Section 250.104/Bonding of Piping Systems and Exposed Structural Steel
NEC 250.104 requires metal water piping, other metal piping that may become energized, and exposed structural steel to be bonded to the electrical grounding system.
Section 250.104(A) requires the interior metal water piping system to be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or the grounding electrode system. The bonding jumper must be sized per Table 250.102(C)(1) based on the size of the largest ungrounded service-entrance conductor. Section 250.104(B) requires other metal piping systems (such as gas piping) that may become energized to be bonded, with the bonding jumper sized per Table 250.122 based on the rating of the circuit that is likely to energize the piping. Section 250.104(C) requires exposed structural metal that is interconnected to form a metal building frame and is not intentionally grounded to be bonded to the grounding electrode system with a conductor sized per Table 250.102(C)(1).
Why this section exists
Metal piping and structural steel can become energized from a fault in any electrical equipment or wiring in contact with the metal. Without bonding, a person touching the energized piping and a grounded surface simultaneously would receive a shock. Bonding provides a low-impedance fault return path that allows overcurrent devices to trip quickly and clear the fault. Water piping bonding is particularly critical because water piping contacts earth at multiple points and occupants frequently touch fixtures and faucets. Gas piping bonding is also required because CSST (corrugated stainless steel tubing) can be damaged by lightning-induced currents if not properly bonded.
What plan reviewers look for
Plan reviewers check the electrical drawings for bonding connections to the metal water piping system, typically shown at or near the water service entrance. They verify the bonding jumper size matches Table 250.102(C)(1) based on the service-entrance conductor size. For gas piping, they verify a bonding connection is shown, sized per Table 250.122. For CSST gas piping specifically, they check for the direct bonding connection required by the CSST manufacturer's installation instructions, which often require a bonding clamp at the gas meter or manifold. For structural steel buildings, they check for the structural steel bonding conductor.