Grounding, Bonding, and Overcurrent Protection
Grounding and bonding
Article 250 separates three conductors: the grounded (neutral) conductor, the grounding electrode conductor (GEC), and the equipment grounding conductor (EGC). At the service, the neutral is bonded to the electrode system (250.24); downstream, neutral and EGC must be separated (250.24(A)(5), 250.142). 250.28 covers the main bonding jumper; 250.30 covers separately derived systems, bonded at the source; 250.32 covers separate buildings, which need their own electrode and an EGC run with the feeder. 250.50-250.53 build the electrode system (8 ft rod, 25 ohms or a supplemental electrode). Size the GEC from Table 250.66 using the service conductor size, and the EGC from Table 250.122 using the overcurrent device rating. 250.118 lists permitted EGCs; 250.148 requires bonding at boxes.
Article 240: 240.4 protects conductors, with 240.4(D) capping 18-10 AWG at 15, 20, and 30 A; 240.21 contains the tap rules (10 ft, 25 ft, and 100 ft industrial taps, plus transformer secondary taps); 110.9 and 110.10 require adequate interrupting and short-circuit ratings; 240.87 requires arc energy reduction above certain thresholds.
Worked example: a 400 A service with 500 kcmil copper service-entrance conductors takes a 1/0 AWG copper GEC (Table 250.66). A 400 A feeder takes a 3 AWG copper EGC (Table 250.122). Sizing the GEC from the breaker instead of the conductor is the most common slip.
Traps: oversizing the EGC beyond the circuit conductors (not required by 250.122(A)); using the neutral as an EGC past the service; believing a ground rod alone clears a fault.