Service Grounding Electrode Conductor and Water Pipe Bonding
A 400 A, 120/240 V single-phase service is being inspected at a small commercial building. The service-entrance conductors are 600 kcmil copper, protected by a 400 A main breaker. The grounding electrode conductor to the metal water pipe is 4 AWG copper, run 35 ft in PVC. A separate 6 AWG copper conductor runs from the service panel to a 5/8 in × 8 ft ground rod. The metal water service has more than 10 ft of buried contact, and the water meter is 12 ft inside the building. The electrician also bonded around the water meter with a 6 AWG jumper. During the inspection, the owner mentions that the water utility may replace the metal service with plastic pipe next year. The panel has a single neutral bar with the main bonding jumper installed.
1. What size grounding electrode conductor is required to the metal water pipe, and is the installed 4 AWG acceptable? Show the table logic.
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The water pipe is a grounding electrode, so the grounding electrode conductor is sized from Table 250.66 using the largest ungrounded service conductor. A 600 kcmil copper service conductor requires a 2/0 copper grounding electrode conductor. The installed 4 AWG is not acceptable. The 6 AWG maximum in 250.66(A) applies only to rod, pipe, and plate electrodes; it does not apply to a metal water pipe electrode. Replace the 4 AWG with 2/0 copper, keep the run continuous, and protect it where exposed. Do not size from the 400 A breaker; the table is based on conductor size, not breaker rating.
2. The electrician used a 6 AWG jumper around the water meter. Is that correct? What section governs, and what size is required?
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250.68(B) requires the bonding jumper around a water meter to be sized per 250.66 because it maintains the grounding electrode conductor path. With 600 kcmil service conductors, the jumper must also be 2/0 copper. The 6 AWG jumper is undersized. Install listed pipe clamps on both sides of the meter, keep the connection accessible, and ensure the path remains continuous if the meter is removed. This jumper is part of the grounding electrode conductor, not a separate interior water-piping bond under 250.104.
3. If the water service is replaced with plastic, what must be done to maintain a compliant grounding electrode system?
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Once the metal water pipe is replaced with plastic, it is no longer a grounding electrode. The ground rod becomes the sole electrode, so 250.53(A)(2) requires a supplemental electrode unless the single rod has 25 ohms or less to earth; a second rod at least 6 ft away is the usual solution. The grounding electrode conductor to the rod is permitted to be 6 AWG copper maximum under 250.66(A). The interior metal water piping must still be bonded under 250.104(A) using Table 250.122 for the 400 A service, which is 3 AWG copper. Check any metal gas piping under 250.104(B) as well.