Grounding, Bonding, Motors, and Controls
Grounding and bonding
Grounding connects the system to earth; bonding connects metal parts together so they stay at the same potential. Article 250 covers the grounding electrode system (250.52, 250.53), the grounding electrode conductor sized from 250.66, and the equipment grounding conductor sized from 250.122, which is based on the overcurrent device, not the conductor size. The main bonding jumper ties the grounded conductor to the equipment grounding system at the service or at a separately derived system, and only there. A single rod, pipe, or plate with resistance over 25 ohms needs a supplemental electrode.
Motors, Article 430
Motor full-load current comes from the tables (430.247 through 430.250), not from the nameplate. Branch-circuit conductors are 125 percent of full-load current; the overload device protects the motor from overload, while the branch-circuit short-circuit and ground-fault device protects the conductors. The disconnecting means must be within sight and rated at least 115 percent of full-load current. Motor control circuits fall under Part VI, with two-wire control giving automatic restart after power loss and three-wire control requiring a manual restart.
Traps
Sizing the equipment grounding conductor from 250.66 instead of 250.122; assuming the motor nameplate current equals the table value; confusing overload protection with short-circuit protection; bonding the neutral again at a subpanel.
Worked example
A 10 hp, 480 V three-phase motor: Table 430.250 gives 14 A. Branch conductors are 14 x 1.25 = 17.5 A, so 12 AWG works. An inverse-time breaker may be up to 250 percent, or 35 A.
Rule of thumb
Motor conductors 125 percent of table current; overload sized to the nameplate; breaker up to 250 percent.