Bonding and Grounding a Detached Garage with a Metal Water Line
A homeowner has a detached garage 25 m from the house, fed by a 60 A, 240 V feeder through PVC conduit with three conductors: two ungrounded and one insulated neutral, plus a bare copper equipment grounding conductor run in the same conduit. The garage contains a 100 A panel with a neutral bus and a separate ground bus that are bonded together by a green screw. A metal water pipe enters the garage from a private well and is bonded to the panel ground bus with No. 6 copper. A concrete-encased electrode (20 ft of bare copper in the footing) is present but not connected. There is also a metal gas line serving a unit heater. The owner has added a 50 A receptacle for a welder and reports a mild tingle when touching the welder case and the concrete floor simultaneously.
1. Identify the grounding and bonding defects in this installation and explain the tingle.
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The tingle is the diagnostic clue: a voltage difference exists between the welder case and the floor, which means the equipment grounding path is compromised or carrying current. Defects: (1) A detached structure fed by a feeder must have a separate equipment grounding conductor, which is present, and the panel's neutral and ground must be separated at the subpanel. The green bonding screw ties neutral to ground at the garage panel, creating a parallel neutral path through the grounding conductor and the metal water pipe. That is a code violation and a shock hazard. (2) The concrete-encased electrode is present but not connected; it should be bonded to the grounding electrode system. (3) The metal gas line must be bonded, but bonding must not rely on the gas piping as a grounding electrode. (4) The water pipe bond is present, but with the neutral-ground bond at the panel, the water line becomes a current-carrying path. The welder case tingles because return current divides through the grounding conductor and the bonded metal piping, producing a voltage drop across that path. Correct by removing the green screw, separating neutrals and grounds, connecting the concrete electrode, and bonding the gas line to the grounding system.
2. The welder circuit is 50 A. What minimum equipment grounding conductor size and bonding jumper considerations apply, and how would you verify the repair?
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For a 50 A circuit, the equipment grounding conductor is selected from the applicable table based on the overcurrent device rating; for copper, 50 A typically requires a No. 10 minimum, though upsizing for voltage drop or physical protection is common. The bonding jumper to the water pipe and gas line must be sized per the grounding electrode conductor table, not simply matched to the circuit. Verify the repair by: confirming neutral and ground buses are isolated at the subpanel with a resistance check, confirming the grounding electrode conductor is continuous to the concrete electrode, measuring voltage between the welder case and a known ground reference under load (should be near zero), and performing a fall-of-potential or clamp-on ground resistance test on the electrode system. A thermal scan of the feeder and bonds under welder load will reveal any remaining high-resistance joints.