Motor Control Centers

Lessons 07-01 to 07-03 covered the six-step motor chain and the transformer/generator OCP basics. This lesson covers the parts of Article 430 that the Unlimited exam reaches for beyond the chain — motor control centers (Part VIII), adjustable-speed drives (Part X), live parts and grounding (Parts XII–XIII) — and the generator installation rules of Article 445 (marking, conductors, disconnects, emergency shutdown, GFCI on portables). Written from the 2023 text.

1. Motor control centers — 430 Part VIII

• Overcurrent (430.94): the MCC is protected per Article 240 Parts I, II and VIII; the OCPD rating ≤ the common power bus rating; provided by an OCPD ahead of the MCC or a main OCPD within it.
• As service equipment (430.95): a single main disconnecting means (a second service disconnect is permitted for additional equipment); a main bonding jumper per 250.28(D) within one section connects the grounded conductor to the equipment ground bus.
• Grounding (430.96): multisection MCCs are connected by an EGC or equivalent bus sized per Table 250.122; 430.97(B) bus arrangement A, B, C — front to back, top to bottom, left to right viewed from the front; B phase is the high leg on a 3-phase 4-wire delta; wire-bending space per 312.6; Table 430.97(D) spacings — ≤ 125 V: ¾ in. between opposite polarity on the same surface / ½ in. held free in air / ½ in. to ground; ≤ 250 V: 1¼ / ¾ / ½ in.; ≤ 600 V: 2 / 1 / 1 in.
• Marking (430.98): per 110.21, including the common power bus rating and the SCCR; feeder-supplied MCCs are field-marked with the identification and location of the means to disconnect all power (110.21(B)).
• SCCR (430.99): not installed where the available fault current exceeds the SCCR; the available fault current and the date of the calculation documented.