Motor FLA & Wiring Calculator — NEC Article 430
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Motor circuits are sized from the NEC tables, not the nameplate amps — the part everyone gets burned on. OhmBase pulls full-load current from Tables 430.248 (single-phase) and 430.250 (three-phase), sizes conductors at 125%, and reads Table 310.16 directly for the wire.
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Branch-circuit conductors for a single continuous-duty motor are sized at 125% of the table FLC (430.22). The wire is then chosen from NEC Table 310.16 at the 75°C column — and because motor branch circuits are exempt from the small-conductor caps (240.4(G)), the raw table value governs. Overload protection is separately set from the nameplate current (430.32).
Worked example
½ HP · 115 V single-phase motor
- Table 430.248 FLA: 9.8 A
- Conductor sizing: 9.8 × 1.25 = 12.25 A
- Table 310.16 (75°C Cu): 14 AWG.
FAQ
- Why use table FLA instead of the nameplate?
- NEC 430.6(A)(1): conductors, and short-circuit and ground-fault protection are sized from the table values. Only the overload setting uses the nameplate current.
- Why can a motor breaker be bigger than the wire’s ampacity?
- The branch-circuit device is sized to let the motor start (up to 250% of FLC for an inverse-time breaker, 430.52); the running overloads protect the wire. Motor circuits are exempt from 240.4(D) per 240.4(G).
- Does OhmBase cover fractional-horsepower motors?
- Yes — the single-phase table includes 1/6 HP through 10 HP at 115 V and 230 V.