Motor Circuit Calculator — NEC Article 430
A motor circuit breaks every rule you learned on lighting circuits. The breaker is deliberately far larger than the conductor's ampacity, and that is not a violation — it is 240.4(G) working as designed, because a separate overload device is what protects the wire. This tool sizes all four parts and shows you the arithmetic.
Size the motor branch circuit
Branch-circuit device
Short-circuit and ground-fault protection only
Conductor
14 AWG
Overload device
16.25 A
Equipment grounding conductor
10 AWG copper
See the breakdown
That is a 35 A device protecting a conductor rated 20 A — and entirely correct.
The method, explained in plain English
Three devices, three jobs
The breaker clears faults. The overload relay protects the windings. The disconnect provides isolation. One breaker does not satisfy all three requirements.
Time-delay fuses need less headroom
A dual-element fuse gets only 175% because its own delay rides through inrush. A non-time-delay fuse needs 300% to do the same job.
The EGC follows the big device
Table 250.122 is keyed to the protective device rating, so an oversized motor breaker gives a larger ground than the phase conductors suggest. That is intentional.
Treat the size as a floor
12 AWG is the normal practical minimum whatever the arithmetic says, and voltage drop on a long feed regularly pushes the conductor larger.
Worked examples
The same 10 HP 460 V motor with a 13.0 A nameplate, protected three different ways.
Inverse-time breaker — the defaults
OCPD: 250% × 14 = 35.0 A → 35 A breaker
overload: 13.0 × 125% = 16.25 A · EGC: 10 AWG Cu
Result: a 35 A breaker on a conductor rated 20 A — 1.8× its ampacity. Legal, standard, and the reason motor circuits confuse people.
Dual-element time-delay fuses instead
EGC now follows a 20 A device: 12 AWG Cu
Result: the fuse's own time delay handles inrush, so it needs far less headroom — and the smaller device permits a smaller ground. Better fault protection, at the cost of carrying spare fuses.
A wound-rotor motor
OCPD: 150% × 14 = 21.0 A → 20 A
Result: a wound-rotor motor starts with resistance in the rotor circuit, so it has far gentler inrush and needs almost no headroom. The motor type matters as much as the device type.
Table 430.52 — device ceiling by type
Maximum rating as a percentage of the motor's table full-load current. The ceiling and device columns below are computed for the 14.00 A full-load current of a 10 HP 460 V Design B motor.
| Device | Design B | Ceiling at 14 A | Largest standard |
|---|---|---|---|
| Non-time-delay fuse | 300% | 42.00 A | 40 A |
| Dual-element (time-delay) fuse | 175% | 24.50 A | 20 A |
| Instantaneous-trip breaker | 1100% | 154.0 A | 150 A |
| Inverse-time breaker | 250% | 35.00 A | 35 A |
| Motor type | Non-delay fuse | Time-delay fuse | Instant trip | Inverse time |
|---|---|---|---|---|
| Single-phase, no code letter | 300% | 175% | 800% | 250% |
| AC polyphase squirrel-cage, other than Design B | 300% | 175% | 800% | 250% |
| Design B energy-efficient squirrel-cage | 300% | 175% | 1100% | 250% |
| Synchronous | 300% | 175% | 800% | 250% |
| Wound rotor | 150% | 150% | 800% | 150% |
| DC, constant voltage | 150% | 150% | 250% | 150% |
Sources & standards: NEC (NFPA 70) 2023 — 430.6(A)(1), 430.22, 430.24 and 430.62 (feeders serving several motors), Table 430.52 with 430.52(C)(1) Exception No. 2 and 430.52(C)(3) (instantaneous-trip breakers only in a listed combination controller), 430.32, 240.4(G), Table 250.122, and Table 310.16. Hermetic refrigerant motor-compressors follow Article 440 instead. Local amendments override the model code, and the AHJ has final say.
Frequently asked questions
Common questions about motor branch circuits.
