Service Replacement Grounding and Bonding
A 200-ampere, 120/240-volt single-phase service is being replaced in a 1960s house. The service-entrance conductors are 3/0 copper. The metal water service enters the basement and is used as a grounding electrode; a driven rod is also present. The electrician drives a second rod 8 feet away but does not measure resistance. He runs a 6 AWG copper conductor from the neutral bar to the water pipe and a 6 AWG copper conductor to the rods. The 60-ampere subpanel for a detached garage is fed with 6/3 NM cable, and the electrician plans to land the neutral and equipment grounding conductors on the same bar in the garage panel. The inspector asks for the grounding electrode conductor size, the water-pipe bonding jumper size, and the reason for the second rod.
1. What size grounding electrode conductor and water-pipe bonding jumper are required for the 3/0 copper service conductors?
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The grounding electrode conductor to the water pipe is sized from Table 250.66 based on the largest service-entrance conductor. For 3/0 copper, Table 250.66 requires 2 AWG copper. The 6 AWG installed is undersized for the water pipe electrode. The connection to a driven rod is different: 250.66(A) permits the grounding electrode conductor to the rod to be no larger than 6 AWG copper, so 6 AWG to the rods can be acceptable. The water-piping bonding jumper is sized by 250.104(A) using Table 250.66, so it also needs to be 2 AWG copper for 3/0 service conductors. The electrician appears to have confused the rod allowance with the water-pipe requirement. The water pipe is both an electrode and a metallic piping system that must be bonded, and the larger 2 AWG requirement controls.
2. Was the second ground rod required, and what must the electrician do about the missing resistance test?
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A single rod is permitted only if it has a resistance to earth of 25 ohms or less. If the resistance is not measured or is greater than 25 ohms, a second rod must be installed and bonded to the first. Driving a second rod without measuring is a conservative approach that satisfies 250.53(A)(2) because the two-rod system is installed. The rods must be at least 6 feet apart; 8 feet is acceptable. The electrician should record the second rod and the connection in the inspection record. He does not need to prove 25 ohms once the second rod is installed, but he must ensure the rod is driven to the required depth, the connection is accessible, and the conductor is protected where exposed. The 6 AWG to the rods is permitted, but the water-pipe GEC still needs to be 2 AWG.
3. The garage subpanel has neutral and equipment grounding conductors on the same bar. Is that acceptable, and what should be changed?
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No. The garage is a separate structure fed by a feeder, so the neutral and equipment grounding conductors must be separated. The equipment grounding conductor must be landed on a separate equipment grounding bar, and the neutral must be isolated from the enclosure and from the equipment grounding conductors. The feeder needs an equipment grounding conductor in addition to the neutral; 6/3 NM cable with a bare equipment grounding conductor can provide that, but the bare conductor must not be used as a neutral. A grounding electrode system is also required at the detached structure under 250.32, typically a rod or other electrode bonded to the equipment grounding bar. The electrician must remove the bonding screw or strap in the garage panel if it is a main-lug panel being used as a subpanel, and must not create a parallel neutral path through the enclosure.