Grounding, Bonding, and Overcurrent Protection
Grounding, Bonding, and Overcurrent Protection
This domain tests Article 250 grounding and bonding and Article 240 overcurrent protection. You must distinguish the grounded conductor, grounding electrode conductor, equipment grounding conductor, and bonding jumper, and size each correctly.
Core rules
- System grounding connects the service neutral to earth; the main bonding jumper connects neutral and equipment ground at the service only. Subpanels keep neutral and ground separate.
- Grounding electrode conductor sizing uses Table 250.66 and is based on the largest service conductor. Equipment grounding conductor sizing uses Table 250.122 and is based on the overcurrent device rating.
- A single ground rod requires 25 Ω or less; if not, a second electrode is required (250.53(A)(2)). The 25 Ω rule does not replace the EGC.
- Standard OCPD ratings are in 240.6. Breakers protect conductors, not equipment; motor and transformer circuits have separate rules.
Worked example
A 200 A service uses 2/0 copper service conductors. Table 250.66 requires a 4 AWG copper grounding electrode conductor. The equipment grounding conductor for the 200 A feeder is sized from Table 250.122: 6 AWG copper. These are different tables and different purposes; using 4 AWG for the EGC is safe but not required, while using 6 AWG for the GEC is a code violation.
Traps
- Using Table 250.66 for an equipment grounding conductor.
- Bonding neutral to ground in a subpanel.
- Assuming a ground rod is a substitute for the EGC.
- Ignoring the second-rod requirement when resistance exceeds 25 Ω.