Q. What are the Code requirements for transfer equipment of emergency systems?
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A. As noted in Sec. 700.4 of the 2014 NEC, an emergency power system must have adequate capacity to carry all emergency loads expected to operate simultaneously. If an alternate power supply has adequate capacity, it’s permitted to supply emergency loads [Art. 700], legally required standby loads [Art. 701], and optional standby system loads [Art. 702]. If the alternate power supply doesn’t have adequate capacity to carry the entire load, it must have automatic selective load pickup and load shedding to ensure adequate power in the following order of priority:
1) The emergency circuits,
2) The legally required standby circuits, and
3) The optional standby circuits.
A temporary alternate source of power must be available whenever the emergency generator is out of service for more than a few hours for maintenance or repair.
As noted in Sec. 700.5, transfer equipment must be automatic, identified for emergency use, and approved by the authority having jurisdiction (AHJ). Automatic transfer switches must be electrically operated, mechanically held, and listed for emergency power system use. Transfer equipment must supply only emergency loads. Note: Multiple transfer switches are required where a single generator is used to supply both emergency loads and other loads.