Motor Full-Load Amps (FLA)
Calculate full-load current of a three-phase or single-phase induction motor from horsepower, line voltage, motor efficiency, and power factor. NEC 430.250 tabulated FLA shown for comparison — use the table value for branch-circuit design, not the calculated value.
Defaults: 50 hp, 460V three-phase, 93% efficiency, 0.86 PF — typical industrial motor. NEC 430.250 tabulated values are conservative — use them for OCPD and conductor sizing per NEC 430.6(A)(1).
NEC tabulated FLA — what it's for
NEC Table 430.250 gives standard FLA values for three-phase induction motors. These are CONSERVATIVE — they assume η × PF = 0.85 for < 5 hp and 0.92 for 100+ hp, neither of which exactly matches a specific nameplate. NEC 430.6(A)(1) requires use of the tabulated value for branch circuit and motor disconnect sizing, not the actual nameplate FLA.
The reason: protective device sizing must accommodate any motor of the given size, not just the specific motor purchased. If a more efficient motor is later substituted, the original wire and breaker must still handle it.
Why FLA isn't the design current
Conductor ampacity per NEC 430.22 = 125% × FLA (for continuous operation). OCPD per NEC 430.52: 250% × FLA for thermal-magnetic breakers, 175% for non-time-delay fuses, 300% × FLA for time-delay fuses.
Why the multipliers? Two reasons:
- Motor running on continuous duty heats up the wire — use 80% factor (= 125% sizing) to limit conductor temperature.
- Motor starting current is 6–8× FLA briefly. Conventional fuses or breakers must let starting current pass without tripping, then trip on a sustained overload.
Locked-rotor current
At startup, before the rotor accelerates, the motor draws locked-rotor amps (LRA), typically 5.5–6.5× FLA for NEMA Design B. NEMA design code letters indicate LRA range:
- Code A: less than 3.15 kVA/hp at LRA
- Code G: 5.6–6.3 kVA/hp (typical 1–10 hp)
- Code H: 6.3–7.1 kVA/hp (typical 10–250 hp)
- Code J: 7.1–8.0 kVA/hp
- Code K-V: progressively higher
Reduced-voltage starters (autotransformer, soft-starter, VFD) cut LRA to 30–60% of full-voltage starting, reducing voltage dips and mechanical shock.
Service factor (SF) overload allowance
Standard motors have SF = 1.15, meaning they can run at 115% of rated load continuously without damage. NEC 430.32 sets motor overload protection at 125% × FLA for SF ≥ 1.15, or 115% × FLA for SF = 1.0. Don't use the SF rating to sustain operation — it's an emergency margin, not a design point.
Reference: National Electrical Code (NFPA 70-2023), Article 430. NEMA MG-1: Motors and Generators. IEEE Std 141 (Red Book).