There is a quiet revolution occurring in the home-building industry. Traditional single- and multiple-dwelling construction methods as we knew them have changed forever. Metal studs are now often being installed in place of wood and steel beams in place of wooden trusses and joists. Telephone, sound, cable television, and computer systems are being interconnected into a structured wiring system. Copper water piping is being replaced with plastic tubing within walls, crawl spaces, attics, and under concrete slabs.
Schedule 40 PVC has replaced copper water pipe as the method of choice for underground water piping systems for dwellings. Cross-linked polyethylene plastic tubing (PEX), polybutylene (PB) plastic tubing, and chlorinated polyvinyl chloride (CPVC) tubing are being used for water distribution inside dwellings. Combinations of these materials, such as polybutylene/aluminum/polyethylene (PEX-AL-PEX), are also being used.
Joints and splices in the plastic tubing system are accomplished by using solvent cement with insert fittings that fit into the tubing or are joined by heat fusion. The transition from nonmetallic water pipe inside the walls to a plumbing fixture, such as a sink, toilet or a hose bibb, is accomplished by using a copper 90-degree elbow and a short piece of copper tubing. The copper elbow is attached to the nonmetallic water pipe by solvent cement, heat fusion or by metal corrosion-resistant compression fittings.
The copper tubing is then soldered to a copper holding plate that spans from one stud to the next with the sole purpose of supporting the copper stub and ensuring the copper pipe cannot be pushed back into the wall. The copper stub is the connection point for the water angle-stop or shut-off valve at the fixture. Often the flexible connection from the angle stop to the plumbing fixture is nonmetallic.
Section 250.50 of the National Electrical Code (NEC) requires that a metal underground water pipe in direct contact with the earth for at least 10 feet or more be used as one of the grounding electrodes for the electrical service to the building. This metal water pipe electrode, along with any other electrodes at the building or structure, provides a zero reference to ground (or as close as possible to zero) for the electrical service.
At what point does the water piping system no longer provide a good electrode reference point? It’s where the water pipe does not have a direct metal-to-earth connection for 10 feet or more. A predominantly nonmetallic piping system is unlikely to have 10 feet of metal pipe in direct contact with the earth.
Section 250.104(A) requires a metal water piping system installed in or attached to a building to be bonded back to the electrical service. The metal water pipe can be bonded to the electrical service enclosure, the grounded conductor at the service, the grounding electrode conductor where it is of sufficient size, or to one or more of the grounding electrodes for the service.
The main purpose of this bond is to ensure that the metal water pipe is at the same zero voltage to ground as the service grounded conductor. A secondary purpose is to ensure that there is a path back to the service for electrical current flow if the metal water pipe becomes energized.
At what point is the water piping system not considered to be metal and thus not required to be bonded? The answer to this question is not as clear as the one for the grounding electrode. An assessment must be made regarding the amount of metal in the system and whether this metal pipe and any metal support for the metal piping may become energized. It is also important to determine if a difference of potential could exist between the metal piping in the water system and an electrical circuit located in proximity to the water pipe.
Could the electrical system energize the metal piping in the predominantly nonmetallic water piping system? If the answer is no, or unlikely, then the metal piping would not be required to be bonded. If the answer is yes, then the metal water piping must be bonded in accordance with 250.104(A).
As an alternative to bonding the metal in the piping system in accordance with Section 250.104(A), an AHJ could permit the metal piping to be bonded in accordance with Section 250.104(B) for “other metal piping systems.” Section 250.104(B) would permit the equipment-grounding conductor for the electrical circuit that may energize the metal water pipe stub to serve as the bonding means.
As construction methods change, a good understanding of grounding and bonding will ensure a safe installation and provide help in dealing with these gray areas. EC
ODE is staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at email@example.com.