Worked problem

Size the Branch Circuit for a 40 hp, 460 V Motor

Conductor size, breaker, overload setting and disconnect rating for a 40 hp, 460 V, 3-phase induction motor, using NEC Article 430 tables.

Electrical & Power · Last reviewed

Given

40 hp induction motor, 460 V, 3-phase, copper conductors, 75 °C terminations. Nameplate full-load current 48 A, service factor 1.15.

Find

Conductor size, short-circuit and ground-fault protection, overload setting, and minimum disconnect rating.

Solution

  1. Use the table value, not the nameplate, to size the conductor and the breaker. NEC Table 430.250: 40 hp at 460 V gives a full-load current of 52 A.
  2. Conductor ampacity is 125% of 52 A = 65 A (NEC 430.22). In Table 310.16, 75 °C copper, 6 AWG is rated 65 A. Use 6 AWG copper.
  3. An inverse-time breaker may be set up to 250% of 52 A = 130 A (Table 430.52). 130 A is not a standard size, so the next size up is allowed (430.52(C)(1), Exception 1): 150 A maximum. A 125 A breaker also complies, but may trip on a hard start.
  4. Overload protection uses the nameplate current. With a service factor of 1.15 or more, the setting is 125% of 48 A = 60 A (NEC 430.32(A)(1)).
  5. The disconnect must be rated at least 115% of 52 A = 59.8 A, so 60 A or more (NEC 430.110(A)).

Answer

6 AWG Cu | 125 to 150 A inverse-time breaker | overload set at 60 A | disconnect 60 A or more

Common mistakes

  • Sizing the conductor from the nameplate current instead of the NEC table value.
  • Applying the 250% breaker limit to the nameplate current.
  • Treating the overload device and the short-circuit device as one device. They protect against different faults and are sized from different currents.

Try a variation: 25 hp at 460 V

  1. NEC Table 430.250 gives a full-load current of 34 A for a 25 hp, 460 V, 3-phase motor.
  2. Conductor ampacity is 125% × 34 = 42.5 A. In the 75 °C copper column, 8 AWG is rated 50 A, so use 8 AWG.
  3. The inverse-time breaker maximum is 250% × 34 = 85 A. The next standard size up (NEC 240.6) is 90 A.
  4. With a nameplate current of 31 A and a service factor of 1.15, the overload setting is 125% × 31 = 38.8 A.
  5. The disconnect must be rated at least 115% × 34 = 39.1 A, so 40 A or more.
8 AWG Cu | up to 90 A breaker | overload 38.8 A | disconnect 40 A or more

Sanity check

At 460 V and three phases, the table full-load current for common motor sizes is roughly 1.2 to 1.4 A per horsepower: 52 A for 40 hp is 1.3 A/hp, and 34 A for 25 hp is 1.36 A/hp. If your answer is far from that, you probably read the wrong row or the wrong voltage column.

Where each number comes from

  • Conductor ampacity of 125% of full-load current: NEC 430.22.
  • Short-circuit and ground-fault protection maximum: NEC 430.52 and Table 430.52.
  • Running overload protection from the nameplate: NEC 430.32.
  • Disconnecting means rating of at least 115% of full-load current: NEC 430.110.

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Questions

Why does the conductor use the table current and the overload use the nameplate?

The conductor and the short-circuit device are sized from the standard table so the circuit works with any motor of that rating. The overload device protects this particular motor, so it follows its nameplate.

Is 150 A always allowed?

Not always. It is the maximum for an inverse-time breaker on this example. Other device types and motor designs have different limits in Table 430.52.

What changes at a different voltage?

The table full-load current changes with voltage, and every number after it changes with it.

What is the difference between table full-load current and nameplate current?

The table value is a standard number for a motor of that horsepower and voltage, used to size conductors, the short-circuit device and the disconnect. The nameplate value is the actual rated current of the motor you have, used for the overload device.

Does a variable frequency drive change the sizing?

Yes. Conductors that supply a drive are sized from the drive's rated input current, and the conductors from the drive to the motor are sized from the motor full-load current. Follow the drive manual and the NEC articles that cover adjustable-speed drives (430.122 and 430.22).

Why can a termination limit the conductor size?

Equipment terminations on smaller circuits are commonly rated for the 60 °C ampacity unless the equipment is marked for 75 °C. Check NEC 110.14(C) and the markings on the equipment before using the 75 °C column.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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