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Motor Circuit Calculator

Size it from the table, not the nameplate.

Motor circuits break the normal wiring intuition: the breaker is deliberately oversized to survive starting inrush, and the wire is sized from NEC table current — not the motor nameplate — so the circuit stays legal when the motor is eventually swapped. This calculator runs the full Article 430 workflow: table FLC by horsepower and voltage, minimum conductor ampacity at 125%, minimum wire size on the correct termination column, maximum breaker or fuse per Table 430.52, and the 115% disconnect rating.

Why people use this

Electricians and maintenance techs use it when a motor gets swapped or a compressor gets added: table FLC, 125% wire, 250% breaker, 115% disconnect — the four numbers on every motor inspection tag. The nameplate-vs-table rule is the part apprentices get wrong, and the calculator makes it impossible.

How it works

1. Pick phase, motor horsepower, and voltage — the FLC comes from NEC Table 430.248 (single-phase) or 430.250 (three-phase).
2. Choose the branch protection device: inverse-time breaker (250% FLC), dual-element fuse (175%), or non-time-delay fuse (300%).
3. Set the termination rating — 60°C for unmarked gear under 100A per 110.14(C), 75°C for marked equipment.
4. Read minimum wire size, max OCPD (rounded up to standard size per the 430.52 exception), and minimum disconnect rating.

Common questions

Why is the motor breaker allowed to be 250% of the running current?

Starting inrush. Motors draw 6–8× their running current for the moments it takes the rotor to spin up, so a breaker sized at the normal 125% rule would trip on every start. The oversized breaker only protects against short circuits and ground faults — the separate overload relay, set at 115–125% of nameplate amps, protects the motor while it runs.

Why can't I just use the nameplate amps?

NEC 430.6 requires the table values for conductor and breaker sizing precisely because nameplates vary — a high-efficiency replacement motor draws less than a cheap one, and the circuit must stay safe across swaps. The nameplate is reserved for the overload relay setting (430.32).

Why does the termination rating change my wire size?

Per 110.14(C), circuits rated 100A or less use the 60°C ampacity column unless the equipment is marked for 75°C. A 5 HP 230V single-phase motor needs 35A conductors — that is 8 AWG on the 60°C column (10 AWG tops out at 30A) but 10 AWG exactly meets it on the 75°C column. When in doubt, use 60°C.

Does this work for air conditioner compressors?

No — hermetic refrigerant motor-compressors follow NEC Article 440, not 430. Use the equipment nameplate MCA and MOP values, which already include the 125% factor. Applying Article 430 multipliers to HVAC equipment is a classic and dangerous mistake.

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