Motor Circuit and Breaker Calculator
NEC 430 motor full-load current, conductor size, breaker or fuse and overload setting — with the table-not-nameplate rule made unmissable.
Use the table, not the nameplate
NEC 430.6(A)(1) is unusually blunt about this: conductor sizing and short-circuit protection come from Table 430.248 or 430.250, not from the number printed on the motor. Only the overload device is sized from the nameplate. A 10 hp motor on 460 V is 14 A for the wire and the breaker even if its plate reads 12.8 — and sizing from the plate leaves you undersized against a motor drawing its table current on a bad day.
A motor circuit has three protective devices doing three different jobs, on three different bases. The breaker is short-circuit and ground-fault protection only — it is sized at 250% of table current because locked-rotor current is six to eight times full load and the motor has to be able to start. It does not protect the motor from overload and will watch one cook without tripping. The overload relay in the starter is what protects the motor. Fitting a breaker and calling the motor protected installs one of the three.
- The breaker is deliberately far larger than the conductors. That looks wrong and is exactly right — the overload relay protects both the motor and the wire against sustained overcurrent, leaving the breaker to handle only faults.
- Overload is 125% or 115% of nameplate, depending on whether the service factor is 1.15 or more, or the temperature rise is 40 °C or less. A motor with a 1.15 service factor is built with margin the code lets you use.
- A dual-element fuse gives the smallest device of the four options, which is why it is preferred where the equipment allows it — a smaller device clears a fault faster and lets through less energy.
- Feeders use 430.24: 125% of the largest motor plus the full-load current of all the others, because only one is assumed to be starting at a time.
A planning aid, not a design. The disconnecting means, controller rating and lockout provisions are separate Article 430 requirements no calculation settles. Anything on a machine or in a commercial building wants a licensed electrician.
How to use
- Pick the phase, horsepower and voltage — the table current appears.
- Choose the protective device; a dual-element fuse gives the smallest one.
- Enter the nameplate amps for the overload, which uses a different figure.
- Read all three: conductors, short-circuit device and overload relay.
Frequently asked questions
Why does this use a table instead of my motor nameplate?
Because NEC 430.6(A)(1) requires it. Conductor sizing and short-circuit protection come from Table 430.248 or 430.250, and the nameplate is used only for overload protection. A 10 horsepower motor on 460 volts is 14 amps for the wire and the breaker even if its plate reads 12.8. Sizing the conductors from a lower nameplate figure leaves them undersized against a motor drawing its table current, which is the whole point of the rule.
Why is the breaker so much bigger than the wire?
Because it is not overload protection. The breaker or fuse on a motor circuit provides short-circuit and ground-fault protection only, and it is sized at 250 percent of full-load current for an inverse-time breaker because locked-rotor current is six to eight times full load and the motor has to be able to start. The overload relay in the starter is what protects the motor and the conductors against sustained overcurrent.
So the breaker will not protect my motor?
Correct, and it is worth being blunt about. A 250 percent breaker will watch a motor draw 150 percent of its rating until the windings fail, because that current is nowhere near its trip threshold. A motor circuit needs three devices doing three separate jobs — conductors at 125 percent of table current, a short-circuit device from the table, and an overload relay from the nameplate. Fitting only the breaker installs one of the three.
How is the overload sized?
From the nameplate, at 125 percent where the service factor is 1.15 or greater or the marked temperature rise is 40 degrees Celsius or less, and 115 percent otherwise. The distinction is not decorative: a motor with a 1.15 service factor is built with headroom the code allows you to use. NEC 430.32(C) permits going to 140 or 130 percent respectively where the motor will not start or run, which is a documented adjustment rather than a free choice.
Which protective device should I choose?
A dual-element time-delay fuse gives the smallest device of the four options at 175 percent, which is worth having — a smaller device clears a fault faster and lets through less energy into the equipment. Inverse-time breakers at 250 percent are the most common in practice because they are resettable and integrate with panels. Non-time-delay fuses at 300 percent and instantaneous-trip breakers at 800 percent are specialised and have their own conditions of use.
What if the motor will not start on the calculated breaker?
NEC 430.52(C)(1) Exception 2(b) permits increasing an inverse-time breaker to 400 percent, or a dual-element fuse to 225 percent. Treat it as a last resort: the further the device sits from the motor current, the more fault energy passes before it clears. On small motors the question does not arise, since 15 amps is the smallest breaker made and the next-size-up permission has usually taken you past the ceiling already.
How do I size a feeder for several motors?
NEC 430.24: 125 percent of the largest motor plus the full-load current of every other motor on the feeder. Only one motor is assumed to be starting at a time, which is why only the largest gets the 125 percent treatment. The feeder overcurrent device has its own rule in 430.62 and is sized from the largest branch-circuit device plus the other motors, not from the feeder conductor ampacity.
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