Motor Control Centers

DOE-HDBK-6004-99

facility auxiliary transformers. The 480V secondary winding of the transformer should be

wye-connected with the neutral solidly grounded, except where the loads are the pressurizer heaters,

in which case the secondary should be delta connected. The transformer primary should be fed from

the 4160 V switchgear by cable. The transformers and the switchgear buses should have sufficient

capacity and should be connected so that overloading under any condition of operation is prevented.

A separate ground fault relay should be provided for the transformer neutral and each load center

feeder.

Motor Control Centers

AC Motor Control Centers:

The 480 V MCCs should be metal enclosed, NEMA Class II, Type B. All 480 VAC motor control

center motor starter circuits should be full-voltage non-reversing and full-voltage reversing,

combination type with breakers, contactors, and individual control transformers. Transformers

supplying the control power should be of uniform rating for similar units and will be sized to provide

for the addition of relays and other loads. The minimum size should be 150 V-amperes. Starter sizes

should be based on motor hp, voltage, and type of service. Table II-3 below covers low-voltage (600

V and below) NEMA Class A (for induction motors) general purpose starters.

Table II-3. Load voltages for general-purpose starters

Size of Starter (480 volts, 3-phase)

Type of Load

Heater

0 - 10 hp motor

15 - 25 hp motor

30 - 50 hp motor

All MCCs should be designed to contain approximately 20% spare circuit breakers, motor starters,

and/or spaces. Essential circuits should be designed to provide for the maximum possible continuity

of unit operation so that after a brief loss of power or voltage dip, the starter will return to its original

position after the power is restored.

DC Motor Control Centers:

The DC MCCs will be similar to the 480 VAC MCCs except that starters will be reduced-voltage

type to limit the motor starting inrush current to 200% of rated current. Some loads will be

controlled locally at the equipment in addition to automatic remote control from the control room.