Electrical · Motors

Three-Phase Motor Calculator

Horsepower and voltage in, conductor and maximum inverse-time breaker out. Sized from the NEC Table 430.250 full-load current, never from the nameplate.

The short answer. A 10 HP motor at 460V three-phase draws 14A from NEC Table 430.250. Its branch-circuit conductor is sized at 125% of that (17.5A), which takes #14 copper, and its maximum inverse-time breaker is 250% of the FLC at 35A.

The breaker is larger than the wire on purpose, and that is the part worth understanding before you use the number. A motor circuit splits two jobs that an ordinary branch circuit gives to one device, so the rule that a breaker never exceeds the conductor ampacity does not apply here.

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Why a motor circuit is not sized like an ordinary branch circuit

On an ordinary branch circuit the breaker protects the wire, so the breaker never exceeds the conductor ampacity. A motor circuit splits that job in two, and once you see the split the numbers stop looking wrong.

The conductor is sized for the motor's continuous running current: NEC 430.22 requires 125% of the table full-load current. The branch-circuit device is sized for something else entirely, the inrush at start, and NEC 430.52 with Table 430.52 permits an inverse-time breaker up to 250% of the FLC. NEC 240.4(G) then exempts these conductors from the small-conductor rules of 240.4(D) that would otherwise cap the breaker.

What protects the conductor from a sustained overload is the running overload device required by NEC 430.32, sized from the motor nameplate. That is the one place the nameplate is used. A 25 HP 460V motor shows the effect clearly: 34A FLC, #8 copper, and a 90A maximum breaker.

Worked example: 25 HP, 460V, three-phase

Every step below is the calculator's own arithmetic, written out. Nothing here is a separate transcription of the answer; the figures are read from the same function the tool reads, so this example cannot drift away from the result above.

Look up the current, do not measure it. NEC Table 430.250, 25 HP at 460V three-phase: 34A. The motor's nameplate may read something different; per 430.6(A)(1) the table value is what sizes this circuit.
Size the conductor at 125%. NEC 430.22: 34A × 1.25 = 42.5A of required ampacity. The smallest copper conductor meeting that in the 75C column of Table 310.16 is #8.
Size the branch device at 250%. NEC 430.52(C)(1) with Table 430.52: 34A × 2.5 = 85A. Where that is not a standard rating, Exception 1 permits the next standard size up, giving 90A from the 240.6(A) ladder.
Protect the conductor separately. The 90A breaker does not protect #8 from overload and is not meant to. The running overload device under NEC 430.32 does that, and it is sized from the nameplate, not from the table.

Single-phase motors use a different table

Switching the phase selector moves the lookup to NEC Table 430.248, which publishes its own currents rather than a conversion of the three-phase ones. The difference is large enough that guessing at it would size the wrong circuit: a 5 HP motor at 230V draws 28A single-phase against 15.2A three-phase at the same horsepower and the same voltage, so it takes #10 copper and a 70A maximum breaker instead of #14 and 40A. Table 430.248 stops at 10 HP; above that the tool offers three-phase only, because the table does.

What this calculator does not do

It sizes a single motor on its own branch circuit. It does not size a feeder for several motors, which follows NEC 430.24 and 430.62 and gives a different answer. It does not include voltage drop, which can require a larger conductor on a long run. It does not size the disconnecting means, the controller, or the running overload device. It returns code minimums and code maximums; your AHJ and any local amendments govern the installation.

The 250% branch-circuit figure is the induction-type squirrel-cage and synchronous row of NEC Table 430.52. Wound-rotor and DC motors take a lower percentage from that same table, so this result does not apply to them and the tool says so beside the answer rather than letting the squirrel-cage figure quietly cover a motor the code treats differently.

The values come from NEC Tables 430.248 and 430.250, which are unchanged across the 2017, 2020 and 2023 editions. Conductors are copper at the 75C column of Table 310.16, with no ambient or conduit-fill correction applied. Where a motor needs more ampacity than the largest single conductor in that table provides, the tool says so and points at parallel conductors under NEC 310.10(G) instead of naming a wire that cannot carry the load.

Frequently asked questions

What size wire and breaker do I need for a 10 HP three-phase motor?

