1 × 285.8 ÷ 10 ≈ 88.6 m³/h
The answer is a fan delivering about 89 m³/h at the real working point, so a catalogue fan of roughly 120 to 150 m³/h free flow is a safe pick. If the ambient rises to 45 °C, the fan can no longer help and a cooling unit is required.
Control Panel Cooling Methods Compared
Once the panel heat load calculation gives you a number, the next choice depends on two facts: is ambient air cooler than the set point, and can dusty or humid outside air enter the panel. The AutomationDirect white paper sorts the options into convection and closed loop methods.
- Lowest cost and power draw
- Simple sizing with one formula
- Easy to replace in the field
- Works well in air conditioned control rooms
- Higher cost and needs a drain
- Draws much more power than a fan
- Must be sized for wall heat gain too
- Can create condensation if set too low
For a cooling unit, add the heat flowing in through the walls: Qcool = Qv + A × k × (Tu minus Ti). Multiply watts by 3.413 to get BTU/hr, the unit used in many Indian catalogues, and compare it with how room units are rated in what is an HVAC system.
Set a cooling unit to about 35 °C, not 20 °C. A cold panel in a humid plant pulls moisture out of the air, and water on terminals is worse than a few extra degrees, a risk similar to the one in dew point in instrument air.
Step by Step Procedure
Field Troubleshooting for Hot Panels
Many hot panels suffer from blocked filters or a dead fan, not a wrong panel heat load calculation. A quick thermal scan, as shown in infrared thermography of electrical panels, locates the hot spot in minutes.
| Symptom | Likely cause | Fix |
|---|---|---|
| Drive trips on heat sink over temperature | Clogged filter mat | Clean or replace the mat on a fixed schedule |
| Top of panel much hotter than bottom | Fan and exhaust at the same level | Fan low, exhaust grille high |
| Water drops inside panel | Cooling unit set too cold | Raise set point to about 35 °C |
| Hot even with a big fan | Ambient above the set point | Change to a cooling unit or heat exchanger |
| One terminal very hot | Loose connection, not panel load | Retighten and check with a thermal camera |
- Datasheet losses for every drive, supply and PLC module
- Mounting type chosen for the IEC 60890 area
- Worst summer ambient used, not the average
- Altitude factor f taken from the site height
- Fan flow checked at the working point, not free flow
Heat also hurts protection devices, because a hot panel shifts the trip point of a thermal overload relay. In power panels, see how heat is handled in a PCC panel with large busbars.
For single hot components, a local heat sink sized with our heat sink calculation tool can move heat to the fan airflow. The basic physics of conduction through walls is covered in what is thermal conductivity.
The capacitor lifetime calculator shows in numbers why a cooler panel lasts longer. Good cooling pays back in fewer power supply failures.
Panel Heat Load Calculation FAQ
Add the power loss of every device inside the panel, taken from datasheets, and include a margin for future items. Then subtract the heat that the enclosure surface can shed, found with the formula A × k × ΔT for the walls.
What remains is the net heat that a fan, heat exchanger or cooling unit must remove. This is the core of every panel heat load calculation you will do.
Pfannenberg estimates that about 3 % of the energy passing through a drive becomes heat, which is close to 22 watts for every horsepower. A 100 HP drive therefore adds roughly 2.2 kW of heat to the panel.
The exact figure is printed in the drive manual as power loss or watt loss. Always use that datasheet number when it is available, because losses vary with switching frequency and load.
Rittal gives the airflow as V = f × Q ÷ ΔT, with V in cubic metres per hour, Q in watts and ΔT in kelvin. The factor f is 3.1 near sea level and rises with altitude.
Use the net heat after subtracting the surface loss as the heat value. Then pick a fan whose real airflow with filters fitted still meets the calculated value at the working point.
A fan can only cool when the ambient air is colder than the maximum allowed inside temperature. When the ambient is equal or hotter, a fan only pushes more heat into the enclosure.
In that case a cooling unit must remove the internal losses plus the heat that leaks in through the walls. Dirty or oily areas also favour closed loop cooling to keep the electronics clean.
EXAIR notes that most electrical components are designed to work at about 35 °C, and the electrical industry rule says life halves for every 10 °C rise above the rating. Many designers therefore target 35 to 40 °C inside.
Check the lowest rated device in the panel, often a drive or power supply, and set the limit from it. Avoid cooling far below ambient, since that can cause condensation on cold surfaces.
Related Articles
- Heat Sink Calculation
- Electronic Cooling Fans and Thermal Efficiency
- VFD Sizing and Selection Guide
- IP Rating Explained
- Infrared Thermography for Electrical Panels
External References
- AutomationDirect: How to Select and Size Enclosure Thermal Management (PDF)
- Rittal Technology Library: Fan and filter unit sizing
- Wikipedia: Electrical enclosure
What We Learn Today
- A panel heat load calculation adds the datasheet losses of drives, power supplies and PLC modules, never their input power, plus a margin.
- The enclosure surface sheds Qs = A × k × ΔT, where A follows IEC 60890 mounting rules and k is 5.5 W/m²K for painted steel.
- Fan airflow is V = f × net heat ÷ ΔT in m³/h, while a hotter ambient than the set point calls for a cooling unit.
