Question 1

Why divide by 3,960?

Accepted Answer

1 hp = 33,000 ft-lb/min. Q (gpm) × 8.34 lb/gal × H (ft) ÷ 33,000 = Q × H ÷ 3,960 for water (SG = 1).

Question 2

What pump efficiency should I assume?

Accepted Answer

Centrifugal end-suction at BEP: 70–85%. Split case: 80–90%. Submersible deep-well: 65–75%. Always use the manufacturer curve at your operating Q/H.

Question 3

Does specific gravity matter for water?

Accepted Answer

Use SG = 1.0 for clean water. Use ≥ 1.05 for thickened sludge, ≥ 1.20 for ferric chloride or 50% NaOH — the same pump moves heavier fluid at higher BHP.

Question 4

What about service factor?

Accepted Answer

NEMA service factor of 1.15 means a motor can briefly handle 115% of nameplate hp. Don't use SF for normal sizing — it's a safety margin, not extra capacity.

Pump WHP / BHP Calculator – Water & Brake Horsepower