Grounding and Bonding
The two systems
Grounding and bonding are different jobs. The grounded conductor (neutral) carries return current and is bonded to earth at the service. The equipment grounding conductor (EGC) carries fault current back to the source to trip the overcurrent device; it normally carries no current. The grounding electrode conductor (GEC) connects the service to the earth electrodes, and the main bonding jumper ties neutral and EGC together at the service only. Beyond the service, neutral and ground must be separated.
Sizing
GEC size comes from Table 250.66 and depends on the largest service conductor. EGC size comes from Table 250.122 and depends on the rating of the overcurrent device ahead of the circuit. This is the single most common mix-up in exam questions. If circuit conductors are increased for voltage drop, the EGC may need to be increased proportionally.
Electrodes
The grounding electrode system includes the metal water pipe (if 10 ft or more in contact with earth), the concrete-encased electrode, the ground ring, and made electrodes such as ground rods. A single rod requires 25 ohms or less, or a second rod.
Worked example
A 200 A service with 2/0 copper service conductors: Table 250.66 gives a 4 AWG copper GEC. The EGC for a 200 A branch circuit or feeder, however, comes from Table 250.122 and is 6 AWG copper. Two different answers for two different conductors.