UFC 3 -520-01
June 10, 2002
B-9
AUTOMATIC TRANSFER SWITCH SIZING. The following example
illustrates the sizing process for an ATS that is UL listed for Total System Load
capability.
EXAMPLE: Determine the required size for an ATS rated for Total System
Loads, for a 208Y/120 volt, three -phase circuit consisting of the following three -
phase balanced loads:
100 kW
1. Resistive heating load: 100 kW or I =
= 278 amperes
3 208 V
50 kW
2. Incandescent lighting load: 50 kW or I =
= 139 amperes
3 208 V
3. Motors (4) at 32 amperes each, or 128 amperes continuous load, and each
motor has approximately 192 amperes inrush on starting.
The total continuous load requirement is 545 amperes. The incandescent
lighting load does not exceed 30 percent of the total load. Select an ATS rated
for 600 amperes (the next standard ATS size above 545 amperes). Verify with
the manufacturer that the ATS is acceptable for the expected motor inrush
currents (although it should be fully capable of this inrush per UL 1008).
B-14
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