What a motor control centre is
A motor control centre (MCC) is a lineup of steel sections that gathers the starters and controls for many motors in one place. Each section shares a common horizontal bus at the top and a vertical bus behind the units. Individual plug-in units, usually called buckets, slide into the sections and connect to the vertical bus through stabs.
They are built to standard frame sizes, so units from the same manufacturer and series are designed to be interchangeable. You will find MCCs in plants, water and wastewater facilities, grain and food processing, and large mechanical rooms where pumps, fans and compressors are grouped. Putting the controls in one lineup makes operation, troubleshooting and lockout simpler than having starters scattered around the building.
What is inside a bucket
A typical full-voltage starter bucket contains:
- A disconnect, usually a circuit breaker or fused switch, operated by the handle on the door
- A contactor that opens and closes the motor circuit
- An overload relay that trips the contactor if the motor draws too much current for too long
- A small control transformer and terminals for start, stop and interlock wiring
- Pilot lights, selector switches or a communication module on the door
Each bucket is a self-contained package. That is the main advantage of an MCC: a failed starter can be swapped for a spare bucket in minutes instead of rewiring a starter on the wall, which shortens downtime on critical pumps and fans.
The door handle is interlocked so the door cannot normally be opened with the disconnect on, and the handle can be padlocked off for lockout.
Types of starters
| Starter type | Where it fits |
|---|---|
| Full voltage (across the line) | Simple, robust; fine where a hard start is acceptable |
| Reversing | Conveyors, hoists, mixers that run both ways |
| Two-speed | Some fans and pumps with two fixed speeds |
| Soft starter | Reduces inrush and mechanical shock at start-up |
| Variable frequency drive | Speed control and energy savings on variable loads |
Modern MCCs often use electronic overload relays that report motor current, trip causes and run hours over a network. That data helps maintenance teams spot a pump that is drawing more current than it used to before it fails. See our guide to variable frequency drives for when a drive is worth it.
Maintaining an MCC
MCCs are reliable, but they live in dusty, warm and sometimes damp rooms, and they are worked on often. A sound maintenance routine includes:
- Annual infrared thermography under load
- During shutdowns: cleaning, checking bucket stabs and bus for heat damage, and torquing connections
- Checking overload settings against the motor nameplate
- Testing door interlocks and lockout points
- Keeping spare buckets and critical parts on the shelf
- Keeping the bucket labels and drawings current
Record what you find each time. Overload trips, hot stabs and worn handles tend to repeat on the same units, and the pattern points to the real cause.
Our guide to infrared thermography covers how scans are done and reported.
Withdrawing or inserting a bucket on an energized MCC is one of the higher-risk tasks in a plant because of arc flash. It should only be done by qualified electricians following CSA Z462, ideally with the section de-energized or using remote racking where available.
Retrofit or replace
Many MCCs in Alberta plants have been in service for decades. Age alone is not the issue. The questions are whether replacement parts are still available, whether the bus and enclosure are sound, whether there is room for growth, and what the arc flash study shows.
Look at what the MCC will need to do in the next ten or twenty years, not only today. If you expect more motors, more drives or a move to networked controls, a new lineup with spare space and smart overload relays may be the better long-term choice even if the old one could be kept running.
Retrofit buckets built to fit older lineups can extend life at lower disruption. Full replacement is planned around a shutdown, with temporary power if some loads must keep running. Either way, update the drawings and the arc flash labels afterward. See motor controls and plant retrofits.
Common questions
What is the difference between an MCC and a panelboard?
A panelboard holds breakers that protect circuits. An MCC holds complete motor starters, including contactors, overload relays and controls, in plug-in units designed to start, stop and protect motors.
Can we add a motor to an existing MCC?
Often, if there is an empty space or spare bucket and the bus has capacity. An electrician checks the rating, available space and the load calculation, then confirms the new bucket is compatible with the lineup.
How often should an MCC be maintained?
It depends on the environment and how critical the loads are. Many facilities scan annually and do internal cleaning and torque checks during planned shutdowns. CSA Z463 and manufacturer guidance are good starting points.
Why do overloads keep tripping on one motor?
Common causes are a motor or driven load that is wearing out, an incorrect overload setting, a loose connection or a voltage problem. It should be investigated rather than simply reset.
Codes and permit rules change and every building is different. For work on your system, talk to a licensed electrician and your local permit authority.