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Motor circuit sizing

Updated · Written for the 2023 NEC

Short answer

To size a motor branch circuit under Article 430 of the 2023 NEC, start with the full-load current from the NEC motor tables, not the nameplate (430.6(A)(1)). Size the conductors at 125% of that table current (430.22), size the overloads from the nameplate current at 115% or 125% (430.32), and size the breaker or fuses for short-circuit and ground-fault protection using the percentages in Table 430.52(C)(1).

What motor circuit sizing is

A motor branch circuit has separate parts that do separate jobs, and Article 430 sizes each part with its own rule. The conductors must carry the running current. The overload device protects the motor and conductors from overheating during a long overload. The breaker or fuses protect against short circuits and ground faults, and they must be large enough to let the motor start.

This split is why motor questions trip people up. A breaker far too big for a lighting circuit is often correct for a motor, because the overloads handle running overcurrent. For general rules, see overcurrent protection.

The key rules (with NEC sections)

  • Use table full-load current, 430.6(A)(1). For general motors, use the full-load current from Table 430.248 (single-phase), Table 430.250 (three-phase), or Table 430.247 (DC) when sizing conductors, disconnect ratings, and branch-circuit short-circuit and ground-fault protection. Do not use the nameplate for these.
  • Use nameplate current for overloads, 430.6(A)(2). Separate overload protection is based on the motor nameplate full-load current, because it must match the actual motor.
  • Branch-circuit conductors, 430.22. Conductors for a single continuous-duty motor must have an ampacity of at least 125% of the table full-load current.
  • Several motors on a feeder, 430.24. Take 125% of the largest motor's full-load current plus 100% of the others.
  • Overload protection, 430.32(A)(1). For a continuous-duty motor over 1 hp with a separate overload device, the device may be rated up to 125% of nameplate current when the service factor is 1.15 or more, or the marked temperature rise is 40°C or less. All other motors are limited to 115%.
  • Overload increase, 430.32(C). If that setting will not let the motor start or carry its load, you may go to the next higher size, up to 140% (for the 125% group) or 130% (for all others).
  • Short-circuit and ground-fault protection, 430.52. The breaker or fuse rating is a percentage of the table full-load current from Table 430.52(C)(1). For common AC motors these are 300% for nontime-delay fuses, 175% for dual-element time-delay fuses, 800% for instantaneous-trip breakers, and 250% for inverse-time breakers.
  • Next size up, 430.52(C)(1) Exception No. 1. If the calculated value is not a standard size from 240.6(A), you may use the next higher standard size.
  • Starting increase, 430.52(C)(1) Exception No. 2. If that size still will not start the motor, it may be increased further. For an inverse-time breaker, the limit is 400% of full-load current for motors of 100 A or less and 300% above 100 A. Time-delay fuses may go to 225%.

The formulas

Conductor ampacity ≥ table FLC × 1.25
Overload ≤ nameplate FLA × 1.25 (SF 1.15 or more, or 40°C rise)
Overload ≤ nameplate FLA × 1.15 (all other motors)
Inverse-time breaker ≤ table FLC × 2.50, then next standard size
Time-delay fuse ≤ table FLC × 1.75, then next standard size

Notice which current goes where. Table FLC drives the conductors and the breaker or fuses. Nameplate current drives only the overloads.

Worked example

A 15 hp, 230-volt, three-phase, continuous-duty squirrel-cage motor has a nameplate current of 38 A and a service factor of 1.15. It is fed with copper THWN conductors on 75°C terminations and protected by an inverse-time breaker.

  1. Table full-load current. Table 430.250 gives 42 A for a 15 hp, 230-volt, three-phase motor. Use 42 A, not the 38 A nameplate.
  2. Conductors. 42 × 1.25 = 52.5 A. Table 310.16 gives 50 A for 8 AWG copper at 75°C, which is too small. 6 AWG copper at 75°C is 65 A, so use 6 AWG.
  3. Overloads. The service factor is 1.15, so use 125% of the nameplate: 38 × 1.25 = 47.5 A maximum. If that will not let the motor start, 430.32(C) allows up to 140%: 38 × 1.40 = 53.2 A.
  4. Inverse-time breaker. 42 × 2.50 = 105 A. That is not a standard size, so the next standard size, 110 A, is permitted.
  5. If fuses were used instead. Dual-element time-delay fuses: 42 × 1.75 = 73.5 A, so the next standard size is 80 A.

A 110 A breaker on 6 AWG conductors looks wrong if you are used to branch circuits, but it is correct here. The breaker only has to clear short circuits and ground faults. The 47.5 A overloads protect the conductors from running overloads. The conductor ampacity page covers reading Table 310.16.

Disconnect. The motor disconnect must have an ampere rating of at least 115% of the table full-load current (430.110(A)): 42 × 1.15 = 48.3 A, and it must be horsepower rated for the motor.

