A common issue during construction for the electrician is proper circuit separation of emergency circuits and normal power.

The general rule in NEC^® 700.10(B) is to keep wiring from an emergency source or emergency source distribution overcurrent device to the emergency loads entirely separate from all other wiring and equipment, unless otherwise permitted in 700.10(B)(1) through (5).

Section 700.10(B)(5) permits emergency and non-emergency wiring to comingle where wiring from the emergency source to supply emergency and other (nonemergency) loads is done in accordance with 700.10(B)(5)a., b., c., and d.

NEC Section 700.10(B)(5)(a) requires separate vertical switchgear or switchboard sections, or individual disconnects mounted in separate enclosures to be used to separate emergency loads from all other loads.

NEC Section 700.10(B)(5)(b) requires the common bus of separate sections of the switchgear or switchboard, or the individual enclosures (mentioned in item a. above) to be either of the following:

(i) Supplied by single or multiple feeders without overcurrent protection at the source

(ii) Supplied by single or multiple feeders with overcurrent protection, provided that the overcurrent protection that is common to an emergency system and any nonemergency system(s) is selectively coordinated with the next downstream overcurrent protective device in the nonemergency system(s)

The reason behind the language in 700.10(B)(5)(b)(ii) above is to ensure that in a configuration where an emergency source feeds both emergency and optional standby loads through a common overcurrent device, the overcurrent devices maintain their selective coordination in order to ensure continuity of power to the more important emergency loads. Selective coordination means selecting overcurrent devices in an electrical system, based on their trip curves and opening times, to ensure that the overcurrent device closest to the fault condition opens before any upstream overcurrent devices do.

When normal power is not present because of an outage, the emergency source intervenes to provide backup power. The emergency source is permitted to supply both emergency loads and non-emergency (optional) loads as long as the wiring is separated as required in 700.10(B). While running on emergency power, a fault coming from an optional load should not cause the main overcurrent device supplying both emergency and optional loads to trip due to lack of overcurrent device selective coordination.

The best way to visualize the above code language is to look at the two illustrations pertaining to this code section. Figure 700.10(B)(a) and Figure 700.10(B)(b). (See our illustration for a reproduction of the NEC^® drawings).

Below is a preview of the NEC^® (NFPA 70). See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the edition of the NEC^® being enforced.

NEC 700.10(B)(5) Wiring from an emergency source to supply emergency and other (nonemergency) loads in accordance with 700.10(B)(5)a., (B)(5)b., (B)(5)c., and (B)(5)d. as follows:

a. Separate vertical switchgear sections or separate vertical switchboard sections, with or without a common bus, or individual disconnects mounted in separate enclosures shall be used to separate emergency loads from all other loads.

b. The common bus of separate sections of the switchgear, separate sections of the switchboard, or the individual enclosures shall be either of the following:

(i) Supplied by single or multiple feeders without overcurrent protection at the source

(ii) Supplied by single or multiple feeders with overcurrent protection, provided that the overcurrent protection that is common to an emergency system and any nonemergency system(s) is selectively coordinated with the next downstream overcurrent protective device in the nonemergency system(s)

c. Emergency circuits shall not originate from the same vertical switchgear section, vertical switchboard section, panelboard enclosure, or individual disconnect enclosure as other circuits.

d. It shall be permissible to utilize single or multiple feeders to supply distribution equipment between an emergency source and the point where the emergency loads are separated from all other loads.

Wiring of two or more emergency circuits supplied from the same source shall be permitted in the same raceway, cable, box, or cabinet.