J1 PROFan motor sizing from airflow and static pressure
How to work out fan absorbed power and choose the motor size from airflow, external static pressure, fan efficiency and a motor margin, with a worked example.
Once the duct external static pressure (ESP) is known, the fan motor size follows from a short chain: air power, then fan absorbed power, then motor power with a margin, then the next standard motor size.
The formulas
For a fan inside an AHU or FAHU, add the internal pressure drop of filters, coils and casing to the duct ESP. A standalone fan only needs the duct ESP.
Typical values
| Fan type | Typical total efficiency |
|---|---|
| Centrifugal, backward curved | ≈ 72 % |
| Plug / EC fan | ≈ 65 % |
| Axial (tube / vane) | ≈ 65 % |
| Centrifugal, forward curved | ≈ 60 % |
| Mixed-flow inline | ≈ 60 % |
Belt drives lose about 5 % (drive efficiency ≈ 0.95); direct drive is taken as 100 %. Use the manufacturer's efficiency at the duty point whenever you have it.
A common motor margin above absorbed power is 25 % up to 1 kW, 20 % up to 5 kW, 15 % up to 20 kW and 10 % above that. The margin covers belt slip, filter loading and small changes in the system curve.
Worked example
Supply fan: 2,000 L/s (2.0 m³/s), ESP 400 Pa, backward-curved centrifugal fan (72 %), direct drive.
The full-load current then comes from the motor rating, efficiency and power factor: I = P ÷ (√3 × V × η × PF) for a three-phase motor.
Common mistakes
- Using the duct ESP alone for an AHU fan and forgetting the internal losses.
- Selecting the motor on air power instead of absorbed power.
- Adding a margin on the airflow and again on the pressure and the motor, which oversizes the fan.
The J1 Pro Fan Sizing calculator gives absorbed power, motor size, full-load current and SFP. Start from the Duct ESP calculator and carry the values across.
Guides are for preliminary design and learning. Always verify final designs against project specifications, local codes and manufacturer data.