Grounding and Bonding a 200 A Service Upgrade
A contractor is upgrading a detached single-family house from a 100 A fuse panel to a 200 A, 120/240 V single-phase service. The service conductors are 2/0 copper. The underground metal water main enters the basement at the front wall and is in earth contact for more than 10 ft; the interior copper runs 15 ft before a CPVC transition, and the water meter is 20 ft inside. A gas-fired furnace is supplied by black iron gas piping that is bonded only by the equipment grounding conductor of the furnace circuit. At the rear, two 8 ft ground rods were driven 6 ft apart and connected with a bare 6 AWG copper conductor. The existing water-pipe grounding electrode conductor is 10 AWG copper, clamped to the pipe 12 ft from the point of entry. The electrical inspector has not yet visited. The contractor must decide what to fix before calling for inspection.
1. Size the grounding electrode conductor to the water pipe and to the ground rods, and explain why the two sizes can differ.
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Table 250.66 sizes the grounding electrode conductor by the largest service conductor. The service conductors are 2/0 copper, and the table row for 2/0 or smaller copper requires a 4 AWG copper grounding electrode conductor. That is the size required for the metal underground water pipe, which is a grounding electrode under 250.52(A)(1) because it is in direct contact with the earth for 10 ft or more. The 15 ft of interior copper before the CPVC transition does not disqualify the underground portion; the CPVC only means the piping downstream of the transition is not part of the electrode.
For a ground rod, 250.66(A) permits the grounding electrode conductor to be no larger than 6 AWG copper. The connection to a single rod does not have to be larger than 6 AWG, so the existing 6 AWG to the rods is acceptable even though the water-pipe conductor must be 4 AWG.
The 10 AWG on the water pipe is undersized and must be replaced with 4 AWG copper or the equivalent. Also verify the conductor is continuous, run as short and straight as practicable, and protected where exposed (250.64(B)).
2. Identify each defect in the existing grounding and bonding and state the correction.
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- Water-pipe grounding electrode conductor: 10 AWG is undersized. Replace with 4 AWG copper per Table 250.66.
- Connection point: clamped 12 ft from the point of entry. The connection must generally be made within 5 ft of the point of entry and must be accessible (250.68). Move the clamp to within 5 ft.
- Water meter: a bonding jumper is required around the meter so the piping does not depend on the meter for continuity (250.53(D)(1)). Install a jumper sized at least as large as the grounding electrode conductor.
- Gas piping: 250.104(B) requires the black iron gas piping to be bonded to the grounding electrode system. Using the furnace equipment grounding conductor is permitted in some cases because that circuit is likely to energize the piping, but the bond must be sized per 250.122. A dedicated, accessible bonding jumper to the grounding electrode system is the clearer, more defensible installation.
- Ground rods: two 8 ft rods spaced 6 ft apart satisfy the supplemental-electrode and spacing rules, but 6 ft is exactly the minimum (250.53(A)(3)). Verify the measurement and confirm the clamps are listed for direct burial and remain accessible.
3. Is a supplemental electrode required because of the water pipe, and does the ground rod installation comply with the rules for a single rod?
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250.53(D)(2) requires a supplemental electrode when the metal underground water pipe is the only grounding electrode. Here the two driven rods serve as the supplemental electrodes, so that requirement is met, provided the rods are actually connected to the grounding electrode system with an acceptable conductor and the water-pipe conductor is not the only path.
For the rods themselves, 250.53(A)(2) requires a second rod or other supplemental electrode unless a single rod measures 25 ohms or less. Because two rods are installed, the 25-ohm test is not required. The spacing rule in 250.53(A)(3) requires the rods to be at least 6 ft apart; 6 ft meets the minimum with no margin, so verify the actual spacing before inspection. Each rod must be at least 8 ft long and listed, and must be driven below permanent moisture level.
The 6 AWG conductor to the rods is the maximum size that can be required, not a minimum, so it is acceptable as installed. The water pipe does not require a separate 'supplemental' conductor beyond the 4 AWG grounding electrode conductor once the rods are part of the system.