UFC 3 -520-01
June 10, 2002
13-5.5
Sizing.
13-5.5.1 Most ATS designs are capable of carrying 100 percent of rated current at an
ambient temperature of 40 C (104 F). Some designs, including those incorporating
integral overcurrent protective devices, might be limited to a continuous load of 80
percent of the ATS rating. Confirm the continuous load rating as part of the sizing and
selection process.
13-5.5.2 When sizing an ATS, specify a switch capable of carrying the total current
load, including anticipated future load additions. The required ATS rating is calculated
by adding the amperes required by all loads. Apply the following guidelines when
determining the total load.
13-5.5.2.1 Only a motor's continuous load current has to be considered. Motor inrush
current need not be included, provided that the ATS or static transfer switch is rated for
inrush capability.
13-5.5.2.2 Resistive loads and incandescent lamp loads are determined from the total
wattage.
13-5.5.2.3 Fluorescent, mercury vapor, and sodium vapor lamp currents must be based
on the current that each ballast or autotransformer draws, not the total watts of the
lamps.
13-5.5.3 In ambient temperatures above 40 C (104 F), the ATS might require
derating. Consult with the manufacturer for sizing requirements for operation above this
temperature.
13-5.5.4 Appendix B provides an ATS sizing example.
13-5.6
Effect of Motor Loads.
13-5.6.1 Motor loads require two co nsiderations with regard to transferring their power
source by an ATS during open transition transfer:
13-5.6.1.1 How to avoid nuisance breaker tripping and possible motor damage when
the motor is switched between two unsynchronized energized sources. Mo tors and
related equipment can be damaged when switched between two energized power
sources. During a retransfer back to normal power or during a system test, both power
sources are at full voltage, which can result in abnormal inrush currents in large mo tors
causing damage to motor windings, insulation, or couplings.
13-5.6.1.2 How to shed motor loads prior to transfer to prevent overloading of the
alternate power source. Engine generators are often used as the alternate power
source to an ATS. When substantial motor load is present, the resulting inrush current
can overload the generator. Engine generators that are intended as an emergency
power source are often sized for the expected full-load current plus a limited margin for
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