What this calculator does

From the airflow and the external static pressure (ESP) it works out the fan air power and the absorbed power. It then picks the motor size with a safety margin and gives the full-load current. For fans inside an AHU or FAHU, add the losses inside the unit.

How it is calculated

Air power P = Q × p_t (ASHRAE; AMCA 203) Absorbed power = air power ÷ fan total efficiency ÷ drive efficiency (belt ≈ 95 %) Motor = absorbed power + margin 25 % ≤ 1 kW, 20 % ≤ 5 kW, 15 % ≤ 20 kW, 10 % above → next standard motor (IEC 60072)

The full-load current uses IE3 efficiency (IEC 60034-30-1, 4-pole) and a typical power factor. Typical fan efficiencies are guidance only, so use the manufacturer's selection when you have it.

Questions

Where do I get the ESP?

Use the Duct ESP Calculator. Its "Next" button carries the airflow and pressure straight into this calculator.

Worked example?

See Fan motor sizing from airflow and static pressure.

Related

Duct ESP Calculator · Duct Sizer · Pump Capacity · Fan motor sizing guide

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Results are for preliminary design. Verify final selections against project specifications, applicable codes and manufacturer data.