Fan Brake Horsepower
Air horsepower, brake horsepower, and the next standard NEMA motor size from airflow, total static pressure.
Example
You enter
- Airflow (CFM) 4000
- Total static pressure (in. w.c.) 2
- Fan static efficiency (0-1) 0.65
- Drive/belt efficiency (0-1) 1
You get
- Brake horsepower (BHP) 1.936
- Next standard motor HP 2
Checked against published worked examples from: AMCA / ASHRAE fan-power relation + NEMA MG 1 sizes.
Details, formula, and sources
ScopeAir horsepower, brake horsepower, and the next standard NEMA motor size from airflow, total static pressure, and fan/drive efficiencies.
Formula or tableAHP = CFM * TSP / 6356; BHP = AHP / eta_fan; motor HP = BHP / eta_drive, rounded up to the next standard NEMA MG 1 size (none is listed above 300 hp).
Source and editionAMCA / ASHRAE fan-power relation BHP = (CFM * SP) / (6356 * eta) (public); standard motor HP sizes per NEMA MG 1, by name.
Free accessFree principles in published HVAC texts; the 6356 constant is a pure unit derivation.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namescfm, tsp_inwc, eta_fan, eta_drive, bhp, next_nema_hp
- Total static pressure external + internal static, not just externalAMCA fan-rating convention