Service Grounding, the Water Pipe, and a Detached Garage
A 400 A, 120/208 V, 3-phase service to a small commercial building uses 500 kcmil Cu service-entrance conductors. A 1-1/4 in copper water service enters the building and runs 45 ft through the building before teeing to fixtures; the water meter sits at the point of entry. The electrician drives one 5/8 in x 8 ft ground rod, runs 6 AWG Cu to it, and runs a 6 AWG Cu bonding jumper from the service panel to the water pipe above the meter, 'because 6 AWG is what we always use for grounds.' The footing contains 40 ft of #4 rebar. A 100 A feeder with three 3 AWG Cu conductors and an 8 AWG Cu equipment grounding conductor supplies a detached garage; there the electrician drives a rod and lands it, along with the feeder neutral, on the same neutral bar.
1. Size the grounding electrode conductor and the water-piping bonding jumper for the service, and list what is missing or wrong on the service side.
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Grounding electrode conductor to the water pipe:
- 500 kcmil Cu service conductors → Table 250.66 requires 1/0 Cu. The installed 6 AWG is badly undersized.
Bonding jumper to interior metal water piping (250.104(A)):
- Also sized from Table 250.66 based on the service conductors = 1/0 Cu. Size alone isn't enough: it must land on the grounding electrode conductor or the grounded service conductor at an accessible point (250.68), and it must be continuous or spliced only with an irreversible connection (250.64(C)).
What else is wrong or missing:
- Water meter must be bonded around with a jumper sized per 250.66 (250.53(D)(1)) so the electrode path does not depend on the meter.
- Interior piping more than 5 ft from the point of entry must be bonded (250.53(D)(1)).
- A supplemental electrode is required because the underground water pipe is a grounding electrode (250.53(D)(2)); the driven rod satisfies this, and 250.66(A) permits the rod conductor to stop at 6 AWG Cu - so 6 AWG to the rod is fine.
- The concrete-encased electrode is missing (see the next task).
The classic error here is applying the '6 AWG rule' to the water pipe. That rule applies to rod/pipe/plate electrodes, not to the water-piping electrode or its bonding jumper.
2. Evaluate the detached-garage grounding and bonding as installed.
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Grounding electrode at the separate structure (250.32(A)): Required for a 100 A feeder; the driven rod is the right idea.
GEC sizing: Table 250.66 is applied to the largest ungrounded feeder conductor (3 AWG Cu), which falls in the '2 AWG or smaller' row → 8 AWG Cu. 250.66(A) says the conductor to a rod need not be larger than 6 AWG, so an 8 AWG is the correct minimum and is acceptable. Installing 6 AWG is also fine (larger), but 8 AWG is not a violation.
The real defect - the neutral bar: Because an equipment grounding conductor is run with the feeder (250.32(B)(1)), the grounded conductor must not be bonded to the enclosure, the EGC, or the grounding electrode at the garage. The neutral must be isolated from the enclosure and terminate on an isolated neutral bar; the 8 AWG EGC and the grounding electrode conductor must land on the equipment grounding terminal bar. Landing the rod on the neutral bar creates a parallel neutral return path through earth and the grounding system - exactly what 250.32(B) prohibits where a separate EGC is provided.
Also verify: any green bonding screw or strap in the garage panel must be removed, and the feeder neutral must be insulated from the enclosure throughout.
3. The electrician says the rod and the water pipe are 'good enough' and that the footing rebar cannot be used because the building is already poured. Is he correct?
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No. 250.50 requires that all available grounding electrodes present at the building be bonded together into a single grounding electrode system. The list in 250.52(A) includes:
- Metal underground water pipe
- Metal frame of the building
- Concrete-encased electrode - defined in 250.52(A)(3) as at least 20 ft of bare copper conductor not smaller than 4 AWG encased in concrete, or at least 20 ft of 1/2 in or larger reinforcing bar (or a listed equivalent)
- Ground ring
- Rod/pipe/plate electrodes
- Other listed electrodes
The footing contains 40 ft of #4 rebar, which satisfies 250.52(A)(3). That electrode is available, so it must be used and bonded. The fact that the concrete is already poured does not exempt it; the bare end of the rebar needs to be exposed and made accessible for the connection (250.68), and the connection must be made with a listed concrete-encased electrode connector or an exothermic weld.
Sizing the conductor to the concrete-encased electrode: Table 250.66, based on the 500 kcmil service conductors → 1/0 Cu, and 250.66(B) permits a 4 AWG Cu where the conductor runs only to a concrete-encased electrode. Either approach must be documented in the grounding electrode conductor routing.