What is the maximum feeder breaker for a group of motors?

NEC 430.62(A) limits the feeder overcurrent device to the largest branch-circuit device of any motor in the group plus the sum of the other motors full-load currents. For the 25, 30, and 40 HP example on inverse-time breakers, the largest branch breaker is 150 A, so the feeder ceiling is 150 + 40 + 34 = 224 A. Because this is a maximum with no next-higher exception, the feeder breaker is the next standard size down: 200 A.

Why does the feeder device round down instead of up?

The branch-circuit rule 430.52(C)(1) Exception No. 1 permits rounding up to the next standard size so the motor can start. The feeder rule 430.62(A) has no such exception: it states the device shall not be greater than the computed value. When that value is not a standard 240.6(A) rating, you take the next standard rating below it.

Which motor gets the 125 percent in the feeder conductor calculation?

Only the single highest-rated motor. Per NEC 430.17 the highest-rated motor is the one with the largest full-load current, and only its current is multiplied by 125 percent. Every other motor in the group is added at 100 percent of its full-load current. If two motors tie for largest, only one is treated as the highest-rated.

Does this calculator size each motor branch circuit too?

It sizes the feeder that supplies the group and shows each motor branch-circuit device it uses in the 430.62(A) calculation, but each motor still needs its own branch-circuit conductor, disconnect, and running-overload protection. Use the Motor Breaker Sizing Calculator for a single motor branch circuit and the Motor Overload Protection Calculator for the overload relay.

Multi-Motor Feeder Calculator

Size the feeder conductor and feeder overcurrent device for a group of motors per NEC Article 430. The tool pulls each motor's full-load current from Table 430.250 or 430.248, sizes each branch-circuit device with Table 430.52, then applies the 430.24 conductor rule and the 430.62(A) feeder-device ceiling.

Feeder system

Phase

Voltage

Motor type

Branch and feeder device (Table 430.52)

Motors on the feeder

All motors share the feeder voltage, phase, motor type, and device family. Set a quantity for identical motors.

Feeder sizing

Min. conductor ampacity (430.24) - A

Max. feeder device (430.62(A)) - A

Highest-rated motor FLC (430.17): -
Sum of the other motors' FLC: -
Total connected FLC: -
Largest branch-circuit device: -
Feeder OCPD ceiling (before rounding): -

How to use this calculator

Set the feeder phase and voltage. Every motor on one feeder shares that system, so these apply to the whole group.
Choose the motor type and the branch-and-feeder device family. Table 430.52 sets the branch percentage; the same family is rounded down for the feeder device.
Add each motor by horsepower, with a quantity for identical motors. The tool reads full-load current from Table 430.250 (three-phase) or Table 430.248 (single-phase), not the nameplate.
Read the minimum feeder conductor ampacity (430.24) and the maximum feeder breaker or fuse (430.62(A)). The breakdown shows the largest motor, the branch devices, and the ceiling before rounding.

NEC reference

This calculator follows NEC 2020 Article 430. Full-load current is taken from Table 430.248 (single-phase) or Table 430.250 (three-phase) per 430.6(A)(1). The feeder conductor is sized under 430.24 at 125 percent of the highest-rated motor full-load current (430.17) plus the sum of the other motors. The feeder overcurrent device is sized under 430.62(A) as the largest branch-circuit device plus the other motors full-load currents, taken as a maximum. Each branch-circuit device uses Table 430.52 with the 430.52(C)(1) next-higher-standard allowance. These values are stable across the 2017, 2020, and 2023 editions.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

The multi-motor feeder method

A feeder that supplies several motors carries the running current of the whole group plus the starting inrush of the largest one. NEC 430.24 captures that by sizing the conductor at 125 percent of the highest-rated motor plus 100 percent of every other motor. Only one motor gets the extra 25 percent, because only one motor starts against a fully loaded feeder.

The feeder overcurrent device follows a parallel logic in 430.62(A): take the largest branch-circuit short-circuit and ground-fault device in the group and add the full-load currents of the other motors. That number is a ceiling, not a target. Where it is not a standard 240.6(A) rating you drop to the next standard size, unlike a branch circuit, which is allowed to round up under 430.52(C)(1) Exception No. 1.

Worked example: 25, 30, and 40 HP at 460V

Full-load currents from Table 430.250: 34 A, 40 A, and 52 A.
Branch inverse-time breakers per Table 430.52 (250 percent, rounded up): 90 A, 100 A, and 150 A.
Feeder conductor per 430.24: 1.25 x 52 + 40 + 34 = 139 A minimum ampacity.
Feeder breaker per 430.62(A): largest branch device 150 A + 40 + 34 = 224 A ceiling.
224 A is not a standard rating, so the feeder breaker rounds down to 200 A.

Why the feeder device can exceed the conductor ampacity

In the example the feeder conductor needs 139 A of ampacity while the feeder breaker is 200 A. That is expected for motor circuits: the branch-circuit and feeder short-circuit devices ride through starting inrush, so Article 430 lets them sit well above the running ampacity. Overload protection, sized near each motor's nameplate current under 430.32, is what actually protects the motors from a sustained overload.

Frequently asked questions

How do you size a feeder for several motors?

Per NEC 430.24 the conductor ampacity must be at least 125 percent of the highest-rated motor full-load current plus the sum of the other motors. For a 25, 30, and 40 HP group at 460V (FLC 34, 40, 52 A), that is 1.25 x 52 + 40 + 34 = 139 A.

What size feeder breaker do the motors need?

NEC 430.62(A) caps the feeder device at the largest branch-circuit device plus the other motors full-load currents. For the same group on inverse-time breakers the largest branch device is 150 A, so the ceiling is 224 A and the feeder breaker rounds down to 200 A.

Does the feeder device round up or down?

Down. The branch-circuit round-up of 430.52(C)(1) Exception No. 1 does not apply to the feeder. 430.62(A) is a maximum, so a non-standard result takes the next standard 240.6(A) rating below it.

Can I put non-motor loads on this feeder?

This version sizes a pure motor group. Continuous non-motor loads on the same feeder add their own 125 percent allowance under 430.24 and change the 430.62 device, which is a planned follow-up. Keep those loads off this calculation for now.

Related calculators

Motor Breaker Sizing Calculator: the branch-circuit device and conductor for a single motor.
Motor Overload Protection Calculator: the running-overload relay per 430.32.
Motor Full-Load Current Calculator: the Table 430.248/430.250 full-load current lookup.
Wire Ampacity Calculator: pick the feeder conductor for the 430.24 ampacity, with derating.

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