Grounding, bonding, overcurrent protection, and safety
What this domain tests
This domain tests Article 250 grounding and bonding, Article 240 overcurrent protection, and the safety rules that keep fault current on a low-impedance path. Questions ask you to distinguish the grounded conductor from the equipment grounding conductor, size grounding and bonding conductors, select overcurrent devices, and apply working-space and PPE rules.
Core rules
- The service neutral is bonded to the equipment grounding conductor at the service disconnecting means through the main bonding jumper. Downstream panels are not bonded again; a separate derived system is bonded at its source.
- Grounding electrode conductors are sized from Table 250.66 using the largest service-entrance conductor. Equipment grounding conductors are sized from Table 250.122 using the overcurrent device rating.
- All grounding electrodes present must be bonded together per 250.50; a supplemental electrode may be required.
- Overcurrent devices must have adequate interrupting rating (110.9, 240.85), and series ratings are permitted only where listed. Standard ratings are in 240.6; small-conductor limits are in 240.4(D).
- Safety: 110.26 working space, lockout/tagout, test before touch, and appropriate arc-rated PPE.
Worked example
A service has 2/0 copper service-entrance conductors. Table 250.66 requires a 4 AWG copper grounding electrode conductor. Reasoning: the GEC is sized from the service conductors, not from the load calculation; 2/0 copper falls in the 1/0-2/0 row, which specifies 4 AWG copper. Using the equipment grounding conductor table instead would be the wrong table.
Traps
Bonding neutral and ground again at a subpanel; using the neutral as an equipment grounding conductor; sizing the GEC from load; ignoring interrupting rating; applying the next-size-up rule without meeting 240.4 conditions.