Short answer: at a service, a standard locknut is never the only bonding means. The locknut supplied with a 3 in [76 mm] compression connector clamps the fitting; the bond needs its own connection. Add an accepted means: a bonding-type locknut, a bonding wedge under the locknut, a grounding-style Myers hub with a lug, or a bushing with a bonding jumper. Around an oversized, concentric or eccentric knockout — or a reducing washer — a bonding jumper goes in as well, at any voltage. Away from services the same jumper demand returns once the circuit runs over 250 V to ground. The standard locknut stays for the mechanical clamp; the bond rides on the fitting you add.
What the NEC demands at a service
The NEC requires the metal parts of a service — raceways, meter bases, CT cabinets, the disconnect enclosure — to be bonded to the grounded service conductor. The bond gives a fault a low-impedance path back to the source, and the earth is not part of that path: NEC 250.4(A)(5) calls the bonded-conductor route the effective ground-fault current path. NEC 250.96(A) sets the enclosure-side duty: metal parts that serve as equipment grounding conductors are bonded together so they can safely conduct any fault current likely to be imposed on them. Then the code names the fitting problem outright: a standard locknut or plain bushing never counts as the only means for the bonding a service needs. Read that sentence as a rule of its own — it covers clean full-size punches as much as damaged holes, on a 120/240 V service and a 277/480 V one alike.
The connections that count, in one table
| Connection at the service | Counts as the bond on its own? | How you use it |
|---|---|---|
| Standard locknut supplied with the connector | No | Keep it as the mechanical clamp on top of an accepted means |
| Bonding-type locknut | Yes | Swap it in; its hardened cone bites the wall as you torque it |
| Bonding wedge under a standard locknut | Yes | Fit it between locknut and wall where a coating or burr sits underneath |
| Grounding-style Myers hub with a lug | Yes | Run a jumper from the lug to the grounded conductor bar |
| Bushing with a bonding jumper | Yes | Land the jumper on the grounded conductor bar |
| Bonding jumper around the knockout | Required when the hole is impaired | Around oversized, concentric and eccentric knockouts and reducing washers |
Pick one accepted means and fit it properly. The bonding-type locknut is the quickest fix on a clean punch, because it stacks exactly where the plain one sat. The wedge earns its keep on coated or galvanized walls, where a plain locknut would ride on paint. The grounding-style hub is the fitting to buy for wet entries: its lug gives the jumper a listed terminal instead of leaving the bond to thread contact through a gasket. Each of these is a listed fitting, tested as a bond path rather than assumed to be one. Hub choice on a wet entry mixes sealing with bonding; the conduit hub vs sealing locknut comparison and the Myers hub with PVC conduit article cover the seal side.
Impaired knockouts: where the jumper is mandatory
A knockout is impaired when the fitting cannot reach full metal contact: the hole is oversized, the ring is concentric or eccentric and partly cut, or a reducing washer spaces the locknut off the wall. At a service the NEC orders a bonding jumper around all four cases, whatever the voltage. The jumper skips the doubtful joint and carries fault current straight to the bar. A reducing washer also hides paint and burrs at its face; the reducing washers as a bonding method article shows why it stays a mechanical part rather than a conductor, and the when a bonding bushing is required at a concentric knockout article walks the bushing choice.
Away from services the same demand returns over 250 V to ground: 277 V on a 277/480 V system, or 347 V on a 347/600 V one, puts a feeder over the line, so concentric and eccentric knockouts take bonding jumpers at any panel. A 120/240 V feeder sits under the line; its mid-run knockouts carry no such demand.
Sizing the bonding jumper
Size an equipment bonding jumper the way the NEC sizes an equipment grounding conductor: from the rating of the overcurrent device ahead of it, with NEC Table 250.122. The table gives 8 AWG copper for a 100 A device and 6 AWG copper for 150 A and 200 A devices. Land the jumper on a terminal listed for the strand count — the grounded conductor bar at a service, the ground bar downstream — and keep the run short, straight and out of the wireway. The bonding jumper size inside a metal junction box article works a case end to end, and the grounding and bonding guide covers bar and lug practice.
Working a 3 in punched knockout at a service
Open the cabinet only after lockout and a check for absence of voltage at the line side. The case: a CT cabinet wall takes a 3 in [76 mm] hole punched full-size for a 3 in compression connector, standard locknut in the bag, raceway running to the main disconnect over 1/0 copper service conductors, good for 150 A at 75 °C.
