Active and Passive Cooling Methods – A Simple Way to Calculate Cooling Requirements

By Jon LaPorta, Pfannenberg USA

Here is a simple way to calculate cooling requirements for both active cooling and passive cooling methods.

Active Cooling Rule of Thumb

VFDs are typically sized in horsepower (HP) and cooling systems are measured in British Thermal Units (BTU, or BTU/h for BTU hours). But how do you convert from HP to BTU/h?

Here is the rule of thumb for air condition and water cooling:

75 BTU/h is required for every 1 HP

In other words, for a 100 HP VFD drive, 7500 BTU/h of cooling is required.

This rule of thumb is derived as follows:

  • 3% of the electrical energy in a VFD is converted to heat
  • 1 HP = 746 watts
  • 746 watts x 3% heat loss = 22 watts of heat loss per 1 HP
  • 1 watt = 4 Btu/h
  • 22 watts x 3.4 BTU/h = 75 BTU/h per 1 HP

Passive Cooling Rule of Thumb

For passive cooling solutions, such as the Pfannenberg Datawind Filterfan®, the rule of thumb is

4 CFM is required for every 1 HP to maintain 10°C above ambient in the enclosure

In other words, for a 100 HP drive, 400 CFM is required.

This rule of thumb is derived from the following equation

1 CFM = 1.82 x watts of heat loss / Δ Temp (°C)


These rule of thumb guides provide a general guide for selecting a cooling method and for sizing the cooling load requirements. For more precise calculations that account for ambient temperature and humidity and other critical considerations, we provide Pfannenberg Sizing Software at no charge.

By simplifying the calculation of cooling requirements, we hope that the adoption of VFD technology will continue grow rapidly, and that users will benefit from maximum performance and service life.