700.10(B) Emergency System Wiring.
Code Change Summary: Revisions were made to the separation requirements for emergency and non-emergency wiring.
The general rule in 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). In the 2017 NEC®, the allowances in 700.10(B)(1) through (4) remain the same but item (5) was revised for clarity.
Section 700.10(B)(5) now 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. as follows:
a. Separate vertical switchgear or switchboard sections, 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 or switchboard, or the individual enclosures (mentioned in item a. above) 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 non-emergency system(s) is selectively coordinated with the next downstream overcurrent protective device in the nonemergency system(s)
c. See the NEC® (this language was unchanged).
d. See the NEC® (this language was unchanged).
The reason behind the new 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.
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 selectivity.
The best way to visualize the code change is to look at the two new illustrations pertaining to this code section. Figure 700.10(B)(5)(b)(1) and Figure 700.10(B)(5)(b)(2). (See our illustration for a reproduction of the NEC® drawings).
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
2014 Code Language:
700.10(B)(5) Wiring from an emergency source to supply emergency and other loads in accordance with 700.10(B)(5)a, b, c, and d as follows:
- 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.
- The common bus of separate sections of the switchgear, separate sections of the switchboard, or the individual enclosures shall be permitted to be supplied by single or multiple feeders without overcurrent protection at the source.
Exception to (5)b: Overcurrent protection shall be permitted at the source or for the equipment, provided that the overcurrent protection complies with the requirements of 700.28.
2017 Code Language:
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., c., and d. as follows:
- 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.
- 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 non-emergency system(s) is selectively coordinated with the next downstream overcurrent protective device in the nonemergency system(s)
Informational Note: For further information, see
Informational Note Figure 700.10(B)(5)(b)(1) and
Informational Note Figure 700.10(B)(5)(b)(2).