Inputs: a clean full-size punch; a compression connector with its supplied standard locknut; no other bonding hardware on hand; a conductor size that sets the jumper row if one is ever needed.
The rule in words: at a service the standard locknut never carries the bond alone, whatever the hole quality, so the connection needs one accepted means, and a jumper enters only when the hole is impaired.
Arithmetic. Impaired test: oversized, no; concentric, no; eccentric, no; reducing washer, none — the hole forces no jumper. Means test: the fitted hardware is one standard locknut, which the NEC excludes as the only means, so the accepted-means count is zero and the connection fails as installed. Fix A: swap in a 3 in bonding-type locknut; accepted means, one, and the connection passes. Fix B: fit a grounding-style hub with a lug and run a jumper to the grounded conductor bar. Had the hole been concentric, size that jumper from the 150 A device with NEC Table 250.122: 8 AWG copper covers 100 A and 6 AWG copper covers 200 A, so 150 A takes the 6 AWG copper row.
Result: a 3 in bonding-type locknut under the fitting, or the grounding hub plus a 6 AWG copper jumper. Decision: keep the standard locknut snug on top as the mechanical clamp and let the bonding-type locknut or the hub lug carry the bond; on this hole no jumper is needed, and any concentric or eccentric version of the same hole adds the 6 AWG copper jumper around it.
What it changes in a real panel
Keep the two cases apart. At service equipment, fit the accepted means every time; the locknut rule stands on its own and reaches clean punches and damaged holes alike. Downstream, a 120/240 V panel on a clean full-size punch can rest on the fitting's metal contact, and the special demand returns over 250 V to ground. Coatings matter next: NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces used for grounding and bonding, unless the fitting is designed to make removal unnecessary. That clause is why a bonding wedge or bonding-type locknut earns its price on a coated box: its points are made to bite through. Our 304 and 316L stainless boxes arrive in #4 brushed finish with no coating at all, so a punched hole lands on bare metal ready for the hub.
What to put on the order
List bonding hardware against each raceway entry: a bonding-type locknut per entry at the service, a grounding-style hub with a lug for wet entries, bonding wedges where a coated wall sits under a standard locknut, and bonding bushings with jumper stock in the Table 250.122 sizes for any impaired hole. Order the holes punched full-size at the factory rather than knocking concentric rings on site; a clean punch settles the impaired-connection question before the box ever leaves. Send hub sizes and positions with the drawing through the custom stainless enclosures with factory cutouts page or the custom enclosure builder, and the cutouts arrive with the box, delivered in 10 business days anywhere in the world, counted for custom work from your approval of the drawing. The cutouts, glands and windows guide lists knockout and hub dimensions to draw against.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is a standard locknut ever the only bond at a service?
A standard locknut is never the sole bond at a service, at any voltage and on any knockout, clean or damaged. Keep it as the mechanical clamp and add a bonding-type locknut, a bonding wedge, or a grounding-style hub with a lug; add a jumper as well around oversized, concentric or eccentric holes.
Does a plain Myers hub satisfy bonding at a service?
Treat the plain gasketed hub as a fitting, not a bond. The version made for the job is the grounding-style hub with a lug: run a jumper from the lug to the grounded conductor bar and the entry carries fault current by listing. On a wet entry it keeps the seal the plain hub gives.
What size jumper goes around an impaired knockout?
Size the jumper from the overcurrent device ahead of the run with NEC Table 250.122, never from the conductor size: 8 AWG copper at 100 A, 6 AWG copper at 150 A and 200 A. Aluminum jumpers go one size larger than the copper row. At a service it lands on the grounded conductor bar; downstream, on the ground bar.
When does a concentric knockout need a jumper away from services?
Once the circuit runs over 250 V to ground — 277 V on a 277/480 V system, 347 V on a 347/600 V one — jumpers go around concentric and eccentric knockouts at any equipment. The line reads voltage to ground, so a 480 V ungrounded circuit sits over it too: land the jumper on the grounding electrode conductor or ground bar instead of a grounded conductor bar.
Do factory-punched holes change any of this?
A factory-punched full-size hole removes the impaired-connection problem: no concentric ring, no reducing washer, no oversized gap. You still fit the listed bonding device at the service; the hole quality settles only the jumper question. Send hub sizes and positions with the order and the cutouts arrive with the box, delivered in 10 business days.
