How to Bond the Water Pipe at the Service
Why this matters
The water pipe clamp is the grounding connection inspectors look at first and techs install last, usually in a hurry at the end of a panel change. Put it on the wrong side of the meter and the next plumber who pulls that meter opens the house's main electrode path. Skip the gas bond and a faulted appliance can leave the gas piping energized with nothing to clear the breaker. This card is the install: where the clamp goes, what gets jumped, how the gas piping is bonded, and the reading that proves it.
Before you touch an existing clamp, clamp the conductor
Treat an existing grounding electrode conductor (GEC) or water bond as current-carrying until a clamp meter says otherwise. On a service with a high-resistance or open neutral, return current goes home through the water pipe, the street main and the neighbor's service. Lifting that clamp opens a live path: it can arc, and it leaves the pipe on each side of the gap at a different potential, with you holding both.
- Clamp the existing GEC and any existing water jumper with the house running normally. A few tenths of an amp is common on a city water system.
- Stop rule: if either reads more than about one amp, do not open it. Install the new conductor and clamp alongside it first, and report the reading to the customer and the utility as a possible neutral problem. One amp is our shop's trigger, not a code value; current on a bond is a symptom to chase before it is a conductor to move.
- Nobody cuts the water line (a meter swap, a filter going in) until your jumper is installed and tested.
You are clamping onto gas pipe, not opening it. If you smell gas at any point, stop, get everyone out of the house without operating switches or the panel, and call the gas utility from outside.
What the code asks for
The water pipe is two things at once. NEC 250.52(A)(1) (2023 edition, as adopted by your jurisdiction) makes metal underground water pipe a grounding electrode when 10 ft or more of it is in direct contact with earth. Separately, NEC 250.104(A) requires the interior metal water piping to be bonded whether or not it is an electrode. A house on a plastic water service still gets its copper bonded; it only loses the pipe as an electrode.
Where the electrode connection may be made. NEC 250.68(C)(1) permits interior metal water piping within 5 ft of the point of entrance to extend the electrode connection. Measure along the pipe. A clamp at 7 ft bonds the piping but is not the electrode connection.
Meters and filters are not conductors. NEC 250.53(D)(1) says continuity of the grounding path cannot depend on water meters, filtering devices or similar equipment. Meters get pulled and filter housings are often plastic. Jump both.
The pipe never stands alone. NEC 250.53(D)(2) requires a supplemental electrode, usually rods; that install is 'How to Drive and Connect a Pair of Ground Rods'.
Gas piping. NEC 250.104(B) requires metal piping likely to become energized, which includes interior gas piping, to be bonded, sized from Table 250.122 on the rating of the circuit likely to energize it, and the equipment grounding conductor (EGC) of that appliance circuit is permitted to be the bond. Many jurisdictions and gas utilities ask for a visible bond at the service anyway, so read the local amendment before relying on the furnace's EGC. Gas pipe is never an electrode (250.52(B)(1)). Yellow corrugated stainless steel tubing follows its maker's bonding instructions; that question is 'Yellow CSST and the Bonding Question'. Do not clamp the corrugated tubing.
The layout
WALL basement side
|
=|=[SHUTOFF]=[A]=[METER]=[FILTER]=[C]=> house
| | |
|<- 24 in ->| |
| +--- 4 Cu jumper -----+
| |
| +--- 4 Cu GEC to panel bar
|
GAS: outside meter ->[SHUTOFF]=[G]=> steel
|
+--- 6 Cu to bar
A = GEC clamp, 24 in from entry (inside 5 ft)
C = jumper clamp, house side of filter
G = gas bond clamp, rigid pipe past shutoff
The steps
1. Size the conductors before you leave the truck. The GEC is sized from NEC Table 250.66 on the service-entrance conductors; the interior water piping bond is sized from Table 250.102(C)(1) on the same basis. For a 200 A dwelling on 4/0 aluminum, both land on 4 AWG copper, and the jumper around the meter and filter carries the same duty, so run it the same size. A 6 AWG coil on the truck turns this job into a red tag. See related: 'Grounding Electrode Conductor Sizing Card'.
2. Mark 5 ft from the point of entrance. Put the GEC clamp on sound pipe inside that 5 ft and on the street side of the meter, so pulling the meter cannot open the electrode connection. Skipped, the clamp lands wherever is convenient, which is usually past the meter.
3. Clean the pipe and use a listed clamp. NEC 250.70 requires listed connectors. Use a clamp listed (UL 467, marked on the clamp) for the pipe material and conductor size, and clean to bright metal with emery cloth where it bears. Paint, corrosion and pipe dope are insulators; a clamp over paint reads fine today and opens the first winter the joint moves.
4. Keep the GEC continuous and protected. NEC 250.64(C) wants it unspliced or joined only by irreversible compression or exothermic means. 4 AWG copper may run exposed, but must be protected where it is exposed to physical damage (250.64(B)). A steel sleeve must be bonded at both ends (250.64(E)) or it chokes fault current; a PVC sleeve avoids the problem.
