Grounding, Bonding, and Ground-Fault Protection
Core concepts
Article 250 separates three ideas that questions deliberately blur. The grounded conductor (neutral) is a circuit conductor that carries current. The grounding electrode conductor (GEC) connects the grounded system to earth, sized by Table 250.66. The equipment grounding conductor (EGC) provides the low-impedance fault path back to the source, sized by Table 250.122 based on the overcurrent device rating - not the conductor ampacity.
Bonding is the intentional joining of metal parts so they stay at the same potential. The main bonding jumper connects the grounded conductor to the enclosure at the service; it is installed only at the service disconnect or at a separately derived system. Neutrals must never be bonded to enclosures downstream of that point.
A grounding electrode system must include any available metal water pipe with at least 10 ft. in earth contact, the structural steel, a concrete-encased electrode, and a ground ring. Where a water pipe is used, a supplemental electrode is required.
Traps
- Sizing an EGC from Table 250.66 (that table is for GECs).
- Re-bonding the neutral at a subpanel, creating parallel neutral paths.
- Using a ground rod as the only electrode when a qualifying water pipe exists.
- Assuming a single rod achieves 25 ohms; if it does not, a supplemental electrode is required.
Rule of thumb
The GEC to a rod or pipe electrode never needs to be larger than 6 AWG copper, and to a concrete-encased electrode never larger than 4 AWG.