How this calculation works
Use the table, not the nameplate
430.6(A) is explicit: conductors and branch-circuit protection are sized from the values in Table 430.248 (single-phase) and Table 430.250 (three-phase), not from the motor nameplate. The nameplate is used for one thing only — overload protection.
This trips up everyone who learns it the first time. The nameplate is right there on the motor, and it is the wrong number for two of the three calculations.
Three devices, three jobs
The branch-circuit device looks absurdly oversized because it is not protecting against overload. Starting inrush on an across-the-line motor is six to eight times running current for a second or two — that is the locked-rotor figure. Anything sized to the running load would trip on every start. So the breaker handles short circuits and ground faults only, and a separate overload device sized close to the nameplate handles sustained overcurrent.
Round-up and the disconnect
430.52(C)(1) Exception 1 permits going to the next standard size when the calculated maximum does not land on one. If the motor still will not start, Exception 2 allows going higher within stated limits.
The disconnecting means gets sized at 115% of full-load current under 430.110, and it has to be within sight of the motor or capable of being locked open.
Multiple motors on one feeder
430.24 sizes a feeder at 125% of the largest motor's FLC plus 100% of all the others. 430.62 sizes the feeder protection at the largest branch-circuit device plus the sum of the other motors' full-load currents. Both of those are outside what this calculator does.
ALL 17 CALCULATORS, OFFLINE, FREE
This one runs in your browser. The app has all seventeen plus the full reference library, and they work in a basement with no signal. Free tier, no card.
Frequently asked questions
Why can the breaker be 250% of the motor current?
Because it is not protecting against overload. Across-the-line starting draws six to eight times running current for a second or more, so a breaker sized to the running load would trip on every start. The branch-circuit device covers short circuits and ground faults; a separate overload relay sized near the nameplate covers sustained overcurrent.
Nameplate or table — which do I use?
Table for conductors and branch-circuit protection, per 430.6(A). Nameplate for overload protection, per 430.32. Getting those backwards is the most common motor circuit error, and it produces either nuisance tripping or an unprotected motor depending on which way you go wrong.
What is locked-rotor current for?
Sizing the disconnect and the controller, and checking that the starting inrush will not trip upstream protection or drag the voltage down. It is the current the motor draws at the instant of starting, before the rotor turns and back-EMF builds.
Does the motor conductor get derated too?
Yes. The 125% figure is the starting point, and ambient temperature correction and conductor bundling adjustment still apply on top of it. A motor feeder in a hot mechanical room with several other circuits can end up needing a size or two more than the raw 125% suggests.
// DisclaimerThe National Electrical Code® (NEC®) and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association (NFPA). This calculator is an independent educational tool and is not affiliated with, endorsed by, or sponsored by the NFPA. All code references are paraphrased for educational purposes only and do not reproduce the official text of any code edition. This tool is not a substitute for the official NFPA 70 publication. Electricians, contractors, and students are encouraged to purchase the official National Electrical Code from the NFPA at
nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).