# Determining Emergency Generator Class in NFPA 110



## Shorty Circuit (Jun 26, 2010)

I'm specifying a gas fired emergency generator as indicated in article 700. It requires dual fuel, that is a manifold that will automatically switch from natural gas to bottled propane if the natural gas supply fails. But there is nothing to indicate how much run time is required and if it is based on connected load or nameplate rating. NFPA article 110 classifies generators according to run time in table 4.1 (B) and puts them in six categories ranging from 5 minuts to 48 hours and "other" which presumably leaves it to the AHJ.

http://static.schneider-electric.us/assets/consultingengineer/appguidedocs/section10_0307.pdf

But there is nothing to tell you which category applies based on the occupancy or hazard. The use of this EPS is in a manufacturing plant and is to sustain a ventillation system to keep part of the facility under negative pressure to prevent escape of a powdered toxic substance (all electrical fixtures are Nema 4X and Nema 12.) I checked NFPA 101 life safety but there is no mention of it there either.

Does anyone know where to find the class of generator backup run time that applies for a given occupancy category and hazard? 

BTW, there are three different definitions of emergency power systems and their purpose, one in NFPA 70 (NEC) one in NFPA 110, and and one in IEEE 446-1995 and naturally they don't agree with each other. You can see that in the link.


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