Section 700.12/Emergency System Power Sources
NEC 700.12 specifies permitted emergency power sources and the 10-second automatic transfer requirement, affecting generator and ATS specifications on drawings.
Emergency systems must have an alternate power source that automatically restores power within 10 seconds of normal supply failure. Generators used for emergency power must have on-site fuel sufficient for not less than 2 hours of full-demand operation. The transfer switch must be automatic and listed for emergency service. Battery systems must maintain the emergency load for a minimum of 1.5 hours. The emergency power source must be located and protected to minimize the possibility of damage from flooding, fire, ice, vandalism, or other events.
Why this section exists
Emergency systems power life safety equipment: egress lighting, exit signs, fire alarm systems, smoke control, and fire pump motors. When normal power fails, these systems must activate within 10 seconds to protect occupants during evacuation. A generator that takes 30 seconds to start, runs out of fuel in an hour, or is damaged by the same event that caused the power failure defeats the purpose of emergency power.
What plan reviewers look for
Plan reviewers check the one-line diagram for the emergency power source, automatic transfer switch, and the emergency distribution system. They verify the 10-second transfer time, the fuel supply duration (2 hours minimum on-site), and the generator location. They also check that the emergency system is kept entirely separate from other systems (legally required standby, optional standby) on the distribution side.
Common violations
Related NEC requirements
Article 701 covers legally required standby systems. Article 702 covers optional standby systems. Section 700.10 covers emergency system wiring requirements. Section 250.30 covers grounding of the generator as a separately derived system.