Modern MCCs Get It Together

Nov. 1, 1999
Today's motor control centers (MCCs) are smaller, more simple, economical, practical, and versatile.Motor control centers (MCCs) provide the most suitable method for grouping electric motor controllers, power distribution devices, and automation equipment in a compact, freestanding installation. Their modular design simplifies installation, wiring, and application; particularly when supplying large

Today's motor control centers (MCCs) are smaller, more simple, economical, practical, and versatile.

Motor control centers (MCCs) provide the most suitable method for grouping electric motor controllers, power distribution devices, and automation equipment in a compact, freestanding installation. Their modular design simplifies installation, wiring, and application; particularly when supplying large numbers of motors. The integrated arrangement permits significant savings in time, cost, labor, and space; even while rewiring.

MCCs house any of the common types of combination starters (full-voltage non-reversing, full-voltage reversing, multispeed, reduced-voltage starting, etc.) in sizes from NEMA 1 through NEMA 7, along with solid-state starters (variable frequency drives and soft starts).

Individual components in MCC units are no different from those found in individually mounted controllers. For example, a combination starter used in an MCC is no different functionally from a separately enclosed combination starter. They both consist of the same components and perform the same work.

Besides starters, other devices often included in an MCC are branch-feeder circuit breakers and fusible switches, distribution transformers, panelboards, drives, and programmable controllers, along with metering and power monitoring systems.

These devices or units, as constructed for use in MCCs, are modular in design. Commonly, they are approximately 14 in. wide and come in varying heights, depending on the components required. The smallest height is generally 6 in., with the size increasing by increments of 3 in. as necessary.

Different-size units can be mixed and matched in a vertical section. Units too large to fit in the standard modular design are given a vertical section to themselves. When required, these sections can accommodate devices that are wider than the standard 20 in. to provide additional mounting space.

The MCC structure is a steel enclosure with 20 in. wide215 in. or 20 in. deep sections capable of connecting together on the job site. You can also add more sections. Standard structures meet NEMA 1 requirements. Special enclosures, such as NEMA 1 Gasketed, NEMA 12 (dust- and oil-resistant) or NEMA 3R (rainproof), provide protection against the environment.

An MCC also includes a network of wiring troughs. A vertical wiring trough runs the full height of each section and connects to horizontal troughs at the top and bottom of the MCC that run the entire length of the lineup. In addition to simplifying installation of power wiring from the units to the top or bottom of the section, these troughs provide a path for the routing of control cables among various units throughout the lineup, without the use of conduit.

Another important feature is their power bus system. A horizontal bus runs the full length of an MCC lineup and distributes power from a central incoming power connection (a main circuit breaker, fusible switch, lugs, or feeder bus way) to each section in the lineup. A horizontal bus consists of tin-plated aluminum, tin-plated copper, or silver-plated copper in ratings from 600A to 2000A or more.

Each vertical section contains a vertical bus system that distributes power throughout that section. Most MCC units have a stab assembly that plugs into the vertical bus when you install the unit. Your main concern is running the load and control wiring to each unit. Vertical bus is typically rated at 300A or 600A.

NEMA classifications and types. The National Electrical Manufacturers Association (NEMA) has basic wiring class and type designations for the wiring of MCCs. These criteria ensure industry-wide uniformity among equipment suppliers. You choose NEMA Class and Type on the basis of the wiring terminations preferred.

• Class 1 has a mechanical grouping of arranged combination motor control units. These MCCs include connections from the common power bus to the units; however, they operate independently and do not include inter-wiring or interlocking among units or to remotely mounted devices.

• Class 2 consists of a grouping of combination motor control units; feeder tap units; and other units designed to form a complete system. These MCCs include the necessary electrical interlocking and inter-wiring among units and interlocking provisions to remotely mounted devices in addition to the connections from the common power bus to the units.

• Type A MCCs contain the minimum wiring and are lowest in initial cost. The user's load and control wiring connects directly to the starter.

• Type B offers the convenience of clearly identified factory-wired terminal boards for control wires. Terminal blocks are placed either in the starter unit or in the vertical wire trough adjacent to the unit. User field load wiring for Size 4 and larger starter units connects directly to the starter terminals. User field load wiring for Size 3 or smaller combination starter units connects as follows: Type B-D: User field load wiring connects to the starter terminals. Type B-T: User field load wiring connects to factory-wired power terminal blocks in or adjacent to units.

• Type C also offers clearly identified factory-wired terminal blocks. However, Type C groups terminal blocks in master terminal compartments. These compartments (one for each vertical section) may be adjacent to conductor entry areas at either the top or bottom of each section.

Short-circuit ratings. NEMA has also set standards for MCC short-circuit ratings. The structure and starter units usually have separate ratings. They have a withstand rating indicating the maximum amount of current the bus structure can carry without damaging the bus components or insulators. Component units have a short-circuit interrupt rating specifying the maximum level of current the starter units or branch-feeder units can interrupt safely. You combine these ratings to obtain a "series combination rating" that specifies the maximum short-circuit current rating for the MCC system.

Unless otherwise specified by the manufacturer, the maximum short-circuit rating of the entire MCC will be the withstand rating of the bus structure or the lowest interrupt rating of any of the MCC units; whichever is smaller. However, in some instances extensive testing by the manufacturer may permit the MCC to carry a higher available fault rating than the ratings of individual MCC components.

Advantages of using MCCs. Using MCCs offers a number of benefits. Instead of handling, mounting and wiring a multitude of individual units, you can install a fully assembled MCC more easily. Each three-section block can control as many as 36 6-in. or 18 12-in. Size 1 combination starters. Because the line-side connection of all the units are tied to one bus, eliminating miscellaneous conduit and wire-way saves more. As a result, the installation costs per starter are typically one half that of costs for individually mounted starters.

MCCs can also be great space savers. As many as 12 standard Size 1, full-voltage, non-reversing starters can fit in one MCC section 20-in. wide. Compare this to several feet of space required for individually mounted combination starters. Because an MCC is freestanding, you can place it closer to the equipment it is controlling.

You can easily enlarge MCCs to accommodate additional components. This avoids mounting and wiring additional enclosures. Adding a new starter is usually just a matter of installing the unit into an existing vacant space in the MCC lineup. When an MCC is completely filled, you can bolt an additional section to either end of the lineup.

The latest technological developments in this industry are incorporated in today's MCCs. These include solid-state reduced-voltage starters, solid-state variable-speed drives, solid-state overload relays, microprocessor-based energy-management systems, and programmable controllers. The MCC is the ideal place to install a programmable controller, because it places the unit near starters and other devices you want to control.

About the Author

Paul L. Johnson and James Grubb

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