Max Motor HP for a Starting-Current Budget (NEC 430.7(B))

The largest motor a starting-current budget (a breaker, a generator's starting kVA, an SCCR / voltage-dip ceiling) can start, max_hp = starting-current / (kVA/hp x 1000 / (sqrt(3) x V)). A 300 A budget on a code-G 460 V feeder tops out at ~38 hp. The code letter is the starting-kVA letter, not the design letter; a soft starter lowers the real inrush. A design aid; the nameplate governs.

max_hp = max_starting_current / (kVA/hp x 1000 / (sqrt(3) x V)) three-phase (kVA/hp x 1000 / V single-phase); the LRA = hp x kVA/hp x 1000 / (sqrt(3) x V) relation solved for the horsepower.

The NEC 2023 Table 430.7(B) locked-rotor indicating code letters, solved for the horsepower, by name; the motor nameplate and measured inrush govern.

NEC is free to read at nfpa.org/freeaccess. Table 430.7(B) and the kVA/hp bands are in the published code; the LRA relation is first-principles.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: max_starting_current_a, code_letter, voltage_v, phase, max_horsepower, kva_per_hp

• Band uses the upper end of each code-letter kVA/hp band for a conservative starting currentNEC Table 430.7(B)
• Linear in hp LRA scales linearly with horsepower for a given code letter and voltagefirst principles
• Not the design letter the code letter is about starting kVA; the design letter (A/B/C/D) describes the torque-speed curveNEC 430.7