What size bonding jumper is required for metal water pipe?

It comes from NEC Table 250.102(C)(1) based on the ungrounded service entrance conductors. A 200 amp service on 3/0 copper requires a 4 AWG copper bonding jumper.

Is a metal water pipe a grounding electrode?

Yes, if at least 10 feet of it is in direct contact with the earth, per NEC 250.52(A)(1). It must be supplemented by an additional electrode under 250.53(D)(2) and connected within 5 feet of where it enters the building.

What are the NEC bonding requirements for metal water pipes?

Q: Does gas piping need to be bonded?

Yes. NEC 250.104(B) requires metal piping likely to become energized to be bonded to the equipment grounding conductor of the circuit likely to energize it, which is usually the EGC of the appliance circuit.

Section 250.104(A)(1) requires that the metal water piping system installed in or attached to a building or structure be bonded to the service equipment enclosure, to the grounded conductor at the service, to the grounding electrode conductor where it is of sufficient size, or to one or more grounding electrodes. The bonding jumper is sized from Table 250.102(C)(1) using the size of the ungrounded service entrance conductors, and the point of connection has to be accessible.

Bonding is not the same as the electrode connection

These two rules get confused constantly because they often land on the same pipe. A metal underground water pipe in direct contact with the earth for 10 feet or more is a grounding electrode under 250.52(A)(1), and it must be part of the grounding electrode system. Section 250.53(D)(2) requires it to be supplemented by an additional electrode, usually a rod or a concrete encased electrode, because the utility may replace the buried line with plastic at any time. That connection is a grounding electrode conductor sized from Table 250.66 and it must be made within the first 5 feet of where the pipe enters the building.

Bonding under 250.104(A) is a different job. Its purpose is to make sure the interior metal piping cannot become energized and stay energized, so that a fault to a pipe clears the overcurrent device instead of leaving a hot pipe in the building. The bonding jumper can be connected anywhere accessible, and it is sized from Table 250.102(C)(1) rather than Table 250.66. For a 200 amp service on 3/0 copper, that table calls for 4 AWG copper.

Practical sizing

Common results for dwelling services: 100 amps on 4 AWG copper gets an 8 AWG bonding jumper, 150 amps on 1/0 copper gets a 6 AWG, 200 amps on 3/0 or 4/0 copper gets a 4 AWG, and 400 amps on two parallel sets of 4/0 copper gets a 1/0. When the service conductors are aluminum, size from the aluminum column of the same table.

Other piping and special cases

Section 250.104(B) covers other metal piping, including gas piping, that is likely to become energized. It must be bonded to the equipment grounding conductor of the circuit that is likely to energize it, which for a gas line usually means the EGC of the furnace or water heater circuit. That EGC is typically permitted to serve as the bonding means without a separate jumper. Section 250.104(A)(2) covers multiple occupancy buildings and 250.104(A)(3) covers separate buildings supplied by a feeder.

Section 250.68(B) requires that any water meter, filter or dielectric union in the path be bypassed with a bonding jumper, since those components interrupt continuity. Use a jumper rated for the connection and listed clamps under 250.70.

Field takeaway

Bond the interior metal water piping with a jumper from Table 250.102(C)(1), connect the underground metal water pipe electrode with a conductor from Table 250.66 within 5 feet of entry, supplement that electrode, and jumper around meters and dielectric unions.