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NEC and standby gensets

4/1/2008
NEC and standby gensets

Generators were not the major point of focus when the National Electric Code was written. As a result, engineers must reference multiple sections of the NEC Handbook when designing power generation systems. I will reference some key code sections, while answering some typical generator code-related questions.

Is the cabling from the generator a feeder or a service? This impacts the interpretation of generator installation requirements. NEC 100 defines a service as being supplied by a serving utility. The conductors from the generator are classified as feeders.

What does NEC require for generator sizing? The answer varies based on the type of load powered. Emergency systems require the generator to be sized for the entire connected load (NEC 700.5). But if the emergency system is a healthcare application, NEC 517.30(D) requires the generator to meet the peak demand of the load and waives the requirements of NEC 700.5. This is a direct result of healthcare applications being too lightly loaded. Legally required standby and optional standby traditionally required the generator to be sized to meet the peak demand of the load intended to be operated. This requirement has been strengthened in the 2008 code to require compliance with Article 220, “Branch-Circuit, Feeder, and Service Calculations,” or another authority having jurisdiction (AHJ) method. This change was driven by a desire to protect the consumer in the residential market from installing a generator smaller than the potential load.

How quickly must a generator startup and transfer? The common answer is 10 sec. Emergency systems (NEC 700.12) and fire pumps (NFPA 20.9.6.2.1) require a 10-sec start and load transfer. But legally required loads require a less demanding 60-sec. response (NEC 701.11), and optional standby does not require an automatic startup and transfer time. Meeting a 10-sec startup time generally is not an issue for single generator applications. However, applications with multiple generators often take a few extra seconds to parallel generators. These applications need to transfer the emergency system loads first and then add lower priority loads later.

Is a disconnect required on the generator? NEC 445.18 shows a preference for the generator to be equipped with a disconnecting means but allows the disconnect—typically the generator breaker—to be removed, provided that the generator can be readily shut down and that it does not operate in parallel. So, even though the NEC allows the generator to be provided without an output breaker, standard industry practice is to include this disconnect.

Is another disconnect required at the point of building entrance, in addition to the generator breaker? The market implementation of this requirement seems to be split—half of the market requires an additional disconnect and half of the market does not. Because the cabling from the generator is a feeder, NEC 225.31 requires a disconnect at the point of building entry unless exempted in another part of the code. Enter the local interpretation of the requirements for an outdoor-housed generator set—NEC 700.12(B)(6), 701.11(B)(5), and 702.11—which state: “When an outdoor housed generator set is equipped with a readily accessible disconnect means located within sight of the building or structure supplied, an additional disconnecting means shall not be required.”

The issue here is what needs to be visible from the building. If your answer is the generator, then an additional disconnect generally isn’t required. But if the answer is the generator disconnect, then an additional disconnect typically be required (most generator breakers are not visible from the building).

What size of generator breaker should feed a fire pump? The utility feeder powering the fire pump must be sized to indefinitely carry locked rotor current per NEC 695.4(B)(1). But does this apply to the generator? Confusion comes from the way NEC 695 interweaves the utility and generator requirements. As a result, generator suppliers receive many requests for magnetic-only breakers and breakers sized for six times rated amps.

But what does the code actually request? The basic requirement is for the generator source and overcurrent protection to be sized for “normal starting and running” (NEC 695.4(B)(1)) and not indefinite locked rotor operation. The NEC Handbook states that “alternative source overcurrent protection device(s) for the electric-driven fire pump are not required to be sized for locked rotor current… the circuit components of the alternative source are permitted to be sized according to Article 430.”

Confusion with respect to fire pumps is multipart. First, most people miss the phrase “remote power sources” when reading NEC 695.4(B)(1). This section, which requires overcurrent devices to be set for lock rotor current, only applies to remote power sources—the utility. Another point of confusion occurs around the topic of no overload protection. NEC 695.6(D) specifies: “Power circuits shall not have automatic protection against overloads. Branch circuits and feeder conductors shall be protected against short circuit only.” Many people mistakenly interpret this to mean a magnetic-only circuit breaker. The NEC defines overload protection in 430.32 and limits overload protection at 125% of rated running amps. Consequently, a breaker larger than 125% would provide short circuit protection only. This is the minimum size of a breaker feeding the fire pump from the generator source. The maximum size of the fire pump breaker typically will be limited to 250% of rated running amps: NEC 695.4(B) requires compliance with NEC 430.62. My interpretation is that the generator fire pump breaker(s) should be sized 125% to 250%, but local interpretation and norms may require a different solution to meet the AHJs approval.

For more detailed information, visit the Web cast archive at csemag.com, and view the Web cast, “Understanding NEC Code for Standby Generators.

Author Information
Kirchner is an electrical engineer and sales training manager with Generac.

AT A GLANCE

Several sections of the National Electrical Code address generators, including:

• NEC 700.5

• NEC 517.30(D)

• NEC 700.12

• NEC 701.11

• NEC 445.18

• NEC 225.31

• NEC 695.4(B)(1)

• NEC 695.6(D)

• NEC 430.32

• NEC 430.62

For a compete discussion, go to the csemag.com Web cast archives and view “Understanding NEC Code for Standby Generators.”

View the original article and related content on Consulting Specifying Engineer

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