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 rather than the nameplate (NEC 430.6(A)(1); 430.6(A)(2) names the exceptions).

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 is sized to the conductor ampacity (NEC 240.4(B) permits the next standard rating above it only under its listed conditions). 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(C)(1) permits an inverse-time breaker up to 250% of the FLC, or the next standard rating above it (430.52(C)(1)(a)). 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. For a motor 430.6(A)(1) sizes from the table, that is the one place the nameplate is used. A 25 HP 460V motor shows the effect clearly: 34A FLC, #8 copper (design B, C or D motor or 75°C-listed equipment; #6 Cu at 60°C), 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.

1. 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.
2. 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. #8 copper holds on the 75°C column only for a motor marked design letter B, C or D (NEC 110.14(C)(1)(a)(4)) or equipment listed and identified for 75°C conductors (110.14(C)(1)(a)(3)), with the breaker and controller terminals rated 75°C too; otherwise the 60°C column applies and it takes #6 copper.
3. Size the branch device at 250%. NEC 430.52(C)(1) with Table 430.52(C)(1): 34A × 2.5 = 85A. Where that is not a standard rating, 430.52(C)(1)(a) permits the next standard size up, giving 90A from the 240.6(A) ladder.
4. 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.

Check your working

Each problem is a single motor on copper conductors in continuous duty, alone on its branch circuit: a three-phase squirrel-cage motor or a single-phase motor, sized the way the calculator above sizes it. A three-phase synchronous motor reads its current from Table 430.250's own synchronous columns, so no answer here applies to it; a wound-rotor or DC motor takes a lower breaker percentage from Table 430.52(C)(1), so the breaker answers do not apply to it; and a motor in short-time, intermittent, periodic or varying duty takes its conductor under 430.22(E), so the conductor answers do not apply to it. Work a problem on paper first, then open its working to check yours. We chose these problems' numbers; your own numbers go in the calculator.

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 #8 copper (#10 Cu only on a design B, C or D motor or 75°C-listed equipment) and a 70A maximum breaker instead of #14 (design B, C or D motor or 75°C-listed equipment; #12 Cu at 60°C) 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 in continuous duty on its own branch circuit (NEC 430.22; a motor in short-time, intermittent, periodic or varying duty is sized under 430.22(E)). 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 comes from the squirrel-cage and synchronous rows of NEC Table 430.52(C)(1). 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. A three-phase synchronous motor's full-load current is in Table 430.250's own synchronous columns, which this tool does not read, so it does not size a three-phase synchronous motor.

The values come from NEC Tables 430.248 and 430.250, which are unchanged across the 2017, 2020 and 2023 editions. Conductors are copper from Table 310.16, with no ambient or conduit-fill correction applied. At 100 A or less and on 14 through 1 AWG terminations, NEC 110.14(C)(1)(a) makes the 60C column the rule: a three-phase answer gives the 75C size only with the condition that permits it (a motor marked design letter B, C or D, or equipment listed and identified for 75C conductors) and the 60C size beside it, and a single-phase answer leads with the 60C size. 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.

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