Grounding and Bonding a Service with a Metal Water Pipe and Ground Rod
A 400 A, 120/240 V single-phase service is being installed at a small commercial building. The service has 500 kcmil copper service-entrance conductors. A metal underground water pipe enters the building and is used as a grounding electrode, and a 10 ft ground rod is also present. The water pipe has 30 ft in contact with the earth. The electrician must size the grounding electrode conductor, bond the water pipe, and connect the ground rod, and must explain why both electrodes are required.
1. Size the grounding electrode conductor and the water pipe bonding jumper.
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Per 250.66, the grounding electrode conductor for 500 kcmil copper service conductors is 1/0 copper (Table 250.66). The metal underground water pipe is a grounding electrode per 250.52(A)(1) if it has at least 10 ft in contact with the earth; 30 ft qualifies. The water pipe must be bonded to the service equipment with a bonding jumper sized per 250.66, also 1/0 copper. The ground rod is a supplemental electrode and requires a grounding electrode conductor sized per 250.66, but 250.66(A) permits it to be no larger than 6 AWG copper for a rod, pipe, or plate electrode. So the rod connection can be 6 AWG copper. The water pipe connection must be made within 5 ft of the point of entry and must not rely on a removable fitting.
2. Why are both the water pipe and the ground rod required, and what is the role of each?
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The metal underground water pipe is the primary grounding electrode and provides a low-impedance path to earth and to the utility water system. The ground rod is a supplemental electrode required by 250.53(D)(2) when the water pipe is the only grounding electrode, to provide a backup path if the water pipe is replaced with plastic or disconnected. The two electrodes must be bonded together with a bonding jumper sized per 250.66 (1/0 copper here). The grounding electrode conductor must be continuous and not spliced except by irreversible connections. The service neutral is bonded to the enclosure and grounding electrode conductor at the service disconnect only; downstream panels must keep neutral isolated.
3. The water utility replaces the metal service line with plastic. What must be done?
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If the metal water pipe is no longer in contact with the earth for 10 ft, it is no longer a grounding electrode under 250.52(A)(1), so the grounding electrode system must be re-established. The ground rod becomes the primary electrode, but a single rod must be supplemented by an additional electrode per 250.53(A)(2) unless it has a resistance of 25 ohms or less. Add a second ground rod at least 6 ft away, or use a concrete-encased electrode or building steel if available. The interior metal water piping must still be bonded per 250.104 even though it is not an electrode, sized per Table 250.122 based on the service overcurrent device. Re-inspect and update the permit.