5. Jump the meter and filter with one continuous conductor. Run the 4 AWG from clamp A past both to clamp C. One conductor landed in a listed clamp at each end survives a meter swap and a cartridge change. Then look for anything else that breaks continuity: a plastic softener loop, a PEX repair, dielectric unions at the water heater. Every metal section past a break needs its own bond, or it is an isolated pipe a faulted appliance can energize.
6. Bond the gas piping at a rigid section. Clamp black steel or rigid copper gas pipe on the house side of the shutoff at an accessible point, and run the conductor to one of the terminations NEC 250.104(B) allows: the service enclosure, the grounded conductor at the service, a GEC of sufficient size, or the electrode. Never the meter, the regulator or a flexible connector.
7. Leave the connections accessible. NEC 250.68(A) requires the electrode connection to be accessible (with exceptions for encased or buried connections). Do not let a new ceiling swallow clamp A. Where the AHJ wants it, tag each clamp as an electrical bond not to be removed.
Worked job: a 1990s basement, panel replacement
The house has a 200 A service on 4/0 aluminum SER and a 3/4 in copper water service entering the basement wall, a shutoff 8 in from the wall, then the meter, then a sediment filter with a plastic housing. The old GEC, 6 AWG copper, is clamped 9 ft along the pipe, past the filter, over basement paint. Gas comes through the rim joist to a black steel header feeding a furnace and a water heater.
Current check. Old GEC: 0.3 A, under the one-amp stop rule, so it may come off. The reading goes on the ticket.
Sizing. Table 250.66 and Table 250.102(C)(1) on 4/0 aluminum: 4 AWG copper for the GEC and the jumper. The old 6 AWG is retired, not reused.
Placement. Clamp A goes 24 in from the wall, between shutoff and meter: inside 5 ft, street side of the meter. The old clamp sat past the meter and the filter, so the electrode path had been running through a plastic housing. That is the finding for the customer: the house was grounded through a part that holds water, not current. The 4 AWG jumper runs from A to clamp C, 18 in past the filter.
Gas. The furnace is on a 15 A circuit and the gas water heater's igniter receptacle is on another 15 A circuit, so Table 250.122 would accept 14 AWG copper. This AHJ wants a visible bond at the service, and our shop standard for it is 6 AWG copper; larger than the minimum is always permitted, and it does not need revisiting if a 20 A gas appliance is added later. Clamp G goes on the steel header 14 in past the shutoff.
The finish test: low-ohm continuity to the bar
Do this with every clamp still exposed, before any ceiling, cabinet or insulation goes back.
- Null the leads. Use a long wander lead on a low-resistance ohmmeter or the lowest range of your meter, short it to the other probe and zero it, or record the lead reading and subtract it. A 50 ft lead can read a large part of an ohm by itself, more than the path you are measuring.
- Measure each clamp to the panel's grounded bar. On this job, after nulling: clamp A 0.04 ohm, clamp C 0.06 ohm, clamp G 0.07 ohm. Expect tiny numbers: 4 AWG copper runs about three-tenths of an ohm per 1,000 ft, so a 30 ft run is under a hundredth of an ohm and the rest is clamp contact. Our shop rejects anything above 0.5 ohm after nulling, a workmanship default rather than a code value.
- Measure across each jumped part, pipe to pipe. Probe bare pipe on each side of the meter and of the filter, not the clamps. Near zero proves the jumper is carrying it. Then measure from a cold-water supply on the top floor back to the bar to prove the interior piping is continuous with the bonded section.
- Clamp the new GEC with the house running. It read 0.2 A against 0.3 A on the old conductor: the same order of current, so no new path has been created.
When a reading is high. A clamp over paint, a clamp the wrong size for the pipe touching on one edge, or a jumper landed on pipe isolated by a dielectric union. Re-clean, re-clamp, measure again. Good readings at the clamps and a high one from the top-floor fixture means a plastic section in between that needs its own jumper.
What changes the answer
- Plastic water service. The pipe is not an electrode, so the GEC goes to the other electrodes, and the interior copper is still bonded under 250.104(A), sized from Table 250.102(C)(1), at any accessible point. The 5 ft rule no longer applies, because that bond is not an electrode connection.
- Commercial or industrial building. NEC 250.68(C)(1) has an exception to the 5 ft limit for industrial, institutional and commercial buildings where conditions of maintenance and supervision ensure only qualified persons service the installation and the interior piping used for the connection is exposed. Every condition has to hold; a strip mall serviced by the landlord's handyman does not meet it.
References
- NFPA 70, National Electrical Code, 2023 edition (2020 where numbering moved) as adopted by your jurisdiction, local amendments checked: 250.52, 250.53(D), 250.64, 250.66, 250.68, 250.70, 250.102(C)(1), 250.104, Table 250.122
- See related: Grounding Electrode Conductor Sizing Card
- See related: How to Drive and Connect a Pair of Ground Rods
- See related: Yellow CSST and the Bonding Question
- Clamp manufacturer installation instructions and listing marks (UL 467)