Motor Branch-Circuit Sizing Calculator
Size the branch conductor, the short-circuit / ground-fault OCPD, and the running-overload protection for a single- or three-phase motor per NEC 430. Pick the motor HP, voltage, and OCPD type — the calculator looks up the table FLC and applies the correct percentages.
FLC
14.0 A
Table 430.250
Branch ampacity
17.5 A
125% of FLC (430.22)
Overload protection
17.5 A
125% of FLC (430.32)
Branch OCPD
35 A
250% → next std (430.52)
Suggested branch conductor
14 AWG copper THWN at 75°C, sized for the 17.5 A branch ampacity.Always re-check ampacity after applying ambient and conductor-count derating per 310.15(B). Run our Ampacity Calculator next.
Show calculation breakdown
- FLC = 14 A (NEC Table 430.250 @ 460 V)
- Branch conductor ampacity = 14 × 1.25 = 17.5 A
- Overload = 14 × 1.25 = 17.5 A
- OCPD calc = 14 × 2.5 = 35.0 A → next standard = 35 A (NEC 240.6(A))
The four NEC 430 calcs at a glance
- FLC — Motor full-load current from NEC Tables 430.248 (1ϕ) and 430.250 (3ϕ). Always use the table value, not the nameplate, for sizing.
- Branch conductor (430.22) — ampacity must be at least 125% of FLC.
- Branch OCPD (430.52) — Inverse-time CB 250% / time-delay fuse 175% / non-time-delay fuse 300% / instantaneous trip 800%. Round up to the next standard rating per NEC 240.6(A).
- Overload (430.32) — 125% of FLC for service factor ≥ 1.15 or temp rise ≤ 40°C, otherwise 115%. Sized to the nameplate FLC, not the table.
Test your knowledge
Educational tool only. Verify against the current NEC. Not a substitute for a licensed engineer.