4.8 Equipment Grounding Conductor Installation & Fault-Path Verification (0304)

The Job of the Equipment Grounding Conductor

250.4(A)(5) states the performance objective plainly: the fault-current path must be permanent and electrically continuous, must be capable of safely carrying the maximum fault current likely to be imposed, and must have sufficiently low impedance to facilitate the operation of the overcurrent device. The same subsection ends with a sentence worth memorizing: the earth shall not be considered as an effective ground-fault current path. Driving another rod does not fix a broken equipment grounding path — soil resistance is far too high to carry enough current to trip a breaker.

What Counts as an Equipment Grounding Conductor — 250.118

Two of these carry the traps. FMC and LFMC are equipment grounding conductors only inside a small envelope — short lengths, small overcurrent devices, listed fittings, and no use where flexibility is needed after installation. The moment a run exceeds 6 ft of flex, or the overcurrent device exceeds the stated rating, or the flex was installed specifically so the equipment could move, a wire-type equipment grounding conductor must be pulled. A 60 A motor circuit ending in 3 ft of liquidtight is fine; the same motor on a 100 A circuit is not, and a vibrating machine that requires flexibility needs the wire regardless of length.

The Grounding Conductor Travels With Its Circuit — 250.134 and 300.3(B)

300.3(B) requires all conductors of the same circuit — every ungrounded conductor, the grounded conductor, and all equipment grounding conductors and bonding conductors — to be contained within the same raceway, trench, cable, or cord, with narrow exceptions. 250.134 applies the same principle to fixed equipment: the equipment grounding conductor must be contained within the same raceway, cable, or cord, or otherwise run with the circuit conductors.

The reason is inductance, not convenience. A conductor carrying fault current and its return path form a loop; the larger the loop, the higher the inductive reactance and the lower the fault current. A grounding conductor pulled through a separate conduit twenty feet away can raise the impedance of the fault path enough to keep a breaker from opening on a bolted ground fault. It is a defect you cannot measure with a continuity tester — the path is continuous, it just cannot deliver current fast enough.

The same physics is why 300.20(A) requires all conductors of a circuit to pass through the same opening in ferrous metal enclosures: a single conductor passing through its own hole in a steel panel induces circulating currents in the steel and heats it.

Bonding Receptacles to Boxes — 250.146

Where a receptacle is mounted to a metal box that is connected to an equipment grounding conductor, an equipment bonding jumper is required between the receptacle grounding terminal and the box, unless one of the recognized alternatives applies:

• 250.146(A) — Surface-mounted box with direct metal-to-metal contact between the device yoke and the box, and the receptacle is not required to be tamper resistant. At least one of the insulating retaining washers must be removed.
• 250.146(B) — Contact devices or yokes listed and identified for the purpose of establishing the grounding circuit between the yoke and flush-type boxes — the listed self-grounding receptacle with its spring clip on the mounting screw.
• 250.146(C) — Floor boxes listed for the purpose to provide the required grounding continuity.
• 250.146(D) — Isolated ground receptacles, where the equipment grounding conductor is run insulated back to the applicable derived-system or service enclosure. Note that the box itself still requires its own equipment grounding connection; the isolated conductor is in addition to it, not instead of it.

Drywall-ring installations are the ones to watch. A receptacle screwed into a plaster ring on a recessed box is not in direct metal-to-metal contact under 250.146(A), so it needs either a bonding jumper or a listed self-grounding device.

Objectionable Current — 250.6

250.6(A) requires grounding and bonding to be arranged so that objectionable current does not flow over grounding conductors or grounding paths. 250.6(B) lists the permitted remedies where objectionable current is found: discontinue one or more but not all of the connections, change the locations of the connections, interrupt the continuity of the conductor or conductive path, or take other suitable remedial and approved action.

The overwhelmingly common cause on a commercial job is a second neutral-to-ground bond in a downstream panelboard — a green bonding screw left installed, or a neutral bar strapped to the enclosure. That puts the equipment grounding conductors, the conduit system, the building steel, and the water piping in parallel with the neutral, so normal load current divides across all of them. The symptom is measurable: clamp an ammeter around the equipment grounding conductor or around the conduit feeding the panel with the load on, and you read current that should be zero.

The fix is to remove the extra bond, and the section is explicit that the remedy is not to add more grounding. 250.6(C) also makes clear that currents introduced for the purpose of operating equipment, such as ground-detection or ground-fault protection systems, are not objectionable currents.

Three-minute field verification. With the panel energized and loaded, clamp the feeder equipment grounding conductor: it should read essentially zero. Open the panel and confirm the neutral bar is isolated from the enclosure on every panel except the service. Then confirm every flex connection over 6 ft, or over its ampere limit, has a wire-type equipment grounding conductor inside it.