Grounding, Bonding, Overcurrent Protection, and Communications Systems
Three distinct ideas
System grounding connects the grounded conductor to earth at the service (250.24) or at a separately derived system (250.30) and requires a main bonding jumper. Equipment grounding connects non-current-carrying metal parts back to the source so a fault trips the overcurrent device. Bonding joins metal parts so they remain at the same potential.
Sizing the conductors
Grounding electrode conductors come from Table 250.66 and are sized on the largest service conductor. Equipment grounding conductors come from Table 250.122 and are sized on the rating of the overcurrent device protecting the circuit, not on the phase conductor size. Electrodes are listed in 250.52; a rod must be 8 feet long and driven so 8 feet contact the soil. Where a single rod exceeds 25 ohms to earth, a second electrode is required (250.53). 250.32 covers separate buildings.
Overcurrent protection
240.4(D) caps small conductors: 14 AWG at 15 A, 12 AWG at 20 A and 10 AWG at 30 A. 240.4(B) permits the next standard size only when the ampacity is not a standard rating and the next size does not exceed 800 A. Standard ratings are in 240.6(A). The tap rules in 240.21 are frequently tested.
Traps
Sizing an equipment grounding conductor from Table 250.66; using the neutral as an equipment ground at a subpanel; forgetting 250.66 keys off the largest ungrounded service conductor while 250.122 keys off the device rating.
Worked example
A 400 A service uses 500 kcmil copper service conductors. Table 250.66: for over 350 through 600 kcmil copper, the grounding electrode conductor is 1/0 copper. A feeder protected at 200 A takes a 6 AWG copper equipment grounding conductor from Table 250.122.