Q. How do I size an optional standby generator when a manual or automatic transfer switch is installed? (Answer based on 2011 NEC.)
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A. The calculated load on the standby source must be in accordance with Art. 220 or by a method approved by the Authority Having Jurisdiction (AHJ) [702.4(B)].
When using manual transfer equipment, the optional standby power source must have adequate capacity for all equipment intended to operate at one time as determined by the user [702.4(B)(1)]. Note: When a manual transfer switch is used, the user of the optional standby system selects the loads to be connected to the system, which determines the system kVA/kW rating.
When using automatic transfer equipment, the optional standby power source must have adequate capacity to supply the full load transferred [702.4(B)(2)]. Note: For a new installation, the load is determined by Art. 220 or an alternate method approved by the AHJ. For existing facilities, the maximum demand data for one year — or the average power demand of a 15-min. period over a minimum of 30 days — can be used to size the power source [220.87].
A transfer switch is required for all fixed or portable optional power systems [702.5].
Exception: Temporary connection of a portable generator without transfer equipment is permitted where written safety procedures are in place and conditions of maintenance and supervision ensure that only qualified persons will service the installation — and where the normal supply is physically isolated by a lockable disconnecting means or by the disconnection of the normal supply conductors.