How to Use the Switchboard Heat Rise and Ventilation Calculator
Every watt lost in breakers, transformers and contactors turns into heat trapped inside the enclosure. Devices are rated at a 40 °C (104 °F) ambient, so once the internal air passes that point breakers must be derated and electronic components age far faster. This calculator estimates internal temperature from total watt loss and enclosure size, then sizes the airflow needed.
Effective dissipation area uses the free-standing enclosure form Ae = 1.8H(W + D) + 1.4WD, and painted steel in still air dissipates roughly 5.5 W per square meter per kelvin, which in imperial units gives ΔT (°F) = 3.52 × P (W) ÷ Ae (ft²). When forced ventilation is required, airflow follows the standard fan sizing rule CFM = 3.16 × P (W) ÷ ΔT allowable (°F).
Enclosures pushed against a wall or lined up in a row lose exposed surface, so real temperatures run higher than this estimate and deserve extra margin. UL 891 also limits temperature rise at specific points, such as 65 °C rise on bus bars and lower limits on accessible surfaces and handles, so meeting the internal air target is not the whole story. Clogged filters cut airflow dramatically, and final selections should always be confirmed against manufacturer thermal data.
Frequently Asked Questions
Add the published losses of each breaker, relay, transformer, drive and power supply from manufacturer data, adjusted for actual load current rather than full rating. When no data exists, assuming 1 to 3 percent of rated throughput as loss is a common conservative starting point.
Fans only work when the surrounding air is cooler than your internal target. If ambient is close to the allowable temperature, or if dust and moisture make outside air unacceptable, a sealed panel air conditioner or air-to-air heat exchanger is required.
A supply fan without enough exhaust area builds back pressure and delivers far less than its rated CFM. Exhaust grille free area should at least match the inlet, and filters need a scheduled replacement interval to keep the calculated airflow intact.