Why is the breaker so much bigger than the wire can carry?
Because on a motor circuit the breaker is not there to protect the conductor against overload — the overload device is. The breaker only has to clear a short circuit or ground fault, and it has to be large enough to let the motor's starting inrush through without tripping. NEC 240.4(G) routes motor conductors out of the normal overcurrent rules to Article 430 precisely so this is permitted. A 10 HP motor on 14 AWG behind a 35 A breaker is fully compliant.
What are the three protective devices on a motor circuit?
Short-circuit and ground-fault protection (the breaker or fuses, sized by Table 430.52) clears faults. Overload protection (heaters or an electronic overload relay in the starter, sized by 430.32 from the nameplate) protects against a mechanically overloaded or stalled motor. The disconnecting means (430 Part IX) provides isolation. All three are separate requirements and a single breaker does not satisfy them all.
Which Table 430.52 column do I use?
The one matching your device. An ordinary molded-case breaker is an inverse-time breaker — 250% of full-load current for most motors. A dual-element (time-delay) fuse is only 175%, because its own time delay handles the inrush. A non-time-delay fuse gets 300%. An instantaneous-trip breaker is 800% (1100% for Design B) but 430.52(C)(3) only permits it as part of a listed combination motor controller.
Can I go above the Table 430.52 percentage?
Yes, where the motor will not start on it — 430.52(C)(1) Exception No. 2 allows an inverse-time breaker up to 400% for full-load currents of 100 A or less (300% above that), a time-delay fuse to 225%, and a non-time-delay fuse to 400%. It is a permission for a demonstrated starting problem, not a default. Start at the table value and only step up if the motor genuinely trips it on acceleration.
Do I use the table FLC or the nameplate for the overload?
The nameplate. This is the single exception in Article 430, set by 430.32: overload devices are matched to the actual machine because they are protecting that machine's windings. Conductors and the branch-circuit device use the table value per 430.6(A)(1). Getting these backwards is the commonest motor-circuit error.
Is 14 AWG really acceptable on a 10 HP motor?
By the letter of the code, yes — 430.22 requires 17.5 A of ampacity for a 14 A table FLC, and 14 AWG copper at 75 °C provides 20 A, with 240.4(D) switched off by 240.4(G). In practice almost nobody does it: 12 AWG is the normal minimum for mechanical robustness, most specifications require it, and voltage drop on a long motor feed frequently pushes it larger still. Treat the calculated size as a floor.
How is the equipment grounding conductor sized?
From Table 250.122, keyed to the rating of the branch-circuit protective device — which on a motor circuit is the large one. A 35 A breaker lands in the 60 A row, giving 10 AWG copper. That is larger than you would expect from the phase conductors, and it is correct: the EGC has to carry fault current until that oversized device operates.
What about several motors on one feeder?
430.24 sizes a feeder at 125% of the largest motor's full-load current plus the sum of the other motors' full-load currents. 430.62 sizes the feeder protective device at the largest branch-circuit device rating plus the sum of the other motors' full-load currents. This calculator handles a single motor branch circuit; the feeder is a separate calculation.
Motor circuit designed? Price the install.
Starter, overloads, disconnect, conduit, conductors, terminations, and commissioning — motor work has a long bill of materials and easy places to lose margin. TradesQuote turns the scope into a line-item estimate with quantities and totals.
AI line-item estimates
Quantities, unit prices, and totals generated instantly.
Knowledge base
Upload past jobs so estimates reflect your real pricing.
Shareable & signable
Clients review, accept, and sign from a public link.
No credit card required · 14-day free trial · Cancel anytime
More electrical calculators
Motor Full Load Amps Calculator
The table FLC every other number derives from.
Available Fault Current Calculator
The interrupting rating the device has to carry.
Ground Wire Size Calculator
EGC and GEC from Tables 250.122 and 250.66.
Conduit Fill Calculator
Fit the motor feed and its ground in a raceway.