NEC Motor Circuit Sizing Calculator (Article 430)

Size the branch-circuit conductors, overload, short-circuit/ground-fault OCPD, and disconnect for a single motor from its NEC table full-load current per Article 430.

  1. Identify the motor Choose the "Motor type" (three-phase AC, single-phase AC, or DC), then pick the "Voltage" and "Horsepower" from the dropdowns. The full-load current (FLC) is looked up from NEC Tables 430.247/248/250 and shown next to the horsepower field.
  2. Set the overload basis Optionally enter the "Nameplate FLA" (used for overload sizing only) and toggle "Service factor ≥1.15 / rise ≤40°C" to choose a 125% versus 115% overload setting per NEC 430.32.
  3. Pick protection and starting details Select the "OCPD type" (non-time-delay fuse, time-delay fuse, instant-trip CB, or inverse-time CB) and the "Starting method". Selecting VFD halts sizing because drive-fed motors fall under NEC 430 Part X.
  4. Calculate the circuit Optionally add "Other motors' FLC sum" for a feeder, then press Calculate for the suggested OCPD, conductor minimum (430.22), overload max, and disconnect minimum (430.110).

What size breaker and wire do I need for a 10 hp 460V three-phase motor?

Select three-phase AC, 460 V, and 10 hp; the calculator uses the FLC from NEC Table 430.250 rather than the nameplate, per 430.6(A)(1). Conductors are sized at a minimum 125% of FLC (430.22), the short-circuit/ground-fault OCPD is capped by Table 430.52 (for example 250% of FLC for an inverse-time breaker), and the disconnect is at least 115% of FLC (430.110). The tool rounds the OCPD up to the next standard size.

Why does the motor calculator use table FLC instead of the nameplate amps?

NEC 430.6(A)(1) requires that branch-circuit conductors, the short-circuit/ground-fault device, and the disconnect be sized from the full-load current values in Tables 430.247, 430.248, and 430.250, not the motor nameplate FLA. The nameplate FLA is used only for overload protection sizing under 430.32. This calculator follows that split, taking the table FLC for everything except the overload.

Can I use this calculator for a VFD or HVAC compressor circuit?

No. Selecting the VFD starting method stops the calculation because variable-frequency-drive-fed motors follow NEC 430 Part X, where the drive nameplate input current governs the supply conductors. Hermetic refrigerant motor-compressors are covered by NEC Article 440, not Article 430, so HVAC compressor circuits are out of scope here.

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