What is a motor control center? They are the industry-standard way of distributing power to motors and equipment in industrial and large commercial buildings. Motor control centers, or MCCs, come in different shapes and sizes and are manufactured by various different brands.
I’m most familiar with Allen-Bradley centerline MCCs, although the principles in this article will apply to any brand of MCC.
MCCs are mainly used to power motors. This can be done with VFDs, soft starts, across the line starters, and more. Disconnect handles are connected to MCC cubicles to allow lockout for safe work on the associated equipment.
Motor Control Center Layout
Every facility will have a slightly different setup for MCCs. And because of their long lifespan, many facilities have MCCs that are older than me!
The Main Lugs
Power comes to the MCC from a power distribution center. The feeder cable from the PDC will enter the MCC and land at the main lugs. The main lugs are typically connected to the live section bus bar and to the main disconnect switch.
The door that covers the main lugs will often have ground fault lights to indicate if any of the phases are grounded.
Bus bars can either be flat or rounded and run through the back of an MCC. Cubicles in the MCC receive power from connections made to the bus bar.
The live section of an MCC stays on even when the main disconnect switch is off. So the only way to lockout this section is from the PDC.
The live section powers equipment that needs to stay running even when a motor control center is being locked out for maintenance. Some examples are:
- Welding machines
- HVAC systems
- Control transformers
- Motor generator sets
Not all MCCs include a live section so it’s important to keep this possibility in mind when performing work on an MCC.
Main Disconnect Switch
The main disconnect switch is connected to the bus bars that feed all cubicles, not in the live section. Locking out the main disconnect switch is common practice when working on large machines centers.
Never throw the main disconnect switch, or any disconnect switch while motors are running because this can create a dangerous arc flash.
MCC cubicles connect to bus bars through stabs in the back of the “bucket”. Cubicles contain the equipment required to start and stop motors. This could be a drive like a VFD, or a classic motor starter.
A normal motor control center cubicle with have a lower control voltage, and the higher voltage used to run the motor. For example 120v control and 480v for the motor.
It can also include a motor starter, fuses, overloads, a door switch, auxiliary contacts, and much more.