A 10 HP motor at 460V three-phase has a full-load current of 14A from NEC Table 430.250. The branch-circuit conductor is sized at 125% of that, so 17.5A, which takes #14 copper at the 75C column. The maximum inverse-time breaker is 250% of the FLC rounded up to the next standard size, which is 35A.

Why is the breaker so much bigger than the wire can carry?

Because a motor draws several times its running current for the first moments of a start, and a breaker sized to the conductor would trip every time. NEC 240.4(G) exempts motor branch-circuit conductors from the small-conductor limits in 240.4(D), so the branch breaker is allowed to exceed the conductor ampacity. The conductor is protected from overload by the running overload device required by 430.32, not by the branch-circuit breaker. This is the single most common reason a motor circuit looks wrong to someone used to sizing ordinary branch circuits.

Do I use the motor nameplate amps or the NEC table?

You use the table. NEC 430.6(A)(1) is explicit that the values in Tables 430.247 through 430.250 are what size the conductors, the ampacity, the disconnect, and the branch-circuit short-circuit and ground-fault protective device. The nameplate full-load amps are used only to size the running overload device, per 430.6(A)(2) and 430.32. Sizing a branch circuit from the nameplate is the most common error on motor work, and it is why this calculator asks for horsepower rather than amps.

What is the full-load current of a 5 HP motor at 230V three-phase?

15.2A, from NEC Table 430.250. That gives a conductor sizing ampacity of 19A at 125%, which takes #14 copper, and a maximum inverse-time breaker of 40A.

Does this work for single-phase motors too?

Yes. Switch the phase selector to single-phase and the lookup moves to NEC Table 430.248, which is a different set of currents rather than a conversion of the three-phase ones. A 5 HP single-phase motor at 230V is 28A, which is nearly double the 15.2A the same horsepower draws at 230V three-phase, and it takes #10 copper with a 70A maximum breaker. Everything else about the method is identical.

Does this cover wound-rotor or DC motors?

No. The 250% branch-circuit figure this tool applies is the row of NEC Table 430.52 for induction-type squirrel-cage and synchronous motors, which is the ordinary case. Table 430.52 gives wound-rotor and DC motors a lower percentage, so applying this result to one would permit a larger branch-circuit device than the code allows. Size those directly from Table 430.52. The full-load current tables cover them; the branch-circuit percentage does not.

Does this calculator include voltage drop?

No. It returns the code-minimum conductor for ampacity under NEC 430.22 and the maximum branch-circuit protection under 430.52. A long run can require a larger conductor to hold voltage drop within the informational 3% branch-circuit recommendation, and motor starting makes drop more noticeable. Size the circuit here first, then check the run length in the voltage drop calculator.

Does this cover more than one motor on a circuit?

No. This sizes a single motor on its own branch circuit, which is the ordinary case. A feeder supplying several motors follows NEC 430.24 and 430.62, which size from 125% of the largest motor plus the sum of the rest, and that is a different calculation with a different result. Do not use a single-motor branch answer for a multi-motor feeder.

Related tools

Motor Full-Load Current Chart

The whole of NEC Tables 430.248 and 430.250 to read across, when you want the table rather than one answer.

Voltage Drop Calculator

The check this tool does not make. A long motor run often needs a conductor larger than ampacity alone requires.

Wire Size Calculator

General branch-circuit conductors, where the 240.4(D) small-conductor rule this motor circuit is exempt from does apply.

MCA and MOCP Calculator

For nameplate-rated equipment such as HVAC, where the nameplate MCA and MOP govern instead of a table lookup.

Standard Breaker Sizes

The NEC 240.6(A) ladder the maximum breaker on this page is rounded up to.

Breaker Size for a Wire Gauge

The ordinary-circuit relationship between wire and breaker, which motor circuits are the exception to.

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Where these motor circuit numbers come from

No answer on this page is hand-typed. The full-load current is read from the NEC table for your phase, horsepower and voltage; the conductor and the branch-circuit device are then computed from that current by the Article 430 rules against the same locked ampacity and standard-rating tables the rest of this site uses, re-derived on every build and recomputed in your browser as you change an input. The chart page and this calculator read the same function, so they cannot disagree. The explanatory copy and the FAQ are hand-authored from the NEC and carry ordinary transcription risk.

