The National Electrical Code article 240.86 addresses series ratings with the following:
Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in (A) or (B), and (C).
(A) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment. For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.
During the early 1980’s there were some problems with series ratings and tested combinations were introduced. Series rated test standards in accordance with U.L were developed and circuit breaker manufacturers begin to provide tables with their listed series ratings like we see today. 240.87(B) states
(B) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.
Informational Note to (A) and (B): See 110.22 for marking of series combination systems.
240.87 (C) addresses the situation where short circuit contribution from motor’s a.k.a. “motor contribution” may occur between the line side and load side devices that make up a series rating. The consideration is as follows:
(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.
An example of a series rating may be a breaker that normally has an interrupting rating of 22,000 Amps but when protected with upstream fuses of a specific size and class, may have a series rating of 100,000 Amps. This must be either a listed combination or determined under engineering supervision as stated above in 240.86 (A)
Those that consider using series ratings should thoroughly review the requirements of 240.87.
With all of that information, here is this week’s question.
Does your facility/client’s facility use series ratings?
– Multiple sites – some do
– Don’t know
– Doesn’t apply to me
CLICK to answer and review discussion.
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