Emergency power sources and the 10-second rule
When normal power fails, emergency lighting and power must be available within the time the application needs, and never more than 10 seconds. Section 700.12 lists the power sources allowed to do that.
What the code requires
When the normal supply fails, emergency lighting, emergency power or both must be available within the time the application requires, and in no case more than 10 seconds. The emergency supply is one or more of the systems listed in 700.12(C) through (H), in addition to the normal service.
The choice of source must consider the occupancy and the type of service. The supply duration rules in 700.12(C) cover every source, including fuel. Stored-energy systems are covered in 700.12(E).
Microgrids used as an emergency source must either isolate emergency loads from all nonemergency loads when normal power is lost or meet 700.4(B). Battery-equipped emergency luminaires, which include unit equipment, follow 700.12(H) and must be listed.
On the job
- The 10-second limit is for emergency systems under Article 700. Legally required standby systems (701.12) and optional standby systems follow different timing rules.
- Emergency systems must now be commissioned under 700.3.
- Battery-equipped emergency luminaires must be listed products, a requirement new in the 2023 edition.
Exceptions that matter
- Where your building department approves, a public utility gas supply may be used instead of an on-site fuel supply, if it is unlikely that the gas supply and utility power would fail at the same time (exception to 700.12(C)(3)).
What changed in 2023
The 2023 NEC reorganized 700.12: a new supply-duration rule in 700.12(C) sets a 2-hour minimum for most sources and takes in the fuel-supply rules, UPS, fuel cells, energy storage and batteries were grouped as stored-energy systems in 700.12(E), the dc-only microgrid rule became a general microgrid rule, and the unit equipment rules became battery-equipped emergency luminaire rules that require listing.
In California
The State Fire Marshal's rulemaking for the 2025 California Electrical Code repeals its 700.12 on-site fuel exceptions and moves them, revised, to new 517.31(H), 517.42(G) and 517.45(H), co-adopted with OSHPD. Confirm with your building department.
Related sections
- NEC 700.3
- NEC 700.4
- NEC 700.10
- NEC 700.32
- NEC 701.12: Legally required standby power sources
- NEC 445.18: Generator disconnect and emergency shutdown
- NEC 517.31
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