Bonding water pipes, gas pipes and structural steel
Metal water piping must be bonded to the building's electrical grounding, and other metal piping that could become energized, including gas piping, must be bonded as well. The bonding jumper sizes come from code tables.
What the code requires
Metal water piping must be bonded to the service enclosure, the service neutral, the grounding electrode conductor (if it is large enough), or one of the grounding electrodes. The jumper is sized from Table 250.102(C)(1) and never needs to exceed 3/0 copper or 250 kcmil aluminum or copper-clad aluminum. For example, a service with 3/0 copper conductors needs a 4 AWG copper jumper. The attachment points must be accessible.
In multiple-occupancy buildings where each unit's metal water piping is isolated by nonmetallic pipe, each unit's piping may be bonded to the equipment grounding terminal of the panel serving that unit, sized per 250.102(D). In a building fed by a feeder or branch circuit, the piping bonds to the building disconnect enclosure, the equipment ground run with the supply, or the building's electrode, sized per 250.102(D); it need not be larger than the largest ungrounded feeder or branch-circuit conductor.
Other metal piping that could become energized, including gas piping, must be bonded. It may bond to the equipment grounding conductor of the circuit likely to energize it (an appliance's ground counts), or to the service, grounding electrode conductor or electrodes, sized from Table 250.122 for that circuit.
Exposed structural metal that is interconnected to form a building frame, is not intentionally grounded or bonded, and could become energized must also be bonded, sized from Table 250.102(C)(1) with the same 3/0 copper cap.
On the job
- A missing or inaccessible water pipe bond is a common correction.
- CSST gas tubing has its own bonding rules. Per Mike Holt, NFPA 54 Section 7.12 requires a CSST bond only for tubing without an arc-resistant jacket. Follow the manufacturer's instructions and your local gas code.
- Bonding gas piping does not make it a grounding electrode. Gas piping may never serve as one (250.52(B)).
Exceptions that matter
- Bonding attachment points on structural metal do not have to be accessible where they are made with exothermic or irreversible compression connections to fireproofed structural metal, as allowed by 250.68(A) Exception No. 2.
Related sections
- NEC 250.52(B)
- NEC 250.64
- NEC 250.68: Grounding electrode connections and the 5 ft rule
- NEC 250.102
- NEC 250.122: Sizing the equipment ground wire
- NEC 680.26: Equipotential bonding for pools
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CSLB #354550 · C-10 Electrical · Licensed since 1978 · $25,000 contractor's bond