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Motor Control Centers, Part 1

Motor Control Centers, Part 1The ampere rating (or setting) of the OCPD for a motor control center can’t exceed the rating of the common bus [430.94]. This rule sometimes creates problems in existing facilities when a production problem is solved with a larger motor, but it’s not possible to supply the motor from the existing MCC due to the increased OCPD that’s needed.

To avoid this problem when designing the original MCC system, do some price-checking and see if it’s feasible to install a larger bus in the original MCC. If not, then another option is to reserve floor space for adding another cabinet or MCC (with its own main bus) in case the extra capacity is needed.

Sometimes, an MCC is used as service equipment. But the 6 disconnect rule [230.71] does not apply to an MCC. Each MCC can have only one disconnect [430.95]. But when using an MCC as service equipment, you can use a second disconnect to supply additional equipment.

Note also that the MCC disconnects each count toward that total of six. The reason for the six disconnect limit is so first responders can disconnect power within a reasonable time. After six, it begins to get unreasonable.

If the MCC has multiple sections, bond these together so there’s no difference of potential between them. The NEC requires doing this with an equipment grounding conductor or an equivalent grounding bus [430.96] sized per 250.28(D).

Part 2 » | Source: Mark Lamendola | Mindconnection