Weekly digest #80: grounding gotchas

Grounding mistakes don't announce themselves. They sit quiet until a fault current finds a path you didn't plan for, or an inspector flips a panel cover and spots a bond where it shouldn't be. This week's digest covers the grounding traps that keep showing up on service calls, rough-ins, and failed inspections.

The neutral-ground bond belongs in exactly one place

On a service, the grounded conductor and equipment grounding conductor bond together at the service disconnect per NEC 250.24(A)(5) and 250.24(B). Past that point, they stay separate. Every sub-panel downstream runs with an isolated neutral bar and a separate EGC bar bonded to the enclosure.

The gotcha shows up on detached buildings and older remodels. Feeders to a detached structure installed before the 2008 cycle sometimes carried a re-bond at the second building. Current code under 250.32(B) requires a four-wire feeder with the neutral isolated at the remote structure, unless you're grandfathered and meet the narrow exception. If you're replacing a panel at a detached garage, pull the bond screw and run a proper EGC.

• Service disconnect: neutral bonded to ground, main bonding jumper installed per 250.28
• Every sub-panel after that: bond screw out, EGC bar bonded to can, neutrals floating
• Detached structure fed by a feeder: treat it like a sub-panel, not a service

Ground rods are not a substitute for an EGC

A driven rod does one job: it ties the system to earth to stabilize voltage and handle lightning or line surges per 250.4(A)(1). It does not clear a ground fault. Earth resistance is too high to trip a breaker on a 120V fault, and 250.4(A)(5) is explicit that the earth shall not be used as the sole equipment grounding conductor.

When a customer says "just drive a rod at the shed and call it grounded," they're describing a code violation and a shock hazard. The EGC back to the source is what clears the fault. The rod is additional, not alternative.

Field tip: if you're troubleshooting a hot chassis on a detached building, measure from the enclosure to a known neutral at the main. If you read significant voltage, the EGC path is compromised. A second rod won't fix it.

Two rods, six feet apart, or prove the single rod

NEC 250.53(A)(2) requires supplemental electrodes unless a single rod tests at 25 ohms or less to earth. Most crews skip the fall-of-potential test and just drive two rods, which is faster and cheaper than renting a clamp meter or 3-point tester. Space them at least six feet apart per 250.53(A)(3), and bond them together with a continuous, unspliced GEC sized from 250.66.

Common failures at inspection:

1. Rods driven three feet apart because that's where the sidewalk ended
2. GEC spliced at a ground bar instead of running continuous, or spliced with an unlisted connector
3. Acorn clamp torqued onto painted rebar or a corroded rod, no listed connection
4. GEC run through ferrous conduit without bonding both ends per 250.64(E)

Bonding around water, gas, and CSST

Metal water piping that's likely to become energized gets bonded per 250.104(A), sized from 250.66 based on the service conductors. If the water service is plastic for the first five feet, you've lost your water pipe electrode and need to make up the electrode system elsewhere.

Gas piping bonding trips people up. Interior metal gas piping that is likely to become energized is bonded per 250.104(B), and the EGC of the circuit that may energize it is typically permitted to serve as the bonding means. CSST is different. Most manufacturers and 250.104(B) require a dedicated bonding jumper, minimum 6 AWG copper, connected to the gas line downstream of the meter and to the grounding electrode system. Check the manufacturer instructions, they carry code weight under 110.3(B).

Receptacle grounding on older work

Two-wire branch circuits without an EGC still exist in plenty of houses. NEC 406.4(D) gives you three legitimate options when replacing a receptacle on an ungrounded circuit: leave it as a two-prong, replace with a GFCI and label "No Equipment Ground," or protect downstream receptacles with a GFCI ahead of them and label the same way.

What you cannot do is land a bootleg ground by jumping neutral to ground at the device. It looks fine on a three-light tester. It kills people when the neutral opens and energizes the chassis of anything plugged in.

Field tip: a plug-in tester that shows "correct" only proves three wires are landed on three screws. It does not prove the EGC is continuous back to the panel. On an old house, put a meter from hot to ground and hot to neutral. If ground reads noticeably lower than neutral under load, you've got a bootleg.

GEC sizing and the irreversible connection

Size the grounding electrode conductor from Table 250.66, based on the largest ungrounded service conductor. The exceptions in 250.66(A), (B), and (C) cap the GEC at 6 AWG to a rod, 4 AWG to a concrete-encased electrode, and 6 AWG to a ground ring, regardless of service size.

Connections to the electrode have to be listed and, if buried or concealed, made by exothermic welding or a listed irreversible compression connector per 250.70. A split-bolt on a buried rod connection will fail inspection and will corrode open in a few seasons. Spend the three minutes on a proper acorn or a cad-weld, and torque it.

• 200A service, copper: 4 AWG GEC to water pipe, 6 AWG permitted to rods
• 400A service, copper: 1/0 AWG to water pipe, 6 AWG still permitted to rods
• Aluminum GEC: allowed per 250.64(A), but not within 18 inches of earth and not in contact with masonry

Grounding is the part of the job that only matters when everything else has already gone wrong. Build the electrode system, bond what needs bonding, keep the neutral and ground separated after the service, and document the ungrounded receptacles you can't fix today. The inspector sees the label. The next electrician sees the label. The homeowner's insurance sees the label.