Grounding and bonding
Scope
Grounding and bonding questions test Article 250 plus the communications grounding articles (800.100, 810.21, 820.93, 830.93). Know the difference: grounding connects to earth; bonding connects metal parts to create an equipotential path.
Core rules
- The grounding electrode system typically includes a metal underground water pipe, metal building frame, concrete-encased electrode, and ground ring, bonded together with irreversible connections.
- The equipment grounding conductor provides the fault-current path and must be sized per Table 250.122, not by the neutral.
- Communications systems use a grounding conductor run as short and straight as practicable to the grounding electrode system, often via an intersystem bonding termination (250.94).
- Do not use a gas piping system as a grounding electrode. Bond metal gas piping only as required and permitted.
- Separately derived systems, separate buildings, and pools have specific bonding requirements; do not apply the main-service rules blindly.
Worked example
A coax service drop enters a house and the shield must be bonded. The installer runs a 14 AWG copper conductor from the coax grounding block to the intersystem bonding termination, keeping it short and straight. The conductor is not the branch-circuit EGC and does not land on the neutral bus. If the IBT is not near the entry, the fix is to relocate the bonding point or use a listed bonding device, not to coil a long conductor.
Traps
Bonding communications grounds to a floating ground rod instead of the electrode system; using the neutral as a ground; undersizing the EGC; mixing Article 250 and Article 800 sizing rules; and assuming a ground rod alone is a complete electrode system.