CQD Spotlight features questions and answers from NECA and ELECTRICAL CONTRACTOR’s Code Question of the Day program. Whether you’re a seasoned contractor, an apprentice or simply a National Electrical Code enthusiast, you’ll gain knowledge with a nod to the legacy of Charles M. “Charlie” Trout, a true NEC legend.
From the archives— June 12, 2000
What is meant by the term “series rated?”?
A circuit breaker can be used on a circuit having an available fault current higher than its marked interrupting rating if it is connected on the load side of an acceptable overcurrent device having the higher rating. As an example, if a service panel
had 14,000A of available fault current at its terminals, circuit breakers marked 10,000A could be used if the main breaker was marked 22,000A. If a fault occurs on the load side of a branch breaker, then the main breaker and the branch breaker act
“in series,” and both trip ”off.” National Electrical Code Section 240.86 has additional information relating to series ratings.
From the archives—Dec. 31, 2002
We have 480V motor control centers connected to 480V switchgear. The 480V switchgear have fused circuit breakers (low-voltage circuit breakers) where the fuses are of current-limiting type and rated 30 kA symmetrical interrupting. The motor control centers are rated for 42 kA symmetrical and consist of molded-case circuit breakers and motor circuit protectors. The available calculated short-circuit current at the motor control centers is 65 kA symmetrical. Our consultant requested the motor control centers be rated above this to meet the NEC requirements for series-rated systems, since current-limiting fuses don’t provide protection to the motor control centers and components. But won’t the current-limiting fused circuit breakers provide the required protection? Also, 240.86 was written for switchgear feeding panelboards, not switchgear that feeds motor control centers. What are the requirements on series-rated systems?
A series-rated system is a “listed” combination of overcurrent devices that may be circuit breakers or fuses and circuit breakers, that can be used at available short-circuit levels that exceed the interrupting rating of the load-side circuit breakers but are not greater than that of the main circuit breaker. Series-rated systems can consist of a combination of main fuses/branch circuit breakers or main breaker/branch breakers. Panelboards are marked with their series rating and the specific overcurrent devices (fuses and circuit breakers) that can be used “in series” per their listing. From your question, it appears as though you are using fused circuit breakers where the internal fuses are of a current-limiting type and rated at 30 kA interrupting capacity. The fuses inside the breaker are really not “fuses“ by definition. They are probably limiters whose current-limiting capability is matched to the withstand rating of the circuit breaker, and the limiters aren’t used to protect other downstream equipment. Your best bet is a fully rated system where all of the overcurrent devices have an interrupting rating as high as the available short-circuit current at each device. You’ll want to study sections 110.10 and 240.86 in the NEC.
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When applying series ratings, attention to detail is critical. In contrast to a series-rated system, a fully-rated system ensures that both upstream and downstream overcurrent protective devices (OCPDs) can independently interrupt the available fault current. Selectively coordinated systems must be fully rated.
The primary advantage of using series ratings lies in cost savings: a downstream circuit breaker with a lower interrupting rating than the available fault current can be used. However, compliance with the requirements of NEC 240.86 is essential for this approach to be valid and safe.
Section 240.86(A) permits engineered series-rated solutions. These are challenging to implement, as the engineer must ensure that downstream circuit breakers in the series combination remain passive during fault interruption. The downstream molded-case circuit breaker must not begin to open and produce an arc, thereby violating the passivity requirement.
Section 240.86(B) allows the use of tested series-rated combinations, which are evaluated and documented under UL 489 standards. These tested pairs eliminate guesswork, as product listings confirm compatibility and performance.
Section 240.86(C) is often overlooked. It prohibits the use of series ratings when motor circuits are connected between the higher-rated OCPD and the lower-rated circuit breaker, and when the combined full-load motor current exceeds 1% of the interrupting rating of the lower-rated breaker.
In summary, while series-rated solutions can offer economic benefits, their application demands rigorous adherence to Code requirements and a thorough understanding of system behavior. Success lies in the details.
