Grounding and Bonding
Grounding and Bonding
This domain tests Article 250: the grounded conductor, equipment grounding conductor, grounding electrode system, bonding, and the fault-current path. The exam separates grounding (earth connection) from bonding (metal parts joined).
Core rules
- The service neutral is grounded at the service disconnect; neutral and equipment ground are bonded there and separated in subpanels.
- Grounding electrode conductor sizing comes from Table 250.66, based on service conductor size.
- A single ground rod must be supplemented unless it measures 25 Ω or less; most installations use two rods.
- Metal water piping, structural steel, and gas piping bonding must follow 250.104 and 250.52.
- Equipment grounding conductors must be sized per 250.122 and run with the circuit.
Worked example
A 200 A service uses 2/0 Cu service conductors. Table 250.66 gives a 4 AWG Cu grounding electrode conductor. If the rod measures 40 Ω, add a second rod; the 25 Ω rule is not met. At a detached garage subpanel, run a 4-wire feeder and keep neutral and ground isolated; the equipment ground is the fault path.
Traps
- Bonding neutral to ground at a subpanel, creating parallel return current.
- Using the earth as the only equipment grounding conductor.
- Assuming one rod always satisfies the electrode requirement.
- Sizing the grounding electrode conductor from the OCPD rather than Table 250.66.