Grounding, Bonding, and Overcurrent Protection
Grounding versus bonding
Grounding connects the electrical system to earth; bonding connects metal parts so they remain at the same potential. The service neutral is bonded to the enclosure and grounding electrode system at one point only (250.24(A)(1) and 250.28). At a separate building or structure, 250.32 governs, and the neutral is not bonded again when an equipment grounding conductor is run with the feeder.
Sizing
- Grounding electrode conductor: Table 250.66, based on the largest service-entrance conductor.
- Equipment grounding conductor: Table 250.122, based on the overcurrent device rating, increased by 250.122(B) when circuit conductors are upsized.
- Main bonding jumper: 250.28, sized from Table 250.66.
- Metal water piping: 250.104, bonded with a conductor sized from Table 250.122 for the service.
Worked example
A 200 A service uses 2/0 Cu service conductors. Table 250.66 gives a 4 AWG Cu grounding electrode conductor; Table 250.122 gives a 6 AWG Cu equipment grounding conductor for the 200 A feeder. The two tables are not interchangeable, and using 6 AWG for the GEC or 4 AWG for the EGC would be a citation error in either direction.
Overcurrent protection: 240.4 protects conductors at their ampacity; 240.4(D) limits small conductors, generally 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG; 240.21 tap rules allow reduced protection only under strict length, location, and ampacity conditions.
Traps
- Sizing the GEC from the breaker rating.
- Re-bonding the neutral at a subpanel.
- Using Table 250.122 for a grounding electrode conductor.
- Ignoring 250.122(B) when conductors are upsized for voltage drop.