400 A Service Grounding Electrode System and a Detached Garage
A 400 A, 120/240 V single-phase service is installed on a rural house. Service-entrance conductors are 500 kcmil copper. The only grounding electrode present is 1-1/4 in. metal underground water pipe with about 12 ft of earth contact; the water meter is 15 ft downstream of where the pipe enters the building. There is also a 3/4 in. metal gas line, a phone/CATV entrance, and a grounding electrode conductor run to a single 8 ft ground rod. A 100 A feeder in 2 in. PVC with four conductors supplies a detached garage containing a 12-circuit panel, a metal water pipe with 10 ft of earth contact, and a concrete slab.
1. Evaluate the grounding electrode system and size each grounding electrode conductor.
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The underground water pipe is a grounding electrode - 250.52(A)(1) requires at least 10 ft of metal pipe in direct earth contact, and 12 ft qualifies.
Supplemental electrode is mandatory. Because the water pipe is the only electrode present, 250.53(D)(2) requires a supplemental electrode; the single 8 ft ground rod satisfies that. However, ensure the rod is installed per 250.53(A) - 8 ft in length, 8 ft of cover, and, since two or more electrodes are present, no earth-resistance measurement is required.
GEC sizing (250.66):
- Water pipe GEC: based on the 500 kcmil Cu service conductors, which fall in the 'over 350 through 600 kcmil' row → 1/0 Cu.
- Rod GEC: 250.66(A) limits the required size to 6 AWG Cu maximum - the rod connection never needs to be larger than 6 AWG.
Water meter bonding (250.68(B)): the meter must be made electrically continuous, either by a bonding jumper sized per 250.66 or by a listed meter yoke. Fifteen feet of unbonded metallic pipe downstream of the service point is a real hazard - the meter must not be the only path.
If a concrete-encased electrode were available it would be preferred, but a slab only qualifies if 20 ft of 1/2 in. rebar or 4 AWG bare copper is encased within the footing itself.
2. Must the gas piping and the communications entrance be bonded, and what must be done for the underground water line in the garage?
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Gas piping: the metal gas piping must be bonded to the grounding electrode system per 250.104(B), sized per 250.122 using the rating of the circuit likely to energize the piping. This is a bonding connection, not a grounding electrode - the gas line cannot be used as an electrode (250.52(B)).
Communications: a listed intersystem bonding termination must be provided at the service equipment (250.94) so the phone/CATV providers can bond their shields; the termination must be accessible and connected to the service enclosure or grounding electrode conductor.
Garage water pipe: the 10 ft of metal underground pipe in the garage makes it a grounding electrode for the separate structure. Under 250.32, the garage panel needs a grounding electrode system with a grounding electrode conductor brought to the panel's equipment grounding bar. The water pipe must be bonded at the panel and, if the garage panel is fed with an equipment grounding conductor from the house (4-wire), the EGC and the panel grounding bar work together - the neutral bar must stay isolated. A water-pipe bonding jumper sized per 250.66 may be needed at the panel to bond the pipe; do not rely on the pipe as the fault-current path back to the house.
3. The owner wants to add a hot tub on the garage slab. What grounding change does this force?
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A hot tub on the concrete requires a equipotential bonding grid (680.26) - a No. 8 bare copper conductor around the tub, bonded to all metal parts, the water, and any metal conduit or equipment within reach. The garage panel's electrode system and the EGC must be solid before the tub is energized.
Key checks:
- 680.26(B): bond all metal parts and equipment; bond any metallic piping within 5 ft.
- 680.26(C): if a concrete slab is used, bond the perimeter and the slab if it contains reinforcing metal - you cannot just 'ground it because it is wet.'
- The hot-tub branch circuit needs its own GFCI protection (680.44) and a disconnect within sight but at least 5 ft away (680.41).
- Verify the garage feeder equipment grounding conductor is sized correctly for the 100 A feeder (8 AWG Cu, 250.122), since the hot tub adds fault-current duty on that path.