Common exam mistakes

  • Sizing conductors from the nameplate. Conductors and short-circuit protection always start from the table full-load current.
  • Sizing overloads from the table. Overloads use the nameplate current.
  • Using 125% overloads on every motor. The 125% value needs a service factor of 1.15 or more, or a marked rise of 40°C or less. Otherwise it is 115%.
  • Rounding overloads up. The 115% or 125% value is a maximum. The next-size-up allowance in 430.52 is for the breaker or fuses, not for this overload step.
  • Mixing up the device percentages. 250% is the inverse-time breaker, 175% is the time-delay fuse, and 300% is the nontime-delay fuse.
  • Reading the wrong voltage column. A 10 hp motor is 28 A at 230 V but 14 A at 460 V in Table 430.250. Three-phase tables also list 200 V and 208 V separately.
  • Applying the small-conductor limits of 240.4(D). Motor circuits are protected under Article 430 instead (240.4(G)).

Practice questions

Question 1 · Motors and controlsSeparate motor overload protection is sized based on which current?
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Answer: C. The motor nameplate full-load current rating. NEC 430.6(A)(2)

Overload protection must match the specific motor installed, so it uses the nameplate full-load current. Conductors and short-circuit protection use the table values.

Question 2 · Motors and controlsA 10 hp, 230-volt, three-phase continuous-duty motor is installed with 75°C-rated terminations. What is the minimum size copper branch-circuit conductor?
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Answer: B. 10 AWG. NEC 430.22

Table FLC is 28 A, and motor branch conductors must be at least 125% of that: 28 × 1.25 = 35 A. 10 AWG copper is rated 35 A at 75°C. The small-conductor rule does not limit motor circuits, which are covered by Article 430.

  1. FLC (Table 430.250) = 28 A
  2. 28 × 1.25 = 35 A
  3. 75°C Cu: 10 AWG = 35 A → 10 AWG
Question 3 · Motors and controlsA motor with a marked service factor of 1.15 has a nameplate full-load current of 52 A. What is the maximum rating of a separate overload device? Use the ordinary separate-overload selection rule before any permitted starting-related increase.
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Answer: A. 65 A. NEC 430.32(A)(1)

Motors with a service factor of 1.15 or more may have overloads sized at up to 125% of nameplate current. 52 × 1.25 = 65 A.

  1. Service factor 1.15 → 125%
  2. 52 × 1.25 = 65 A
  3. Maximum overload = 65 A
Question 4 · Motors and controlsA 25 hp, 460-volt, three-phase squirrel-cage motor (not Design B energy-efficient) is protected by an inverse-time breaker. What is the maximum breaker rating using the next-size-up allowance?
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Answer: A. 90 A. NEC 430.52(C)(1)

FLC is 34 A. Inverse-time breakers may be sized up to 250%: 34 × 2.5 = 85 A, which is not standard, so the next higher size, 90 A, is permitted.

  1. FLC = 34 A
  2. 34 × 2.50 = 85 A
  3. Next standard size = 90 A
Question 5 · Motors and controlsA 10 hp, 230-volt, three-phase motor is protected with nontime-delay fuses. What is the maximum fuse rating using the next-size-up allowance?
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Answer: C. 90 A. NEC 430.52(C)(1)

FLC is 28 A. Nontime-delay fuses may be sized at 300%: 28 × 3 = 84 A, which rounds up to the next standard size of 90 A.

  1. FLC = 28 A
  2. 28 × 3.00 = 84 A
  3. Next standard size = 90 A

For more mixed practice, take a practice test. Motors on feeders with other loads are covered in dwelling load calculations and services and feeders.

Frequently asked questions

Why do you use the table FLC instead of the nameplate for motor conductors?

Section 430.6(A)(1) requires the table value so the circuit stays adequate if the motor is replaced later by another motor of the same horsepower with a different nameplate current. The nameplate is used only for overload sizing.

What size breaker do I need for a motor?

For an inverse-time breaker, multiply the table full-load current by 250% (Table 430.52(C)(1)). If the result is not a standard size, you may use the next higher standard size. If the motor still will not start, the breaker may go up to 400% for motors of 100 A or less.

Why is a motor breaker larger than the wire ampacity?

The motor branch-circuit breaker only protects against short circuits and ground faults, so it must be large enough to carry the high starting current. Running overloads are handled by the separate overload device, which is sized close to the motor's actual current.

What does a 1.15 service factor mean for overloads?

A 1.15 service factor means the motor can handle 15% more than its rated load without damage. Under 430.32(A)(1), that lets you size the overload device up to 125% of nameplate current instead of 115%.

How do you size conductors for several motors on one feeder?

Under 430.24, add 125% of the largest motor's table full-load current to 100% of the full-load currents of all the other motors, plus any other loads on the feeder.

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