Why round up to the next standard motor size?

Q: How do I calculate pump horsepower from GPM and head?

Brake horsepower is BHP = (GPM × head in feet) ÷ (3960 × efficiency). The 3960 constant is the imperial conversion factor for water (1 HP equals 550 ft·lbf/s at 62.4 lbf/ft³). This calculator computes hydraulic power first, then divides by pump efficiency to get the brake horsepower the motor must supply.

Q: What does pump efficiency mean here?

Efficiency is the fraction of input shaft power that becomes useful hydraulic power, entered as a decimal between 0 and 1. Small centrifugal pumps often run around 0.5–0.7, so lower efficiency means more brake horsepower for the same flow and head. The figure should come from the pump curve at your design operating point.

Question

Why round up to the next standard motor size?

Accepted Answer

Motors are manufactured in discrete sizes (¼, ⅓, ½, ¾, 1, 1½ HP and up), so you select the next size at or above the computed brake horsepower. You should also verify NPSH-available against the pump curve and aim to operate near the best-efficiency point to avoid overload and premature wear.

Why round up to the next standard motor size?

Answers