Bonding jumper for rigid conduit through an eccentric knockout

Short answer: bond the raceway at every knockout ring left in the wall. On a circuit over 250 V to ground, the NEC calls for a bonding jumper or a listed bonding fitting at every concentric or eccentric knockout a metal raceway passes through, and a 480Y/277 V feed sits at 277 V to ground. Punch the knockout clean, crescent and all, and standard listed fittings make the bond on their own. Size the jumper from the breaker ahead of your run, not from the load: 10 AWG copper behind a 50 A breaker, 8 AWG behind 100 A.

What the NEC asks for over 250 volts to ground

The trouble is mechanical, and you can see it on any scrap of enclosure wall. An eccentric knockout leaves a crescent of sheet steel attached on one side of the hole; a concentric one can leave a full ring. A standard locknut grips that crescent, not the wall. The scored joint under the ring cannot be counted on to carry fault current, and that is the whole problem. The NEC closes it for circuits over 250 volts to ground: bond around the ring with a jumper, or land the raceway on a fitting listed for bonding. UL's guide to concentric and eccentric knockouts applies the section the same way, asking for the bonding means wherever a ring survives and taking standard fittings where the knockout is fully removed.

Below 250 V to ground the ring sits outside that rule, and the fault-path rules still bind. The raceway and its terminations have to conduct the fault current they are likely to carry (NEC 250.96(A)). The bond runs through metal; the earth takes no part of it (NEC 250.4(A)(5)). So the bonding locknut earns its minute at an eccentric ring on any voltage, and a clean hole needs none of it. NEC 250.12 shapes the contact surfaces: paint, lacquer and enamel come off the threads and faces at bonding connections, unless the fitting is made to bite through them. On a coated box, that line decides the entry. The grounding and bonding guide lays out the full picture, from the effective fault path to bonding on coated walls.

Volts to ground picks your column. On a grounded wye, divide line-to-line by 1.732: 480 ÷ 1.732 = 277 V, over the line, and 208 V lands at 120 V. A corner-grounded 240 V delta sits at 240 V. An ungrounded 480 V delta can measure a full 480 V to ground.

What you need

Most of the kit is one fitting and one wire:

• A grounding (bonding) bushing with a lug sized for the jumper, or a bonding locknut listed for the job
• A threaded hub or a standard locknut pair for holes punched clean
• Green insulated wire for the jumper; NEC 250.119 wants green or green with yellow stripes on insulated grounding conductors 6 AWG and smaller
• Bonding fittings with biting teeth, or a file, for painted and powder-coated walls
• A low-resistance ohmmeter for the checks afterwards

Read the row at the device rating, not at the load: a 20 A drive behind a 50 A breaker still takes the 10 AWG jumper. Aluminum runs larger, 4 AWG aluminum behind a 200 A device where copper takes 6 AWG.

How to install the bond

1. Lock the circuit out and verify absence of voltage before the first wrench turns.
2. Read volts to ground off the system: line-to-line ÷ 1.732 on a wye. Over 250 V means the jumper or bonding fitting is required at every ring.
3. Knock the ring out whole, crescent included, and deburr the hole.
4. Bare the contact faces on your painted or powder-coated wall, or fit bonding fittings with biting teeth.
5. On a clean hole, land the conduit on two standard locknuts and a bushing, or on a threaded hub.
6. Where a ring survived, or the circuit runs over 250 V to ground, add the grounding bushing with its jumper to the enclosure ground terminal, or set a listed bonding locknut instead.
7. Keep your jumper short and direct, land it on the ground terminal or the EGC, and dress it clear of door edges and moving parts.

Worked example: a 460 V drive feed into a disconnect

A 20 hp drive feeder leaves the MCC on a 50 A breaker, 480Y/277 V, in rigid metal conduit. The conduit lands in the drive disconnect through a factory eccentric knockout, crescent left in the wall. Here is the whole decision in four lines:

• Volts to ground: 480 ÷ 1.732 = 277 V, over 250, so the bonding rule applies at this termination.
• Ring in the wall, so a standard locknut fails the rule; a bonding means is required.
• Jumper size: 50 A device, so NEC Table 250.122 reads 10 AWG copper.
• Painted disconnect body, so the locknut faces get bared or the fitting carries biting teeth.

Built one way, a grounding bushing sits inside with its 10 AWG jumper on the ground terminal, and the standard locknut outside grips bare metal. Built the other way, a single bonding locknut does the job and the jumper disappears. The feed runs on to a second disconnect, and that entry gets the same treatment; each termination stands on its own bond. Where the rigid conduit itself carries the ground, the bond at each termination carries the whole fault, and rigid conduit as a grounding conductor with NEMA 4X boxes covers that arrangement.

Order the entry differently and the problem never starts. A custom box takes the entry as a plain 1-3/8 in (35 mm) hole for a 1 in hub; the ring never exists, a gasketed standard hub makes the bond, and the jumper leaves the parts list. Hub sizes, sealing rings and spacing on a drive box have their own article: knockouts and fittings on a NEMA 4X drive enclosure.

Checks when done

• Sight the hole: no crescent, no ring, bare metal under both locknut faces.
• Measure from conduit to the enclosure ground terminal, circuit dead, with a low-resistance ohmmeter. A direct metal joint of a square inch or two reads milliohms; anything above about an ohm tells you paint or a ring remnant sits in the path.
• Check the jumper end to end, bushing lug to ground terminal, in the same session.
• Walk the box and test every raceway entry, not just the first one you wired.

Mistakes that undo the bond

• Trusting the crescent. The scored joint is the failure the rule was written to remove.
• Sizing from the load. The 20 A drive behind the 50 A breaker still takes the 10 AWG jumper, not a 12 AWG leftover.
• Bonding one entry of two. Every termination on the box needs its own bond.
• Leaving paint under the locknut on a coated box; the ohmmeter finds it in seconds.
• Dropping the seal. A bare bonding locknut on a Type 4X box leaks at the first hose-down, and the entry fitting sets the assembly rating. The hub-versus-sealing-locknut decision for outdoor top entries covers the seal side.

Order the entry so the ring never exists

Custom enclosures take cutouts from your drawing, so each entry leaves as a plain round hole sized to the punch: 7/8 in (22 mm) for a 1/2 in hub, 1-3/32 in (28 mm) for 3/4 in, 1-3/8 in (35 mm) for 1 in. No ring survives, so the jumper question closes before the box ships. The cutouts and glands guide walks through the hole sizes and sealing hardware entry by entry. On 304 or 316L stainless the wall is bare metal and the coating rule never comes up; on powder-coated carbon steel, use the biting fittings or wipe the faces to bare metal. Non-metallic fiberglass and polycarbonate boxes change the geometry once more: with metal raceways entering, the metal parts bond to each other inside the box (NEC 314.3).

The wall-mount and HMI stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Where a spec asks for a listed box, the manufacturer rating versus UL 50E listing difference has its own guide, and an unlisted enclosure inside a listed control panel has its own path too. Every box, custom included, is delivered in 10 business days, counted from your approval of the drawing.

The decision, case by case

Whatever the case, the test is the same: ring gone or bond fitted, bare metal under the fitting, and milliohms from conduit to ground terminal.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions