Emergency lighting systems consist of circuits and equipment intended to provide power to required facilities when normal power is interrupted. Municipal, state, federal or any governing agencies having jurisdiction typically are the entities that classify an emergency system. NFPA 101, the Life Safety Code, designates specific locations of occupancies where emergency lighting is necessary for life safety. The type of emergency lighting system often is a matter of design, but it always has to conform to National Electrical Code (NEC) minimums regarding the source, type of luminaire and circuit wiring.
Emergency lighting is not required for every room in a facility but usually is required for any portion of the egress path to the exterior exits. Section 700.12(F) provides some detail about unit equipment that is installed as emergency lighting. The term unit equipment refers to self-contained equipment that includes a source (usually a battery) for emergency lighting. The NEC describes it equipment as emergency lighting equipment that consists of a rechargeable battery, battery charger, provisions for one or more lamps on the unit, and a relay device that will energize the emergency luminaires automatically in the event of interruption of power to the equipment.
An important general requirement in 700.12(F) is that the supply circuit to the unit be connected to the same circuit supply as the normal lighting in the area. This allows the emergency illumination to function in the event of a lighting branch-circuit failure to that area rather than just interruption of all power to the building. The equipment must be supplied ahead of any local switches for the lighting in that area. This is required to keep the batteries charged and ready for operation at all times. The batteries must have sufficient rating and capacity to maintain not less than 87.5 percent of the normal battery voltage for the total lamp load for at least 1.5 hours. The unit equipment also is permitted to supply and maintain at least 60 percent of the initial emergency illumination for a period of at least 1.5 hours. See the exception for alternatives relating to areas supplied by a minimum of three circuits originating in the same panelboard.
Some unit equipment designs allow provisions for supplying remote luminaires from the DC (emergency output) circuit of the equipment. There are a few important requirements that have to be applied here. First, the supply circuit to the unit equipment is required to be installed in a wiring method according to Chapter 3 of the NEC. Always verify the construction type used for the building, as this may affect what wiring methods are suitable for use.
A second important design consideration for emergency DC lighting circuits (low-voltage circuits) is that they are subject to the effects of voltage drop. This is an important consideration in meeting the minimum number of lumens at the floor line in an egress path.
The third requirement is that the DC circuit wiring to the remote luminaire(s) of the unit equipment be installed in a wiring method according to Chapter 3 of the NEC, even though it is low voltage. This low-voltage wiring is not permitted to be installed using low-voltage cable or limit energy cable assemblies that are not a wiring method covered in Chapter 3. Unit equipment is required to be approved for emergency use according to Section 700.3. Unit equipment is usually listed by a qualified electrical testing laboratory and, as such, must be installed according to the installation instructions included in the listing or labeling. Listed equipment installed according to manufacturers’ instructions serves as a basis for inspector approvals. The number of remote luminaires that can be supplied by the low-voltage output circuit usually is limited because of the capacity limitations of the battery. Verify this with the installation instructions or through the manufacturer.
Emergency lighting systems that include unit equipment and remote luminaires connected to a DC output circuit have to meet minimum requirements related to capacity, branch-circuit supply and wiring methods on both connected input (AC) and output circuits (DC). Another important rule to follow related to this type of emergency lighting system is to verify the operation by conducting or witnessing a performance test of the completed installation as required by 700.4(A). It is a good idea to simulate a power failure and correct operation of the equipment before the actual witness test procedure overseen by the authority having jurisdiction (AHJ). When in doubt about requirements in the NEC or any local rules, it is best to consult the AHJ.
JOHNSTON is NECA’s executive director of standards and safety. He is former director of education, codes and standards for IAEI; a member of the IBEW; and an active member of the NFPA Electrical Section, Education Section and the UL Electrical Council. Reach him at email@example.com.