How it works
Min conductor = FLC × 125% (430.22(A)). Max breaker/fuse = FLC × Table 430.52 multiplier, snapped up to the next standard size (240.6(A)). Overload ≤ nameplate FLA × 125% (SF ≥ 1.15) or × 115% (430.32).
FLC is the Full-Load Current read straight from NEC Table 430.250 or 430.248 by horsepower and voltage — not computed from efficiency and power factor. The branch-circuit conductor is sized at a minimum of 125% of that table FLC (430.22(A)); pick a wire whose 75 °C ampacity meets it on the wire ampacity chart. The branch-circuit short-circuit and ground-fault protective device uses a much larger multiplier from Table 430.52 — 250% for an inverse-time breaker, 175% for a time-delay fuse, 300% for a non-time-delay fuse, 800% for an instantaneous-trip breaker — because it must let the motor draw its locked-rotor inrush at start without tripping; it is NOT the overload protection. When that calculation does not land on a standard ampere rating from 240.6(A), 430.52(C)(1) Exception No. 1 permits the next standard size up. The overload relay is the thermal protection and is the only number sized from the actual nameplate FLA, at 125% (service factor ≥ 1.15 or temperature rise ≤ 40 °C) or 115% otherwise (430.32(A)(1)). That FLC-for-the-breaker, FLA-for-the-overload split is the crux of NEC 430.
Code references
- Three-phase full-load currents NEC 2023, Table 430.250
- Single-phase full-load currents NEC 2023, Table 430.248
- Use table FLC (not nameplate) for conductor and device sizing NEC 2023, 430.6(A)(1)
- Branch-circuit conductor minimum 125% of FLC NEC 2023, 430.22(A)
- Maximum branch-circuit short-circuit / ground-fault device NEC 2023, Table 430.52
- Next standard size up; increased rating for starting NEC 2023, 430.52(C)(1) Exception No. 1 and Exception No. 2
- Overload device 125% / 115% of nameplate FLA NEC 2023, 430.32(A)(1)
- Standard ampere ratings for fuses and inverse-time breakers NEC 2023, 240.6(A)
FAQ
Why is the breaker sized from the NEC table but the overload from the nameplate?
They protect against two different faults and NEC 430.6(A)(1) splits the job. The branch-circuit device (breaker or fuse) only has to clear short-circuits and ground faults, and it must ignore the brief locked-rotor inrush when the motor starts — so it is sized large (250% of FLC for an inverse-time breaker) from the standardized Table 430.250 / 430.248 value, which the code requires you to use instead of the nameplate. The overload relay handles the slow thermal overload of the running motor, so it is sized tight — 125% of the actual nameplate FLA (430.32). Using the nameplate for the breaker, or the table value for the overload, is the most common motor-circuit code violation.
Why is the motor breaker sized at 250%?
A motor draws six to eight times its running current for a fraction of a second at start (locked-rotor inrush). A breaker sized to the running current would trip every time the motor started, so NEC Table 430.52 lets an inverse-time breaker be set up to 250% of the full-load current — enough to ride through the inrush. That device is short-circuit and ground-fault protection only; the thermal overload relay, set near 125% of nameplate FLA, is what actually protects the motor from a sustained overload.
What is the difference between FLC and FLA?
FLC (Full-Load Current) is the standardized value from NEC Table 430.250 or 430.248, looked up by horsepower and voltage; the NEC requires you to use it for sizing conductors and the branch-circuit protective device (430.6(A)(1)). FLA (Full-Load Amperes) is the actual current stamped on the motor nameplate and is used only for sizing the overload relay (430.32). They are often close but rarely identical — and mixing them up is a code violation.
How do I size an overload relay for a motor?
Take the motor nameplate FLA (not the table FLC). For a motor with a marked service factor of 1.15 or more, or a temperature rise of 40 °C or less, the overload device may be set at no more than 125% of that FLA. For any other motor it is 115% (NEC 430.32(A)(1)). For a 10 HP / 460 V motor with a 14 A nameplate FLA and SF 1.15, the overload is set at 14 × 1.25 = 17.5 A maximum.
What is the next standard breaker size for a motor?
NEC 240.6(A) lists the standard fuse and inverse-time-breaker ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and up. After you multiply the table FLC by the Table 430.52 factor, if the result is not one of those numbers, 430.52(C)(1) Exception No. 1 lets you go up to the next standard size. For example a 7.5 HP / 460 V motor (FLC 11 A) on an inverse-time breaker calculates to 11 × 250% = 27.5 A, which rounds up to a standard 30 A breaker.
Can I go above the 250% breaker multiplier?
Only when the motor will not start on the standard rating. NEC 430.52(C)(1) Exception No. 2 then permits an inverse-time breaker up to 400% for motors of 100 A or less (300% above 100 A), a time-delay fuse up to 225%, and a non-time-delay fuse up to 400%. This calculator shows that ceiling in the verdict so you can confirm you are still within the absolute maximum an inspector will accept.
This calculator is provided for estimation purposes. Always verify motor circuit sizing against the current NEC edition, the motor nameplate and listing, and local amendments with a licensed electrician or electrical engineer before installation.