Series rating
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Introduction
The intent of this article is to provide an understanding of series rating and the fundamental NEC requirements for series rating of circuit breakers. Series rating current protections are employed to reduce cost of materials for new installations as well as modifications to existing installations. This paper will aid the reader to distinguish between series rated systems and fully rated systems by posing and answering some common questions.
Series Rated Systems
A. What is series rating?
Series rating is a short circuit interrupting rating assigned to a combination of two or more overcurrent protective devices (OCPDs) which are connected in series. This differs from a fully rated system in that the individual interrupting capacity of the downstream device(s) in the combination is less than the series rating interrupting capacity. (i,ii) Figure 1 shows such a combination.
The application of series ratings has many practical limitations and code restrictions that must be adhered to for proper application. The most common application of series rating is with branch circuit lighting panels.(iii) Applications including large motor loads are seldom suitable for series rating.
B. What are the NEC requirements for series rated combinations (excluding motor contribution)?
1. NEC Articles 240.86 and 110.22 govern series rated combinations. The code requires that the following be observed for series rating to be used if there is no motor contribution: NEC 240.86(A), 240.86(B) and 110.22
a. The upstream OCPD shall meet or exceed the available fault current (fully rated). In practice, such an OCPD should have an AIC rating that exceeds the available fault current by a suitable margin.
b. The downstream OCPD shall have a UL series rating for use with the fully rated OCPD in the combination.
c. For combinations of more than two devices, every OCPD to be included in the series combination shall be series rated to the combination of devices upstream of it. (Figure 2.)
d. Downstream OCPDs shall be field marked indicating the upstream OCPD to which it is series rated.
e. All devices shall be factory marked for the series rated combination employed. (Figure 5)
f. A professional engineer shall approve, document and stamp any design using series rating.
2. The series rating between OCPD’s can not be determined by theoretical means, testing is the only way to demonstrate the combination can safely interrupt the available fault current.(iv) In practice, manufacturer specified series ratings should be used.(v) (It should be noted that documented combinations only exist for OCPD’s of the same manufacturer. This is especially important if the project is a renovation of an existing structure.)
C. How might series ratings be used incorrectly?
It is a common mistake when selecting downstream OCPDs to use a device that is series rated to the nearest upstream OCPD. Doing so can lead to a violation of code when there are more than two devices in the series rated combination. Figure 2 illustrates such an error.
Using Figure 2 as our example; the branch OCPD has a series rating with the main OCPD, which is fully rated. If we are to add an additional OCPD down stream of the branch to the series combination, we must be certain to select a device that has a series rating with the combination of the branch and main OCPDs rather than either OCPD individually.
D. Can motors be used in series rated combinations?
240.86(C) (1) Motors shall not be connected on the load side of the higher rated overcurrent device and on the line side of the lower-rated overcurrent device.(vi)
1. Under part 1 of 240.86(C) a motor may be placed before or after a series rated system but may not be placed within the combination. This is due to potential motor contribution to available fault current. Figure 3 demonstrates incorrect and correct motor placement. (A discussion of motor contribution and short circuit calculations is beyond the scope of this paper.)
240.86(C) (2) The sum of the motor full-load currents shall not exceed 1 percent of the interrupting rating of the lower-rated circuit breaker.(vi)
2. Part 2 of 240.86(C) limits the total motor load to 1% of the load side OCPD’s individual interrupt rating. For example in a system with 25,000 Amp air interrupting rated in series combination, motor full load Amps shall not exceed 250 Amps.
Figure 4 illustrates the possible motor contribution to the fault current, which is independent of the fault current from the utility.
Figure 5 - Example of required field marking
Image:Series rating fig5.1.gif
References
i. UL 489 and UL 67, ‘Engineering Series Ratings: Is It Practical?’, NEMA Publication
ii. Clive W. Kimblin and Yun-Ko Chien, Series Ratings of Protective Devices, IEEE Publication
iii. ‘Component Protection’, pg. 81, Electrical Protection Handbook, Cooper Bussman
iv. IAEI News, March / April 1994, ‘Series Ratings, ’, NEMA publication
v. Documentation of UL approved series ratings for their products and can be found at http://www.bussmann.com/apen/pubs/series.asp.
vi. National Electric Code Handbook, 2005
