When deciding to go with a series rated combination over a fully rated system, motor contribution needs to be considered if motors are located in specific locations relative to the overcurrent devices that are part of a series combination.

The reason: If a fault occurs such as a short-circuit, running motors can momentarily contribute current to the short-circuit condition (four to six times the motor full load current rating).

The additional motor contribution can result in a short-circuit current exceeding what the load side (protected) circuit breaker was tested to handle during the series rated combination testing.

NEC 240.86(C) does not allow a series rated system to be used if motors are connected between overcurrent devices used in a series combination where the sum of all motor full load current (FLC) exceeds 1% of the interrupting rating (AIC rating) of the lowest rated circuit breaker in the series combination.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014-2017 Code Language:

240.86(C) Motor Contribution. Series ratings shall not be used where

(1) Motors are connected on the load side of the higher rated overcurrent device and on the line side of the lower-rated overcurrent device, and

(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.