Grounding and Bonding
The system
Article 250 separates two ideas: grounding connects the electrical system to earth, and bonding connects metal parts together to create a low-impedance fault path. The service neutral is the grounded conductor; at the service, it is bonded to the enclosure with the main bonding jumper (250.28) and connected to the grounding electrode conductor. Downstream of the service disconnecting means, the grounded conductor may never be re-grounded or bonded to equipment enclosures (250.24(A)(5)). Subpanels in separate buildings require their own grounding electrode system and a disconnecting means (250.32).
Sizing
Table 250.66 sizes the grounding electrode conductor from the largest service-entrance conductor; Table 250.122 sizes equipment grounding conductors from the rating of the overcurrent device ahead of the circuit. A 200 A circuit needs a 6 AWG copper equipment grounding conductor. Where conductors are paralleled, each raceway must carry a full-size equipment grounding conductor (250.122(F)). The grounding electrode system must include all available electrodes (250.50); a single driven rod needs a supplemental electrode unless it measures 25 ohms or less (250.53(D)(2)). Metal water piping, structural steel, concrete-encased electrodes, and ground rings are all qualifying electrodes (250.52).
Traps
Swapping the two tables, using the neutral as an equipment ground on a subpanel, omitting bonding around water meters and gas piping, and assuming an isolated ground receptacle removes the safety ground.
Worked example
A service with 3/0 copper service-entrance conductors: Table 250.66 gives a 4 AWG copper grounding electrode conductor. Containers, boxes, and fittings on the circuit still need bonding per 250.104 and 250.148.