Critical Power: Generators and Generator System Design: Q&A Session

James Ferris and Robert JonesWebcast Q&A session with presenters Robert Jones Jr., JBA Consulting Engineers; and James Ferris, TLC Engineering for Architecture

  1. Q: When do you need a higher rating breaker on the generator than the rating of the generator itself?

    • A: James Ferris: The generator breaker should be sized at 100% to 115% to match the NEC requirement for conductor size. The onboard controllers for most generators do include overload features for the generator to protected from over current situation. Generator breaker should always coordinate with the damage and the decrement curve. 

  2. Q: What is considered as life safety separation between the breakers? Is it just the load side considered as separation or the top to bottom of the panel needs to be separated?

    • A: Robert Jones: NEC 700.10(B)(5), which addresses separation of feeders, allows for a common bus within switchgear serving all generator loads, but the feeder for emergency loads must originate in a separate vertical section from those that serve legally required and optional standby loads.

  3. Q: Because most generators are rated at 0.8 power factor, should a full load bank be sized at 125% of the kW rating of the generator?

    • A: James Ferris: A reactive load bank test should be at the rated kVA and power factor, meeting the comprehensive full load requirements for the generator. For the resistive load bank, the generator has a kW rating therefore, I would not recommend using a load bank bigger than the generator nameplate.

  4. Q: What is the process for ensuring adequate generator power to address generator down time, including scheduled maintenance and generator failure? Are there reliable sources to determine probability of generator failure?

    • A: James Ferris: I have heard generator reliability expressed as 98% reliable. How accurate that is probably would depend on manufactures individual data. This does simply imply that a generator in your system will be off for several days each year. Redundancy can be accomplished via more generators, a load shedding plan, for the times when the generators are off line. There is also an approach by several manufacturers to use smaller generators but more of them in a modular set-up. There are applications where anyone of these solutions could be the correct choice and would depend on specific site conditions.

  5. Q: To what minimum percentage load should a generator be to minimize wet-staking conditions?

    • A: Robert Jones: A generator should not be operated below 30% to 40% of rated capacity for prolonged periods. The engine will not operate at the optimum temperature and result in unburned fuel passing through with the exhaust. Over time the engine fuel injection and exhaust systems will foul and negatively affect performance, emissions, and life expectancy of the generator. NFPA 110 recognizes this condition and contains minimum loading requirements during monthly exercising in Section 8.4.2.

  6. Q: Because of severe weather conditions, are there new standards for hurricane-affected areas?

    • A: James Ferris: It would depend on the state code, but yes, there are specific hurricane-related requirements. For example, Florida requires an EPSS system to be protected from a missile impact such as a flying 2 x 4 at specific hurricane speeds depended upon where in the state you are. Also the flood level would be of key importance even on the national level.

  7. Q: How do you select between a diesel generator versus a natural gas generator? Which one has better benefits in terms of reliability and economics?

    • A: Robert Jones: Per NEC 700.12(B)(3) and 701.12(B)(3), the generator fuel source serving emergency and/or legally required standby loads may not be solely reliable on a public utility gas supply. At the discretion of the authority having jurisdiction (AHJ), some jurisdictions recognize public utility natural gas as an acceptable fuel source if historical statistics indicate the unlikelihood of loss of both utility power and utility gas. If the generator will only serve optional standby loads, a natural gas generator would be acceptable. In regards to reliability, the testing and maintenance is significant in determining the reliability of the system. If properly maintained, on-site diesel fuel oil storage is more reliable than a public utility natural gas system as the fuel stored on-site would be under the control of the site operator. If the site operator is more concerned with extending the run-time and minimizing or eliminating the on-site fuel storage, natural gas would provide this benefit at some costs. A natural gas generator can consume up to three times the fuel of a comparable diesel generator and in colder climates natural gas generators tend to perform poorly below freezing temperatures.

  8. Q: How do you write energy efficiency into the contract's specifications when specifying generators?

    • A: James Ferris: Energy efficiency typically isn't a major concern in design of the EPSS itself. If you do a parallel system, it's important to make sure you include a generator demand protocol to conserve fuel and keep the engines operating at a higher more efficient load. Although that does need to be balanced against the requirement for capacity to start large motor load such as a chiller.

  9. Q: Are emergency/standby generators required to comply with Tier requirements?

    • A: James Ferris: The way the current code is written standby generators can participate in an emergency demand response program for a certain amount of hours (50 hours currently). Participation in this type of program would not require the upgrade to Tier 4. That said, where the agreement with the utility does not limit the number of hours to coincide with this or where the utility uses the program to sell back to the grid, upgrade to Tier 4 would be required.

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