Comparing Series-Rated and Fully Rated Electrical Systems

There is so much misunderstanding and biased information concerning series-rated systems. A fully rated system has advantages over a series-rated system, but the latter is a viable option and can provide savings to owners, especially in areas of high available fault current and in a network utility or large service area with high AIC ratings.

11/01/2004


There is so much misunderstanding and biased information concerning series-rated systems. A fully rated system has advantages over a series-rated system, but the latter is a viable option and can provide savings to owners, especially in areas of high available fault current and in a network utility or large service area with high AIC ratings. In most cases, the savings for a series-rated system outweighs its potential drawbacks. An electrical engineer must be aware of all the important issues to fully evaluate both:

  1. Full selective coordination. Series-rated breakers cannot completely, selectively coordinate. For that matter, fully rated breakers cannot fully, selectively coordinate. Unless fuses are used, selective coordination is not possible anyway. A breaker's instantaneous region starts at five to 10 times the breaker rating. Therefore, breakers will trip at high-fault current regardless of whether they are series- or fully rated systems.

  2. Future build-out restrictions. An owner can get stuck using the same vendor for future build-out. There are currently no inter-vendor series-rated systems. Future additions to the electrical system would have to use the same vendor to keep a series rating.

  3. Potential equipment replacement mistakes. Future contractors can potentially make mistakes when replacing breakers, such as replacing breakers with non-series-rated breakers, which could cause potential safety hazards.

  4. Lowest series-rated breakers (i.e., 10,000 amps) cannot exceed 1% of the total motor contribution served from the panel. If the total motor load from the panel is more than 100 amps or about 75 hp, the code will not allow you to use 10,000 AIC secondary breakers. But only in rare instances will this be a problem. (See Section 240%%MDASSML%%86 of the NEC.)

  5. Extra labeling is required. Per NEC Section 110%%MDASSML%%22, field labeling must state: CAUTION—SERIES COMBINATION SYSTEM RATED ____ AMPERE. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.

Series rating is a viable option to a fully rated system and should be evaluated to offer the client the most cost-effective project.





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