Grounding and Bonding
The Grounding and Bonding domain tests Article 250: system grounding, grounding electrode conductors, equipment grounding conductors, bonding of metal piping and structures, and electrode requirements. The exam separates grounding, the earth connection, from bonding, the fault-current path.
Core rules
- The service grounded conductor must connect to the grounding electrode conductor at the service per 250.24(A). The main bonding jumper bonds neutral and equipment grounding at the service only per 250.28.
- Grounding electrode conductor sizing uses Table 250.66 based on the largest ungrounded service conductor, with a minimum of 8 AWG copper or 6 AWG aluminum. For parallel service conductors, 250.66(B) permits a maximum of 3/0 copper or 250 kcmil aluminum.
- Equipment grounding conductor sizing uses Table 250.122 based on the overcurrent device rating. If circuit conductors are increased in size, 250.122(B) may require a proportional increase in the EGC.
- Electrodes in 250.52 include underground water pipe with at least 10 ft in contact with earth, metal frame, concrete-encased electrode with at least 20 ft of rebar or conductor, ground ring with at least 20 ft, rod or pipe at least 8 ft, and plate electrodes. A supplemental electrode is required where a water pipe is the only electrode per 250.53(D)(2).
- Bond metal water piping per 250.104, structural steel, and other metal systems likely to become energized.
Worked example
A 400 A service uses 600 kcmil copper ungrounded conductors. Table 250.66 requires a 1/0 copper grounding electrode conductor. A 400 A feeder overcurrent device requires a 3 AWG copper equipment grounding conductor from Table 250.122. If the feeder conductors are increased from 500 kcmil to 750 kcmil for voltage drop, the EGC must be increased proportionally under 250.122(B); leaving it at 3 AWG is a violation.
Traps
- Using Table 250.122 to size the grounding electrode conductor.
- Bonding neutral to ground downstream of the service.
- Treating a ground rod as a substitute for an equipment grounding conductor.
- Omitting the supplemental electrode when a water pipe is the only electrode.
- Sizing the GEC from one parallel set instead of the total service conductor area.