Motors, Transformers, and Special Equipment and Occupancies
Motors (Article 430)
Motor questions hinge on using the right number. Tables 430.248 (single-phase) and 430.250 (three-phase) give full-load current by horsepower and voltage. Use table FLC - not nameplate current - for branch-circuit conductors and short-circuit protection; nameplate current is used for overload sizing. Branch conductors are 125% of FLC (430.22). Overload protection (430.32) is set from the nameplate at roughly 115–125%. Short-circuit and ground-fault protection comes from Table 430.52: 250% for an inverse-time breaker, 175% for a dual-element fuse, 300% for a non-time-delay fuse, with next-size-up permitted up to the listed caps. A feeder supplying several motors is 125% of the largest FLC plus 100% of the others (430.24). The disconnecting means must be within sight and rated at least 115% of FLC (430.102, 430.110).
Transformers and special occupancies
Article 450 sets transformer overcurrent protection - for example, primary-only protection at 125% for transformers over 1000 V, with the 125%/167% rules for 600 V and below - and limits loading under 450.21. Separately derived systems are grounded per 250.30. Know where to find special occupancies: 500–506 classified locations, 511 commercial garages, 514 fuel dispensing, 517 health care, 547 agricultural buildings, 550 mobile homes, 680 pools and spas (equipotential bonding and GFCI), 690 solar photovoltaic, and 695 fire pumps.
Trap: do not size motor branch-circuit conductors from the nameplate amperes, and do not use the table FLC for overload sizing - those two errors account for many missed items.
Worked example: 10 hp, 208 V, three-phase motor - Table 430.250 FLC = 30.8 A. Conductors: 30.8 × 1.25 = 38.5 A. Inverse-time breaker at 250% = 77 A, so the next standard size is 80 A.