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How to Specify the Right Motor Control Center For the Job

June 1, 2005
Motor control centers (MCCs) were first introduced in 1937 as a way to conserve wall and/or floor space in industrial facilities. Before 1960, wall-mounted motor starters were used even if only a few motors were involved. Relays were housed in separate control cabinets. Today, even if a few motor starters are needed, they're typically installed within a standardized vertical enclosure with all the

Motor control centers (MCCs) were first introduced in 1937 as a way to conserve wall and/or floor space in industrial facilities. Before 1960, wall-mounted motor starters were used even if only a few motors were involved. Relays were housed in separate control cabinets. Today, even if a few motor starters are needed, they're typically installed within a standardized vertical enclosure with all the required relays, instruments, and controls. Overall, the same requirement today often dictates the use of MCCs in new installations.

However, unlike the '50s, when the vast majority of loads served by MCCs were electric motors with across-the-line starters, today's MCCs can accommodate a wide variety of different devices required in modern facilities. But specification of this popular piece of equipment shouldn't be taken for granted. A few key line items in your spec, including ampacity, bussing material, and feeder cables, could mean the difference between a long, reliable service life and an early, abrupt failure.

Ampacity. One of the first items to be determined is the ampacity of the MCC. Basically, the ampacity is the maximum amount of current that the main horizontal bus can accommodate without overheating. The minimum ampacity of an MCC main horizontal bus is 600A. Smaller sizes are generally not available and are considered “special order” items. Often, these special order MCCs with smaller bus ampacity simply contain 600A bussing with a revised nameplate.

Following the horizontal bus ampacity is the specification of the vertical bus ampacity. The minimum ampacity of a vertical bus is 300A. For most manufacturers, the maximum ampacity of an MCC horizontal bus is 2,000A and the maximum ampacity of a vertical bus is 1,600A, although some manufacturers claim an ampacity up to 5,000A for a horizontal bus. Generally, if a group of loads requires more than 2,000A, then it would be best to use two separately fed MCC line-ups as opposed to one.

Another issue to consider when specifying an MCC is the available fault current levels on the power distribution system. The available fault current is simply the amount of electric current that would flow if a direct short were to occur between phases or from phase to ground (enclosure) within the MCC.

A couple of definitions are in order at this point. The interrupting rating of an electrical device applies primarily to circuit breakers and fuses and other devices designed to interrupt the flow of current during a ground fault and/or a short circuit. This rating signifies that the current interrupting device will open and break the flow of current to the piece of equipment it's protecting without rupturing and damaging nearby equipment or personnel. The short-circuit rating of electrical equipment is the maximum fault current that the device can handle without extensive damage to the electrical components of the circuit.

Per 110.10 of the 2005 NEC, “Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.” Although an MCC may have a nameplate that indicates the short-circuit withstand rating as 42,000A, for example, the overall short-circuit rating of an MCC is based on the installed circuit interrupting device with the lowest current-interrupting rating.

The amount of current available varies from installation to installation, depending on the impedance of the electric source. For example, if the available fault current is determined to be 12,000A, the MCC must be rated to withstand 12,000A of fault current during the time it takes the overcurrent protective device (OCPD) immediately upstream of the fault to open and clear the fault. Fault-current ratings of electrical devices are available in semi-standard ratings. Some typical ratings are 10,000A, 14,000A, 18,000A, and 22,000A.

The fault current withstand rating of an MCC is determined by the lowest rated device within the MCC. If an MCC bus structure is rated to withstand 42,000A and a circuit breaker is installed with a fault-withstand rating of 18,000A, then the entire MCC assembly is rated 18,000A. Bear in mind that most NEMA class motor contactors are rated for a fault-current withstand of 5,000A; some are rated for 10,000A. In order to account for the lower fault-current withstand rating of motor contactors (and avoid costly fault damage), a current-limiting motor protector circuit breaker or current-limiting fuses should be considered.

Bear in mind that the higher the interrupting rating of a device, the higher the cost (Figure on page 18). Therefore, significant cost savings can be obtained by doing a short-circuit analysis of an electric power circuit.

Bussing material. The material used for the electrical bussing within the MCC is another item you should carefully consider during specification.