NEC 2023 Table 430.250, Full-Load Current, Three-Phase AC Motors: Read verbatim from the NFPA 70-2023 primary, printed page 70-364, on 2026-07-21. A subset of these cells is re-asserted on every build against that transcription, which guards against drift in our own file rather than re-reading the book. One value is worth naming because the web gets it wrong: 1-1/2 HP at 575 V is 2.4 A on the primary, not the 2.1 A that circulates elsewhere, and that one is checked against a render of the page on every build. The 115 V column exists only through 2 HP and is blank above it, and blank is published as blank rather than filled in.
NEC 2023 Table 430.248, Full-Load Currents, Single-Phase AC Motors: Read verbatim from the NFPA 70-2023 primary, printed page 70-363, on 2026-07-21. Some of its cells are re-asserted on every build against that transcription; that guards against drift in our own file and is not a second reading of the code book. Selecting single-phase on this tool moves the lookup to this table, which is a different set of currents, not a conversion of the three-phase ones.
NEC 2023 430.6(A)(1), which current to use: NFPA 70-2023 primary, printed page 70-340. The values in Tables 430.247 through 430.250 are what size the conductors, the ampacity, the disconnecting means and the branch-circuit short-circuit and ground-fault device. The motor's nameplate full-load amps are used ONLY for the running overload device, under 430.6(A)(2) and 430.32. This is why the tool asks for horsepower and will not accept a nameplate amp figure: accepting one would return a confidently wrong circuit.
NEC 2023 430.22 and Table 310.16, the conductor: NFPA 70-2023 primary, printed page 70-344. The branch-circuit conductor for a single motor is sized to at least 125 percent of the table full-load current, and the tool then takes the smallest copper conductor in the 75C column of Table 310.16 that meets it. A subsection nuance that is easy to get wrong and is guarded here: the general single-motor rule is the bare 430.22 parent. 430.22(A) is Direct-Current Motor-Rectifier Supplied, a different rule, so citing 430.22(A) for an AC motor is a wrong-subsection error.
NEC 2023 430.52(C)(1) with Table 430.52, and 240.6(A) standard ratings: NFPA 70-2023 primary, printed pages 70-349 and 70-350, with the standard ampere ratings on 70-124. For an induction-type squirrel-cage motor the inverse-time circuit breaker may be up to 250 percent of the table full-load current, and 430.52(C)(1) Exception 1 permits the next higher standard rating where that value is not itself a standard rating. Table 430.52 gives DIFFERENT percentages to other motor types: a wound-rotor or DC motor takes a lower one, so this result does not apply to them and the tool says so on the page rather than quietly extending the squirrel-cage figure to a motor the table treats differently.
NEC 2023 240.4(G) and Table 240.4(G), why a motor branch circuit may sit under a larger breaker: NFPA 70-2023 primary, printed pages 70-123 and 70-124. A motor branch circuit is one of the specific applications Table 240.4(G) exempts from the small-conductor limits of 240.4(D), which is what makes a legitimately small conductor under a much larger breaker code-correct here and never a general branch circuit. What protects that conductor from a sustained overload is the running overload device of 430.32, printed page 70-346, not the branch-circuit breaker.

This sizes ONE motor on its own branch circuit, and it returns code minimums and code maximums rather than a design. It does not size a feeder supplying several motors, which follows 430.24 and 430.62 and gives a different answer, so a single-motor result must not be reused for a multi-motor feeder. It does not include voltage drop, which can require a larger conductor on a long run and is more noticeable during motor starting. It does not size the disconnecting means, the controller, or the running overload device, and the overload device is the one that is sized from your motor's nameplate rather than from the table. The 250 percent branch-circuit figure is the induction-type squirrel-cage and synchronous row of Table 430.52; wound-rotor and DC motors take a lower percentage from that table and are out of scope here. Conductors are copper at the 75C column of Table 310.16, ambient and conduit fill uncorrected. Where the required ampacity exceeds the largest single conductor the table lists, the tool says so and points at parallel conductors under 310.10(G) rather than naming a wire that cannot carry the load. Your AHJ and any local amendments govern the installation.

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