Motor Branch Circuit Calculator
Pick the motor and the protective device to get the full-load current, minimum conductor, breaker or fuse maximum and overload setting.
Step-by-step
- Enter valid values to begin.
FLC values come from NEC Table 430.250 (three-phase) and 430.248 (single-phase), which you must use for sizing conductors and short-circuit protection — not the nameplate. The nameplate current is used only for overload relays. Verify against the adopted NEC edition.
Motor Branch Circuits: Why Four Different Numbers Apply to One Motor
Core Engineering Principles
A motor isn’t like a heater. At start it pulls five to eight times its running current for a moment, and the NEC knows that. So it splits the circuit into separate jobs: the conductors carry the running current, the overload relay watches for running overloads, and the branch-circuit short-circuit and ground-fault device is allowed to be much larger than the running current so it doesn’t trip on starting inrush. If you size everything off the nameplate you’ll get at least two of them wrong.
Here is the part that trips people up. For sizing conductors, the breaker and the disconnect, the Code tells you to ignore the nameplate and use the full-load currents in Table 430.250 (three-phase) or 430.248 (single-phase). The nameplate amps are used only for the overload relay. That means two motors with the same HP but different nameplates get identical conductors and breakers.
Overload: 125% of nameplate (SF ≥ 1.15) or 115% • Disconnect: ≥ 115% of FLC (430.110)
NEC & Standard References
430.6(A)(1) sends you to the tables for FLC. 430.22 sets the conductor at 125%. 430.52 and Table 430.52 give the maximum short-circuit device, and 430.52(C)(1) Exception 1 lets you round up to the next standard size from 240.6. Exception 2 allows 400% for an inverse-time breaker on motors up to 100 A FLC (300% above 100 A) when the motor won’t start on the smaller size. 430.32 controls the overload, and 430.110 the disconnecting means. For standard sizes check 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, and so on.1. Table 430.250: FLC = 14 A.
2. Conductor: 14 × 1.25 = 17.5 A → 14 AWG Cu at 75°C (20 A) works on paper. Most electricians would pull 12 AWG for mechanical strength and the voltage drop on a long run.
3. Breaker: 14 × 2.5 = 35 A → 35 A maximum. If the motor trips on start, the exception allows 4.0 × 14 = 56 A → 60 A.
4. Overload: nameplate FLA 14 A × 1.25 = 17.5 A maximum.
5. Disconnect: 14 × 1.15 = 16.1 A minimum.
- Those are maximums, not targets. The breaker can be smaller than 250%, and often should be if the motor starts fine.
- Check the nameplate for the overload. Use the real number there, or the table FLC if you don’t have it, and set the relay to what the motor says.
- VFDs change the rules. Use the drive manufacturer’s input current and Article 430 Part X.
- Multiple motors: the feeder gets 125% of the largest motor plus the rest at 100% (430.24). This calculator handles one motor.
- Terminals again: 110.14(C) limits you to the column that matches the lug, even if the wire is rated higher.