The most common type of bus material is copper. Aluminum can be used to reduce costs, but aluminum has its own set of problems. For example, its expansion and contraction is much greater than copper, which can result in the gradual loosening of bolts and other fasteners. To account for this effect, special washers with spring-type tensioning are used to connect the aluminum bus to the insulated bus supports. These fasteners must be properly torqued per the manufacturer's recommendations, or the spring effect will be lost and subsequent loosening of the fasteners will occur over time. Also, due to its lower conductivity than copper, aluminum bus must be larger to handle the same level of current.

Copper bus, on the other hand, can be bolted tightly to the bus supports, and fasteners seldom loosen over time. However, copper does have its own shortcomings. Some environments may present a corrosive atmosphere to the copper bussing, which will lead to eventual MCC failure. For example, ammonia gas, which is often used in refrigerant or large chiller loops, will attack copper. It will also cause stress corrosion cracking of copper alloys.

It's possible to protect the copper bussing from corrosion by coating the bus with tin. Although a tin coating will corrode, the corrosion products of tin are relatively thin and friable and will break down mechanically, allowing for a continuously reliable metal-to-metal connection. Tin is also anodic with respect to copper. In other words, if a tin-coated copper bus is scratched and the underlying copper is exposed, the tin will corrode while the copper bus won't. Other examples of corrosive industrial environments that you should pay attention to are pulp plants, wastewater treatment facilities, fossil fuel-based power generating plants, and marine environments.

Coating copper bus with silver is effective in some environments, but it can form relatively thick, insulating layers of silver sulfide (tarnish). It can also be aggressively attacked by chloride sources. Copper is also anodic to silver, meaning that given a copper/silver metallic junction, the copper will corrode while the silver wont.

Feeder cables. One of the commonly overlooked issues involved in specifying and purchasing an MCC is how the main feeder cables will enter the MCC. You typically have two choices: overhead or underground. Due to the fact that the electric power cables that feed an MCC are typically numerous and large, knowing beforehand how an MCC will be fed will eliminate difficult wire bends and/or costly field changes required to properly terminate an awkward cable feed. One can specify an MCC that will accommodate both types of feeds, but again, costs will increase and some MCC space will be wasted.

If automatic controls are going to be implemented for operation of the motor loads, then the type of control wiring should be indicated. The various types of control wiring available are as follows:

Type A — No terminal blocks within the MCC. Only wiring on the motor starter itself is required. No wiring external to the control device.

Type B — Control wiring to terminal blocks within motor starter buckets while user field load wiring terminates to the device adjacent to the vertical wire-way.

Type B-D — For Size 3 and smaller starters, field load wiring connects directly to the device terminals.

Type B-T — For Size 3 and smaller starters, field load wiring connects to factory-wired power terminal blocks located in or adjacent to each unit.

Type C — Same as B except all factory-wired to master terminal boards at the top or bottom of the MCC.

Class I — Doesn't include interwiring or interlocking between units or to remotely mounted devices. Doesn't include control system engineering diagrams if individual units are supplied. Types A, B, or C wiring available.

Class II — Factory-wired controls and interlocks. Full wiring diagrams are provided, factory system engineering. Types B or C. Class II type C wiring is expensive and requires a longer lead time.

By no means are the preceding items meant to be an all-encompassing list of specifications necessary to successfully purchase or spec an MCC for a specific application. However, it's a good starting point. Other issues that must be considered when specifying an MCC include:

  • Enclosure type: indoor (NEMA 1) or outdoor (NEMA 3R)
  • Ground-fault detection
  • Metering
  • Wire marking
  • Indication lights
  • Hands-off auto switches
  • Voltage rating

Additional help is available through a review of pre-written specifications. However, the selected document will need to be carefully reviewed and possibly altered in order to produce the specification that will meet you and your client's specific needs. If the task seems time consuming or difficult, it's recommended that the services of a professional engineer be sought for expertise in this area. The MCC will be in service for the next 25 to 30 years, so such an investment will be well worth the effort.

Daley is an electrical engineer with Devon Engineering in Colorado Springs, Colo.

About the Author

Patrick Daley | P.E.

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