Technical Corner

This month’s article speaks to the challenges of sizing multiple motor circuits. First some common vocabulary/abbreviations: MCA-minimum circuit ampacity-minimum wire size required FLA/FLC-full load amps/current at rated load and voltage

Motor branch circuits and feeders must have overcurrent protection devices to protect the circuits from short circuits and ground faults. Overload protection is a separate calculation and often puts the overload protection device at the motor itself.

In this example, the motor information is given in the photo. Assume the branch and feeder protection are both breakers, not fuses.

Step-1: Determine the overcurrent protection for the largest motor by using NEC Table 430.52 and the Inverse Time Breaker ratings of 250%. (FLC x 2.5 = 70A)

Note-if the above equated to, say, 68 Amps then using the next standard size listed in NEC 240.6 could have been used which would put us at 70A anyways.

Step-2: Repeat step 1 for the 2nd motor. (17 x 2.5 = 42.5) Next size up per NEC 240.6 is 45A.

Step-3: Add the FLC of ALL the other motors on the same circuit to the rating of the breaker calculated for the largest motor. (70 + 17=87 Amps) Using the next standard size DOWN per NEC 240.6 gives you a feeder breaker rated at 80A.

The confusing part here is using the next size down. Typically sizing a breaker at 250% is unheard of and would add additional cost to the owner. Using this to size the branch protection already adds a safety factor so using the next size down is warranted.

This article was not intended to go over wire sizing, but you might have noticed the use of #6AWG wire on the 80A feeder circuit. For motor circuits the wire sizes only need to be sized per the MCA (definition above) which equates to 125% of the FLC (definition above) of the LARGEST motor and the sum of the FLC of ALL other motors. (28 x 1.25) + 17 = 52 amps. This represents the minimum ampacity the wiring must be rated for. So using the 60 degree column #6 AWG is rated for 55 amps and exceeds the 52 amps calculated.

Personally, I prefer to size the wire per the feeder breaker rating (80 amps) which is acceptable as well.

Hopefully this article finds you well and can be used as a reference for your project needs.

If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Image Credits: Mike Holt Enterprises Inc.

Brett Eliasz, P.E., LEED AP BD+C , RES Director

res - technical corner JUNE 2019 The ROCHESTER ENGINEER | 5