Selective Coordination and Emergency Power Supply Systems

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Welcome to the third in a series of posts focused on National Electrical Code® (NEC®) requirements for selective coordination. In my previous post, I covered some of the specifics important to engineers and designers working on health-care projects. This post is a bit more general, and covers other types of installations in which emergency power supply systems (EPSS) are being employed. As you’ll read, assuring your designs comply with these requirements requires a clear understanding of both which edition is in effect in any particular project’s jurisdiction, and how the authorities having jurisdiction (AHJs) are interpreting the requirements of that edition.

Multiple revisions could mean confusion

The reason for this need for clarity is the fact that the NEC requirements in Article 700 (Emergency Power Systems) and Article 701 (Legally Required Power Systems) have evolved over the course of the last several revision cycles. These articles apply to systems intended to supply power “when the normal electrical supply or system is interrupted.” Such language would seem to indicate that the selective coordination requirements in those articles only apply when the system is fed from an alternate, backup-power source. Figure 1, which is included in Appendix B.1 of the National Fire Protection Association’s NFPA Standard 110, “Standard for Standby and Emergency Power Systems,” identifies how such equipment would physically relate to the rest of an electrical system and seems to support this interpretation.

However, diving into the details of Articles 700 and 701, one finds their requirements state that “overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices”. So, what, exactly, does “all” really mean in this context? This question has arisen in the last two NEC revision cycles.

When these selective coordination requirements were first initiated in the 2005 NEC, “all” was interpreted by some to include all devices upstream of the transfer switches connecting the EPSS loads to both normal (grid-supplied) and alternate power sources. A statement by Code Making Panel 13 (CMP 13) for the 2011 cycle, however, indicated that requirements for “normal” system devices were actually outside the scope of Article 700.

With this decision, it seemed the panel had settled the issue, and that the selective coordination requirements in Sections 700.27, 701.18 and 708.54 applied to overcurrent devices on the load side of the automatic transfer switch and the overcurrent devices on the alternate source line side of the automatic transfer switch. However, this decision was reversed in the 2014 revision cycle, with CMP 13 stating that selective coordination was required up to both the alternate and “normal” power sources. Referring to Figure 1, then, this requirement adds a selective coordination requirement for the overcurrent devices directly above the three automatic transfer switches serving the three emergency power supply system loads.

Understand your AHJ’s requirements

Obviously, this change could have a significant impact on how an AHJ might view proposed electrical designs, so it’s critical that engineers understand which NEC edition is in effect in the jurisdiction in which any given project is located. Additionally, it’s important to know what local amendments to NEC selective coordination requirements might be in effect. Consider how the following major jurisdictions have enacted their own guidelines:

• Phoenix: “Selective coordination is… the responsibility and judgment of the design professional”
• Chicago: Required only up to the alternate source
• Las Vegas, New York City, Oregon, Washington State (may vary by city): Several significant differences
• Memphis: Coordination down to 0.1s
• Wisconsin: Enforced to the extent practicable

Additionally, designers working on projects requiring EPSS installation also should be clear on the relationship between the NEC and two other National Fire Protection Association (NFPA) standards: NFPA 99, “Health Care Facilities Code,” and NFPA 110, “Standard for Emergency and Standby Power Systems. There is no conflict between the NEC and these two standards – as the NFPA Standards Council has stated, NFPA 99 and 110 outline performance requirements of their respective systems, while the NEC covers installation requirements of those systems.

My next post will begin a discussion of molded-case circuit breakers, their functions and related design guidelines. Also, should you have questions regarding selective coordination in any current or future project, please visit Schneider Electric’s dedicated consulting engineer portal for a wide range of resources, including access to one-on-one support from expert staff engineers.