Weekly digest #20: grounding gotchas

Why grounding trips up seasoned hands

Grounding and bonding carry more callbacks than almost any other rough-in category. The rules read clean on paper, but panels get added, subpanels get fed from detached structures, and someone swaps a water heater without re-evaluating the bonding jumper. The result is a system that passes a visual but fails under fault.

This digest walks through the recurring field traps, the article numbers that settle arguments on site, and the quick checks that keep your work out of the correction stack.

Grounding electrode conductor sizing

NEC 250.66 sizes the grounding electrode conductor based on the largest ungrounded service conductor. The catch is the exceptions in 250.66(A), (B), and (C). A GEC run solely to a rod, pipe, or plate electrode never needs to be larger than 6 AWG copper. A GEC run solely to a concrete-encased electrode never needs to be larger than 4 AWG copper. To a ground ring, never larger than the ring conductor itself.

Get this wrong in the wrong direction and you burn copper for no reason. Get it wrong the other way and you fail inspection on a 400 A service because someone ran 6 AWG to the ground bar instead of 1/0.

• Service conductor 2/0 copper or 4/0 aluminum: GEC is 4 AWG copper.
• Service conductor 3/0 to 350 kcmil copper: GEC is 2 AWG copper.
• Run to rod or pipe only: cap at 6 AWG copper regardless of service size.
• Run to Ufer only: cap at 4 AWG copper regardless of service size.

Bonding at separately derived systems

Transformers, generators with a switched neutral, and solar inverters configured as SDS all need a system bonding jumper per 250.30(A)(1) and a supply-side bonding jumper per 250.30(A)(2). One location only. Bond the grounded conductor to the equipment grounding conductor at either the source or the first disconnect, not both.

The second most common miss here is the grounding electrode conductor under 250.30(A)(4). You need a GEC to the nearest effectively grounded structural metal or metal water pipe within 5 feet of entry, or to another qualifying electrode. A transformer bolted to a basement pad with no electrode connection is a violation, not an oversight.

If the neutral is switched in your transfer switch, the generator is a separately derived system and needs its own bonding jumper and electrode. If the neutral is solid through, it is not. Check the switch, not the nameplate.

Feeders to detached structures

Since the 2008 cycle, 250.32(B) requires an equipment grounding conductor run with the feeder to a detached building. The old three-wire feeder with a neutral-ground bond at the second building is gone for new work. Four wires, neutral isolated from ground at the detached panel, EGC bonded to the enclosure.

Existing installations with the old bond can remain under 250.32(B)(1) Exception, but only if there is no continuous metallic path between structures and no ground fault equipment. Add a metal gas line, a phone ground, or a GFCI-protected circuit and that exception evaporates.

• Pull the EGC, size it per 250.122 based on the feeder OCPD.
• Remove the main bonding jumper at the detached panel.
• Drive electrodes at the detached structure per 250.32(A), minimum two rods 6 feet apart unless a single rod reads 25 ohms or less.
• Isolate the neutral bar from the can.

Metal water piping, 5 feet, and supplemental electrodes

A metal underground water pipe in direct contact with earth for 10 feet or more is an electrode under 250.52(A)(1), and it must be supplemented per 250.53(D)(2). That supplemental electrode is almost always a driven rod or the building steel. The connection to the water pipe must be made within the first 5 feet from where it enters the building, per 250.52(A)(1).

If the first 5 feet is inaccessible, the AHJ can allow a connection further in, but document it. The bonding jumper around the water meter required by 250.68(C)(1) is separate from the GEC and still gets pulled on plenty of services where someone assumed the meter strap was enough.

Receptacle-level grounding issues

Two-wire circuits in older homes do not get a bootleg ground from the neutral. Ever. The legitimate retrofits under 250.130(C) are to run an EGC back to the panel, to the grounding electrode conductor, to a grounded enclosure on the same circuit, or to install a GFCI and label it "No Equipment Ground" per 406.4(D)(2)(b).

On the commercial side, isolated ground receptacles per 250.146(D) still need an insulated EGC run back to the service or SDS ground point. The branch raceway EGC bonds the box; the insulated conductor bonds the receptacle ground terminal. Two paths, one purpose.

A tester that shows "correct" on a bootlegged receptacle is lying. Plug-in testers cannot distinguish a real EGC from a neutral tied to the ground screw. Pull the device or ring out to the panel if you are auditing existing work.

Quick field checks before you close up

Most grounding callbacks come down to four measurements and one visual. Run them on every service and SDS before the cover goes on.

1. Continuity from the furthest receptacle EGC to the grounding bus, less than 1 ohm on a healthy branch.
2. Resistance to earth at the rod, 25 ohms or less to skip the second rod, per 250.53(A)(2).
3. Neutral-to-ground voltage at a loaded receptacle, under 2 volts on a typical branch. Anything higher suggests a neutral problem or a second bond downstream.
4. GEC continuity from the electrode to the neutral bar, no splices except as permitted by 250.64(C).
5. Visual: one main bonding jumper, one location, labeled or obvious.

Catch these on the front end and the inspector becomes a formality. Miss them and you are pulling a cover in the rain next Tuesday.