Grounding and Bonding
The grounding path
Article 250 separates system grounding from equipment grounding and bonding. The service neutral is the grounded conductor and is connected to the grounding electrode system at the service through the main bonding jumper. Downstream, the neutral must not be regrounded except at a separately derived system. The equipment grounding conductor provides the low-impedance fault path and is sized from the overcurrent device using the equipment grounding conductor table. The grounding electrode conductor is sized from the service conductors. Metal water piping, structural steel, and grounding electrodes must be bonded, but gas piping may not serve as a grounding electrode.
Common traps
Questions often mix up the grounding electrode conductor and the equipment grounding conductor. A larger service does not automatically require a larger equipment grounding conductor; that conductor follows the overcurrent protective device rating. Another trap is bonding the neutral at a subpanel, which creates parallel neutral paths and objectionable current. A ground rod must be at least 8 ft long and, if a single rod does not achieve 25 ohms or less, a second electrode is generally required.
Worked example
A 200 A service uses 2/0 Cu service conductors. The grounding electrode conductor is 4 AWG Cu. The equipment grounding conductor for a 200 A circuit is 6 AWG Cu. These are different because they are selected from different tables. Rule of thumb: GEC follows service conductor size; EGC follows OCPD size.