Grounding, Bonding, and Electrical Safety
Three words, three jobs
Grounded means connected to earth. Grounding is the earth connection itself. Bonding is joining metal parts so they stay at the same potential. Most missed questions come from confusing the grounding electrode conductor (GEC) with the equipment grounding conductor (EGC). The GEC carries fault current only in unusual events and is sized from the service conductors. The EGC is the low-impedance fault path and is sized from the overcurrent device.
System grounding
A service supplied by a grounded system must have the grounded conductor connected to a grounding electrode system, with the main bonding jumper at the service (250.24, 250.28). Separately derived systems bond at the source. The electrode system must use all available electrodes (250.50); a metal underground water pipe qualifying under 250.52(A)(1) must be supplemented by another electrode. Ground rods need 8 ft of contact and, when a single rod exceeds 25 ohms to ground, a second rod is required.
Sizing tables
Table 250.66 sizes the GEC from the largest service conductor: 2 AWG or smaller copper service conductors take 8 AWG copper; over 3/0 through 350 kcmil takes 2 AWG copper. Table 250.122 sizes the EGC from the rating of the overcurrent device: 15 A to 14 AWG, 20 A to 12 AWG, 60 A to 10 AWG, 100 A to 8 AWG, 200 A to 6 AWG.
Worked example
A 400 A service with 500 kcmil copper service-entrance conductors: Table 250.66 (over 350 through 600 kcmil) requires a 1/0 copper GEC. The EGC for the 400 A feeder, by Table 250.122, is 3 AWG copper. Different tables, different inputs - that contrast is the most common exam trap.