Motor Breaker Sizing Calculator
Size the branch-circuit short-circuit and ground-fault device for a single motor per NEC Article 430. Pick the motor and the device type; the tool pulls full-load current from Table 430.250 or 430.248, applies the Table 430.52 percentage, and reports the maximum breaker or fuse plus the 430.22 conductor ampacity.
How to use this calculator
- Choose phase, voltage, and horsepower. The tool reads the full-load current from NEC Table 430.250 (three-phase) or Table 430.248 (single-phase), not the nameplate.
- Set the motor type. Squirrel cage and Design B differ only for instantaneous-trip breakers; wound rotor uses lower percentages across the board.
- Pick the protective device. Table 430.52 sets a different percentage of full-load current for each of the four device types.
- Read the maximum device rating, the percentage applied, and the 430.22 conductor ampacity. The notes flag any 430.52(C)(1) Exception No. 1 round-up.
NEC reference
This calculator follows NEC 2020 Article 430. Full-load current is taken from Table 430.248 (single-phase) or Table 430.250 (three-phase induction motors) per 430.6(A)(1). The branch-circuit device is limited to the Table 430.52 percentage of that current, with 430.52(C)(1) Exception No. 1 permitting the next higher standard 240.6 rating. The branch-circuit conductor is sized to at least 125 percent of full-load current per 430.22. These values are stable across the 2017, 2020, and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
The motor branch-circuit sizing method
Motor circuits split protection into two jobs. The branch-circuit device (this tool) handles short circuits and ground faults and must survive starting inrush, so it is sized as a large multiple of full-load current. Running overload is handled separately by an overload relay or heater sized near the nameplate current under NEC 430.32. That division is why a motor breaker can legally sit far above the conductor ampacity.
Two rules drive the numbers. First, NEC 430.6(A)(1) says to use the table full-load current, not the nameplate, for both conductor and device sizing. Second, Table 430.52 gives the maximum device rating as a percentage of that current, and the percentage depends on both the motor type and the device type.
Table 430.52 maximum percentages
| Motor type | Non-time-delay fuse | Time-delay fuse | Inst.-trip breaker | Inverse-time breaker |
|---|---|---|---|---|
| Squirrel cage (standard) | 300% | 175% | 800% | 250% |
| Squirrel cage, Design B | 300% | 175% | 1100% | 250% |
| Wound rotor | 150% | 150% | 800% | 150% |
Percentages are the maximum permitted rating or setting. Synchronous and DC motors have their own rows and full-load-current tables and are out of scope for this tool.
Worked example: 5 HP, 230V, three-phase
- Full-load current from NEC Table 430.250 is 15.2 A.
- Conductor per 430.22: 15.2 x 1.25 = 19 A minimum ampacity, which a 12 AWG copper conductor at 75 degrees C satisfies.
- Inverse-time breaker per Table 430.52: 15.2 x 250 percent = 38 A.
- 38 A is not a standard 240.6 rating, so Exception No. 1 permits the next size up: a 40 A breaker.
Frequently asked questions
What size breaker for a 10 HP, 230V three-phase motor?
Table 430.250 gives 28 A full-load current. On an inverse-time breaker a standard squirrel-cage motor is allowed up to 250 percent, so 28 x 250 percent = 70 A, which is a standard rating and the maximum breaker. The conductor must carry at least 35 A (28 x 1.25) per 430.22.
Why not just use the nameplate amps?
NEC 430.6(A)(1) requires the table full-load current for conductor and branch-circuit device sizing. The nameplate full-load amperes drives only the separate overload device under 430.32. Using the nameplate for branch-circuit protection is a common and code-incorrect shortcut.
Where does overload protection fit?
Overload (running) protection is a separate device, typically an overload relay in the starter, sized at 115 to 125 percent of the nameplate current under NEC 430.32. This tool sizes only the branch-circuit short-circuit and ground-fault device and the conductor.
Does this handle more than one motor on a feeder?
No. This calculator sizes one motor branch circuit. Feeders serving several motors are sized under NEC 430.24, 430.25, 430.28, and 430.62, which add the largest branch-circuit device to the sum of the other full-load currents. That is a planned follow-up.
Related calculators
- Breaker Size Calculator: general-use branch and feeder loads under 210.20(A) and 240.6(A).
- Wire Gauge Calculator: minimum AWG for the 430.22 conductor ampacity.
- Wire Ampacity Calculator: derated ampacity of a specific conductor.
- Wire Size Calculator: conductor sizing with voltage drop and OCPD together.
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