Size a pump motor from brake power, service factor, and margin — with the next standard NEMA (hp) and IEC (kW) size, a full-load current estimate, and starting current by starting method.
US + metric
Formula shown
Shareable results
Motor Sizing
hp
ft
SG
%
SF
%
V
Recommended motor20 hp(15 kW)
Brake power
16 hp
Required power
16 hp(11.9 kW)
Full-load current (est.)
23.7 A
Starting current (est.)
142 A (600%)
Both policies: Non-overloading: 20 hp / 15 kW · Service-factor: 15 hp / 11 kW
Formula & method
Preq = BHP · (1 + margin) [ ÷ SF for service-factor sizing ] · IFL = P / (√3 · V · η · PF)
Full-load current is a formula estimate from power, voltage, efficiency class, and an assumed 0.85 power factor — for code work use the motor nameplate or NEC 430.250 tables. Starting multiples are representative and depend on the motor code letter and starter.
For preliminary sizing and educational use. Verify against motor nameplate data, NEC/IEC requirements, and applicable codes; final designs should be reviewed by a licensed professional engineer.
Sizing a pump motor means choosing the next standard frame above the brake power the pump demands, with a margin. Take the brake power (or derive it from flow, head, specific gravity, and pump efficiency), apply your service factor and design margin, and step up to the nearest standard NEMA horsepower or IEC kilowatt rating. This tool also estimates the full-load current and the inrush at start.
Worked example
A pump needs 16 brake horsepower with a 1.15 service-factor motor:
Non-overloading sizing: nameplate must cover 16 hp → next standard is a 20 hp (15 kW) motor
Service-factor sizing: 16 ÷ 1.15 = 13.9 hp → a 15 hp (11 kW) motor, using its service factor to carry the load
At 20 hp, 460 V, 3-phase, IE3 efficiency: full-load current ≈ 24 A; direct-on-line start draws ≈ 6 × that ≈ 142 A
Two sizing philosophies — and why they differ
The two answers above are both correct; they reflect different policies. Non-overloading sizing picks a nameplate rating at or above the brake power so the motor never runs into its service factor — the conservative, longest-life choice. Service-factor sizing allows the motor to use its service-factor headroom, so a smaller (cheaper, more efficient at load) motor can carry the same pump. Running continuously into the service factor raises temperature and shortens insulation life, so many specifiers reserve it for intermittent or capped-load duty. Pick the policy deliberately.
About the current estimates
The full-load current here is a formula estimate from power, voltage, efficiency, and an assumed power factor — useful for a first pass, but for code work use the motor nameplate or the NEC 430.250 full-load-current tables, which govern conductor and overload sizing. Starting current depends on the motor’s code letter and the starting method: direct-on-line draws roughly six times full-load current, while star-delta, soft starters, and variable-frequency drives cut that inrush substantially, which matters for supply capacity and voltage dip.
