Plastic Water Service and a Detached Shop With Voltage on the Enclosure

The electrode is gone; the electrode system must be rebuilt.

• A metal underground water pipe is an electrode only if it is in direct contact with the earth for 10 ft or more (250.52(A)(1)). A 1 in HDPE service is not metal, so the house no longer has a water-pipe electrode. The 6 AWG clamp on a plastic pipe is a decoration and must be removed - a grounding electrode conductor connected to nothing creates a false sense of a grounded system.
• If the water pipe was the only electrode, a grounding electrode system must now be installed (ground rods, or another 250.52 electrode). Where a single rod is used, a supplemental electrode is required unless the single rod measures 25 ohms or less (250.53(A)(2)), so plan on two rods.
• Even if the water pipe had remained metal, a supplemental electrode would have been required because a water pipe electrode is not permitted to be the sole electrode (250.53(D)(2)); and a bonding jumper around the meter is required because the grounding path may not depend on the meter (250.53(D)(1)).

Was the 6 AWG correct? No. The conductor to a metal water pipe electrode is sized from Table 250.66 on the service conductors. With 4/0 Al service-entrance conductors, the table calls for 2 AWG copper (the 3/0 through 350 kcmil aluminum row). The allowance for 6 AWG copper applies only to a rod, pipe, or plate electrode (250.66(A)), not to a water pipe electrode.

Correct scope now: drive two rods, run a grounding electrode conductor from the service to them (6 AWG copper is permitted to the rods), keep the service neutral bonding jumper in the service equipment, and remove the abandoned clamp from the plastic pipe.

The shop is wired correctly - which is the clue.

• Equipment grounding conductor size: for a 100 A overcurrent device, Table 250.122 requires 8 AWG copper or 6 AWG aluminum. The 6 AWG Al EGC is compliant.
• Separate building, separate electrode: 250.32(A) requires a grounding electrode system at the separate structure - two rods with a 6 AWG copper grounding electrode conductor satisfy 250.66(A).
• Equipment grounding conductor run with the feeder: 250.32(B) requires an EGC with the feeder; the older practice of using the neutral as the grounding path at a second building is no longer permitted.
• Neutral isolated at the shop: correct and important. The grounded conductor is bonded to the enclosure and to the grounding electrode system at the service only. If the neutral were bonded again at the shop, normal neutral current would divide onto the EGC and the shop's grounding electrode, energizing every metal enclosure and defeating the GFCIs.

So the defect is upstream. When the second building's grounding and bonding are right but the enclosure sits several volts above remote earth under load, look for a compromised service neutral, a missing or deteriorated main bonding jumper, loose lug terminations on the 1/0 Al neutral (aluminum terminations need proper torque and, per manufacturer instructions, an antioxidant where specified), or an EGC termination that has been disturbed.

What the reading means

A steady 4 to 6 V between the shop panel enclosure and a remote reference rod, appearing only when a large single-phase load runs, means current is flowing on the equipment grounding conductor or the earth path. With the shop neutral isolated, neutral current has no legitimate reason to be on the enclosure. That current is looking for a return path because the intended neutral path has elevated resistance.

Check, in order:

1. Service neutral integrity - terminations at the meter base, the service equipment neutral/grounded bus, and the main bonding jumper. Measure voltage drop across the neutral under the welder load; a few volts of drop from the service equipment to the panel is normal, volts across a single lug are not.
2. Shop feeder neutral lugs and the 1/0 Al terminations - re-torque per the lug manufacturer's instructions, inspect for aluminum oxide and overheating.
3. EGC continuity between the shop enclosure and the service grounded bus (a low-resistance measurement, not just a voltage check).
4. The welder circuit itself - verify the equipment grounding conductor is continuous at the receptacle and the welder frame; welding transformers and high-frequency starts can produce leakage that trips GFCIs.
5. The newly plastic water line - with the metal water pipe gone, the house lost an earth connection; this did not create the fault, but it removed a parallel path and made an existing neutral problem show up as voltage on metal.

Do not "fix" it by:

• Driving more ground rods at the shop - a ground rod cannot clear a fault or carry neutral current.
• Bonding the neutral to the enclosure at the shop panel to "stabilize" it - that creates a parallel neutral on the EGC and the grounding electrode and is a serious shock hazard.
• Removing or upsizing the GFCI devices, which are reporting a real condition.

Reportable conclusion: the shop's grounding and bonding are compliant per 250.32 as installed; the indicated fault is a deterioration of the service neutral/grounded conductor path or its terminations, and it must be corrected at the service before the shop is returned to service.