240.86 Series Ratings and Motor Contribution.
Code Change Summary: Revised code language on motor contribution in a series rated system.
Overcurrent protective devices can be either Fully Rated or selected as a series combination rated system or Series Rated System.
In a series rated system, two overcurrent devices are used. The upstream overcurrent device can be either a fuse or circuit breaker and it will have an interrupting rating (AIC rating) equal to or greater than the Available Fault Current (AFC) at its line terminals. The downstream overcurrent device will always be a circuit breaker and will have an AIC rating less than the actual AFC at its line terminals but the two devices will have been tested by a nationally recognized testing laboratory, and found to work together in order to eliminate a fault condition that subjects the downstream circuit breaker to more AFC than it is rated for.
When deciding to go with a series rated combination or a fully rated system, motor contribution needs to be considered if motors are 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.
In the 2020 NEC®, a proposal was accepted to make Section 240.86(C) easier to understand.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.
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.
2020 Code Language:
240.86(C) Motor Contribution. Series ratings shall not be used where
(1) Motor circuits are connected between the higher-rated overcurrent device of a series-rated combination and on the lower-rated circuit breaker, and
(2) The sum of these motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.