What Is a Motor Control Center (MCC)?

A motor control center (MCC) is a floor-standing assembly of standardized, plug-in units — called buckets — that house the motor starters, contactors, overload relays and protection devices that run electric motors throughout an industrial facility. MCCs centralize motor control into one accessible location, simplifying wiring, maintenance and troubleshooting.

How an MCC Is Built

A typical MCC consists of a steel enclosure divided into vertical sections. Each section holds multiple removable buckets stacked on a common power bus. The bus bar runs vertically through the section and delivers three-phase power to every bucket. Each bucket slides in on guides, makes contact with the bus through "stabs" (spring-loaded clips), and can be withdrawn for maintenance without de-energizing the rest of the lineup.

What's Inside an MCC Bucket?

A standard combination motor starter bucket includes:

• A disconnect — often a molded-case circuit breaker (MCCB) or fusible disconnect that isolates the motor circuit.
• A contactor or motor starter — the electromechanical device that actually switches the motor on and off, controlled by the control circuit.
• An overload relay — protects the motor against sustained overcurrent (Class 10 or Class 20 trip depending on the motor type).
• A control transformer — steps bus voltage down to 120 V for the control circuit (pushbuttons, PLC I/O, pilot lights).

Bucket sizes (NEMA Size 1, 2, 3, etc.) correspond to the maximum horsepower the starter can handle. Larger motors require larger contactors and correspondingly larger buckets.

Major MCC Manufacturers

The most common MCC platforms in North America are:

• Square D — Model 6 is the dominant platform; buckets are identified by a class/type code (e.g. 8536, 8538).
• Cutler-Hammer / Eaton — Freedom and Advantage series; uses the "F" designation for front-accessible buckets.
• Siemens / ITE — Tiastar platform, with NEMA and IEC-rated options.
• General Electric — 8000 Line and Evolution series.
• Allen-Bradley / Rockwell — CENTERLINE 2100 and 2500 platforms.

Sourcing Replacement MCC Buckets

When a bucket fails or you need to add a motor to an existing MCC, the replacement must match the bus configuration, form factor and stab arrangement of the original lineup. Key compatibility points:

• Bus amperage and voltage — the new bucket must be rated for the main bus (typically 480 V / 600 A or 800 A).
• Bucket height — measured in "units" (one unit ≈ 6"). A Size 1 starter is usually a 1-unit bucket; larger starters need 2 or 3 units.
• Stab pattern — the bus stab clips must align with the vertical bus bars. This is manufacturer-specific.
• Form factor — NEMA 1 (indoor), NEMA 12 (dust-tight) or NEMA 3R (outdoor rain-tight).

Because many MCC platforms are decades old, replacement buckets are often available only on the surplus market. We carry one of the largest inventories of surplus MCC buckets and components in the country.