Motor Control Centers

Motor Control Centers

NEMA MCCs

CENTERLINE 2100

Why Use MCC?

• Standard Motor Control Components in One Large Enclosure

  The idea behind a motor control center (MCC) is very simple. An MCC is a large enclosure designed to house standard motor control equipment. The unique characteristic of an MCC is that the motor controls are contained in plug-in units. You may already be familiar with some of Rockwell Automation's Allen-Bradley motor control components, such as:

  • Bulletin 509 starters
  • Bulletin 500 lighting contactors
  • Bulletin 1336 PLUS II and PowerFlex AC drives
  • Bulletin 150 SMC solid-state controllers
  • Pilot lights and push buttons

  If you have some knowledge of these products, then you have some understanding of motor control centers.

• Ability to Plug In

  MCCs have a unique characteristic: control components can plug in and unplug instead of having to wire each device. For example, a combination starter can be plugged into a motor control center just like you would plug in a household device such as a lamp. However, in an MCC, the user can feed much larger loads.

  Note that industrial motors operate on three phase power, unlike a typical household device; this is why MCC units have three power stabs plus a fourth stab to provide a means for grounding.

• Mounting Alternatives to Motor Control Centers

  • Mounting Motor Control — Plugging units into an MCC is not the only way to mount motor control components. Let's look at two other alternatives and see why so many customers prefer to use MCCs.

  • Wall-Mounted Motor Control — The drawing to the right illustrates how combination starters might be wall-mounted in separate enclosures. Drawbacks inherent to wall-mounted motor controls include the following:

    • Increased mounting time — Each enclosure must be mounted to the wall individually, resulting in increased mounting time. Wall-mounted enclosures are generally used when there is a small number of starters.
    • Additional wiring time and materials — Incoming power must be routed to each enclosure.
    • Additional wall space is required — Enclosures generally are oversized, plus space must be allowed to route conduit between enclosures.
    • The mass of conduit detracts from the appearance of the installation.

  • Custom Panels — Motor controls also can be mounted and wired in a single large enclosure. The drawing to the right shows a large cabinet with some starters and circuit breakers. The panel is fed by a main circuit breaker. Custom panel drawbacks include:

    • The entire panel must be de-energized in order to service an individual component — The alternative is to service a component without de-energizing and risk exposure to hazardous voltage.
    • Poor isolation — Because no barriers separate components, a fault is likely to spread throughout the panel. Delivery is typically long because of the custom design.
    • Untested for fault currents — Panels are generally only UL listed for 5,000 amperes available fault current.
    • Expansion is very difficult — Often requires an additional enclosure when more components are needed.