Introduced in 1937, the motor control center maximized the use of space inside industrial facilities. Even though similar specifications dictate the design of today's motor control centers, well-designed systems accommodate a vast variety of devices required in modern facilities. Therefore, if you want a long and reliable service life with maximum return on investment, make sure you address the following five design specifications.
One of the first specifications to consider is the ampacity of the main horizontal bus of your motor control center. The ampacity represents the maximum current the main horizontal bus can accommodate before overheating occurs. The minimum horizontal bus ampacity is 600 A.
Secondly, determine the ampacity for the vertical bus. The minimum vertical bus ampacity is 300 A. Though, many manufacturers use a maximum vertical bus ampacity of 1,600 A, and a maximum ampacity for horizontal bus of 2,000 A.
Fault Current Levels
A fault current level is simply the amount of electric current that flows if a short were to occur between phases. The fault current level rating is determined by the lowest-rated device in the motor control center. For example, if the bus structure was designed to withstand 30,000 A and a breaker with a fault-withstanding rating of 12,000 A is installed, then the entire system rating is 12,000 A.
Copper has many advantages over aluminum as a bussing material. Firstly, copper has a higher conductivity rate than aluminum, which allows for a smaller bus to handle the same current. Additionally, copper doesn't expand and contract to the degree of aluminum.
Though, copper is not a one-size-fits-all bussing material. Aluminum costs less and is less likely to corrode in some industrial environments. For example, ammonia gas used in refrigerant systems or chiller loops attacks copper and cause stress cracking of copper alloys.
Careful not to overlook how the main feeder cables enter the control center. If you determine beforehand how feeder cables enter your system (e.g. underground or overhead), you may avoid difficult wire bends and potentially costly field changes. You can design your system to accommodate both overhead and underground feeds. Though, your costs will increase.
Make sure cables and components are up to fire code. Motor control centers generally sit on floors. If your floors (and walls) are required to have a fire-resistance rating, fire-stops may be necessary for floor- and wall-penetrating feeder cables.