Two similar installations have been proposed recently. The first is a proposed large hotel on a mountaintop at a distance of several miles from the nearest utility distribution line, and in a location subject to frequent lightning storms. Contemplating that there would be frequent interruptions in the utility electric service, it was proposed that a generating set be installed with sufficient capacity to serve the entire load through an automatic transfer switch.
The other proposal was for a new combination city hall and office building for a large city. The purpose here was to have a generating plant large enough to carry the entire load so that there would be no interruption of city services to the public even when the electric utility service failed.
In both cases, it was decided that no special emergency circuits were necessary because the entire load would be transferred to the on-site generator in case of utility service interruption.
Both of these proposals are impractical for meeting the National Electrical Code (NEC). In both cases, there are public rooms, meeting rooms, assembly areas, etc., which are large enough to require emergency lighting and the associated exit signs and exitway illumination.
Among the many NEC requirements that are difficult to meet with these proposals are:
· "700-3. Equipment Approval
All equipment shall be approved for use on emergency systems.
· "700-7. Signals
Audible and visual signal devices shall be provided, where practicable, for the following purposes.
(a) Derangement. To indicate derangement of the emergency source.
(b) Carrying Load. To indicate that the battery is carrying load.
(c) Not Functioning. To indicate that the battery charger is not functioning.
(d) Ground Fault. To indicate a ground fault in solidly grounded wye emergency systems of more than 150 volts to ground and circuit-protective devices rated 1,000 amperes or more. The sensor for the ground-fault signal devices shall be located at, or ahead of, the main system disconnecting means for the emergency source, and the maximum setting of the signal devices shall be for a ground-fault current of 1200 amperes. Instructions on the course of action to be taken in event of indicated ground fault shall be located at or near the sensor location.
FPN: For signals for generator sets, see Standard for Emergency and Standby Power Systems, NFPA 110-1996
· "700-9. Wiring, Emergency System
(a) Identification. All boxes and enclosures (including transfer switches, generators, and power panels) for emergency circuits shall be permanently marked so they will be readily identified as a component of an emergency circuit or system.
(b) Wiring. Unless otherwise permitted in (1) through (4), wiring from an emergency source or emergency source distribution overcurrent protection to emergency loads shall be kept entirely independent of all other wiring and equipment. Wiring of two or more emergency circuits supplied from the same source shall be permitted in the same raceway, cable, box, or cabinet.
1. The normal power source wiring shall be permitted to be located in transfer equipment enclosures.
2. In exit or emergency lighting fixtures, wiring supplied from two sources shall be permitted.
3. In a common junction box, attached to exit or emergency lighting fixtures, wiring supplied from two sources shall be permitted.
4. The wiring within a common junction box attached to unit equipment, containing only the branch circuit supplying the unit equipment and the emergency circuit supplied by the unit equipment shall be permitted.
· "700-15. Loads on Emergency Branch Circuits
No appliances and no lamps, other than those specified as required for emergency use, shall be supplied by emergency lighting circuits.
· "700-20. Switch Requirements
The switch or switches installed in emergency lighting circuits shall be arranged so that only authorized persons will have control of emergency lighting. . . "
There is one way to essentially accomplish the aims of these two proposed installations, and that is by identifying all of the required emergency feeders and circuits and keeping the wiring for them separate from all of the other wiring, and then following Section 700-5 and especially -5(b):
· "700-5. Capacity
(a) Capacity and Rating. An emergency system shall have adequate capacity and rating for all loads to be operated simultaneously. The emergency system equipment shall be suitable for the maximum available fault current at its terminals.
(b) Selective Load Pickup, Load Shedding, and Peak Load Shaving. The alternate power source shall be permitted to supply emergency, legally required standby and optional standby system loads where automatic selective load pickup and load shedding is provided as needed to ensure adequate power to (1) the emergency circuits, (2) the legally required standby circuits, and (3) the optional standby circuits, in that order of priority.
A portable or temporary alternate source shall be available whenever the emergency generator is out of service for major maintenance or repair."
Calling all the wiring other than the required emergency wiring "optional standby" and providing automatic load shedding ensures that, in the case of trouble in the premises wiring, the required emergency circuits will still be in service after the rest of the premises wiring has been interrupted while being served by the generator.
SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.