Motor control centers (MCCs) offer a means of grounding electric motor controllers, freestanding installs, automation equipment and power distribution components. Manufacturers typically make MCCs from heavy-duty steel. The National Electrical Manufacturers Association (NEMA) oversees the two classifications of the control centers and the types of wiring in the United States.
Class I Motor Control Centers
Class I motor control centers are a group of feeder tap, combination motor control or other units in an orderly arrangement. They feature connections from the units to the power bus. These centers do not have interwiring between units or devices, and do not control system engineering.
Class II Motor Control Centers
Although Class II motor control centers are the same as Class I motor control centers, Class II centers create a complete control system that features the interwiring and electrical interlocking that fits in the middle of the units and for remote-mounted devices.
Type A Wiring
Type A wiring for Class I motor control centers include fusible disconnect units and feeder circuit breakers. The factory wires the combination line starters inside the motor control center. Additionally, the factory does not supply wires outside the unit. The motor control center maker joins the unit to vertical bus with stabs situated on the backside of the unit. The vertical bus supplies power to the circuit breaker. The circuit breaker is pre-wired to the motor starter.
Type B Wiring
Type B wiring for Class I motor control centers is the same as Type A wiring, except the control wires stop near the lower section or on the side of the motor control center unit. Indicator lights, selector switches, pushbutton or other pilot devices use Type B wiring. The manufacturer wires the various pilot devices and overload relay to the terminal block. The customer needs to connect the external control wires and motor to the block. Additionally, Class II Type B wiring is similar to Class I Type B wiring, except it features interwiring between the units.
Type C Wiring
Type C wiring for a Class I motor control center can sit in a Type B motor control center unit. A master terminal block sits in the upper or lower section of the wiring gutter. An installer connects the wires supplied by the motor control center manufacturer to the master terminal block. The load wiring for size three and up connect to the device terminal. Load wires smaller than size three connect to the master terminal block.
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