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Motor circuit calculator

Enter the motor's full-load current from the NEC tables, the motor type, and the protective device. You get the minimum conductor ampacity, the maximum breaker or fuse, and the feeder if there are other motors.

Built on the 2026 NEC · Free, no sign-up · Updated October 2026

From Table 430.247, 430.248, 430.249, or 430.250, not the nameplate FLA.

Type of branch-circuit short-circuit and ground-fault OCPD.

Optional. FLAs of the other motors, leaving out the largest one above. Adds the 430.24 feeder ampacity.

The result updates as you type.

Result

Branch ≥ 35.0 A · OCPD ≤ 70 A (250%)

How it was calculated

  1. Branch-circuit conductor: 125% × 28 A = 35.00 A (430.22).
  2. Branch-circuit OCPD: 250% × 28 A = 70.00 A (Table 430.52(C)(1), inverse-time-cb).

Assumptions

  • Motor FLA 28 A taken from the appropriate NEC table for three-phase-induction (430.247-430.250).
  • 250% multiplier from Table 430.52(C)(1) for three-phase-induction + inverse-time-cb.
  • Where the calculated value does not match a standard rating in 240.6, 430.52(C)(1)(a) permits the next higher standard rating. If that rating will not carry the starting current, 430.52(C)(1)(b) permits further increases up to its limits.
  • Branch-circuit only; no multi-motor feeder calculation requested.

Sections used

430.22430.52Table 430.52(C)(1)

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Start with the table current, not the nameplate

For general motor applications, 430.6(A)(1) says to size conductors, switches, and short-circuit protection from the full-load current tables (Tables 430.247 through 430.250), not from the motor's nameplate. Use the nameplate current for the overload setting instead.

The three numbers

  • Branch-circuit conductors: at least 125% of the full-load current (430.22).
  • Short-circuit and ground-fault protection: up to the percentage in Table 430.52(C)(1) for the motor type and device. For a typical three-phase induction motor, that is 250% for an inverse-time breaker, 175% for a dual-element fuse, and 300% for a nontime-delay fuse.
  • Feeder for several motors: 125% of the largest motor's current plus 100% of the others (430.24).

The breaker can be larger than the wire's ampacity. That's normal for motors. The overload device protects the conductors from overload, and the breaker or fuse handles short circuits.

Size a feeder for several motors

Use the same steps for any group of motors on one feeder. Here the group is three 460 V three-phase induction motors: 25 hp, 15 hp, and 10 hp.

  • For standard motors, find each motor's full-load current in the table, not on the nameplate (430.6(A)(1), Table 430.250). At 460 V the three motors read 34 A, 21 A, and 14 A.
  • Pick the motor with the highest rating. Here that is the 25 hp motor, at 34 A.
  • Take 125% of that one motor: 34 A x 1.25 = 42.5 A (430.24, item 1).
  • Add the full-load current of every other motor at 100%: 21 A + 14 A = 35 A (430.24, item 2). The feeder ampacity is 42.5 A + 35 A = 77.5 A.

If the feeder also carries lighting or other non-motor load, 430.24 adds it too: 100% of the noncontinuous load and 125% of the continuous load. Then choose a conductor with at least that ampacity, using your usual temperature and derating checks.

Change the motors and the method stays the same. The highest rated motor takes the 125%, and every other motor counts once. Use the calculator's optional field for the other motors to check your arithmetic.

When the result isn't a standard size

If the calculated maximum doesn't land on a standard rating, 430.52(C)(1)(a) lets you go to the next standard size up. If the motor still won't start, 430.52(C)(1)(b) allows a further increase within limits. (Older editions called these Exceptions No. 1 and No. 2.)

2023 to 2026: our list has 8 changes in Article 430. See the changes

Worked example

A three-phase induction motor with a 28 A table full-load current on an inverse-time breaker:

  • Full-load amps (amps): 28
  • Motor type: 3-phase induction
  • OCPD type: Inverse-time CB

Result

Branch ≥ 35.0 A · OCPD ≤ 70 A (250%)

How it was calculated

  1. Branch-circuit conductor: 125% × 28 A = 35.00 A (430.22).
  2. Branch-circuit OCPD: 250% × 28 A = 70.00 A (Table 430.52(C)(1), inverse-time-cb).

Assumptions

  • Motor FLA 28 A taken from the appropriate NEC table for three-phase-induction (430.247-430.250).
  • 250% multiplier from Table 430.52(C)(1) for three-phase-induction + inverse-time-cb.
  • Where the calculated value does not match a standard rating in 240.6, 430.52(C)(1)(a) permits the next higher standard rating. If that rating will not carry the starting current, 430.52(C)(1)(b) permits further increases up to its limits.
  • Branch-circuit only; no multi-motor feeder calculation requested.

Sections used

430.22430.52Table 430.52(C)(1)

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Questions electricians ask

Why can a motor breaker be bigger than the wire?

The motor breaker or fuse only has to protect against short circuits and ground faults. The overload device protects the wire and motor from overload. That's why Table 430.52(C)(1) allows ratings well above the conductor ampacity.

Do I use the nameplate current or the table current?

For standard motors, use the table current from 430.247 through 430.250 for conductors and short-circuit protection. Use the nameplate current for the overload setting. Low-speed, high-torque, multispeed, and canned-pump motors are among the exceptions: 430.6(A) sends you to the nameplate for those.

How do I size a motor feeder for several motors?

For motor feeder sizing, take 125% of the largest motor's full-load current and add the full-load current of every other motor (430.24). Enter the other motors in the optional field.

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How this calculator was built

The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.

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