To determine the motor contribution
during the first half-cycle fault current,
when individual motor horsepower
load is known, the subtransient reactances
found in the IEEE Red should
be used in the calculations. When the
system motor load is unknown, the
following assumptions generally
are made:
Induction Motors ? Use 4.0 times
motor full load current (impedance
value of 25%).
Note: For motors fed through adjustable
frequency drives or solid-state soft starters,
there is no contribution to fault current, unless
1) they have an internal run contactor or
2) they have a bypass contactor.
Synchronous Motors ? Use 5.0 times
motor full load current (impedance
value of 20%).
When the motor load is not known,
the following assumptions generally
are made:
208Y/120 V Systems
■ Assume 50% lighting and 50%
motor load.
or
■ Assume motor feedback contribution
of twice full load current of
transformer.
or
240/480/600 V 3-Phase, 3-Wire or
4-Wire Systems
■ Assume 100% motor load.
or
■ Assume motors 25% synchronous
and 75% induction.
or
■ Assume motor feedback contribution
of four times full load current
of transformer.
480Y/277 V Systems in Commercial Buildings
■ Assume 50% induction motor load.
or
■ Assume motor feedback contribution
of two times full load current
of transformer or source.
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