Short answer: a snap-in MC connector bonds the enclosure wall by itself only when the knockout is punched fully open and the connector is listed and marked as a bonding means. Where the concentric or eccentric rings stay in the hole, at service equipment, or over 250 V to ground in an enclosure that is not listed for the purpose, you run a bonding jumper around the ring. Size it from NEC Table 250.122: 10 AWG copper covers a 30 A or 60 A feeder. The worked example below bonds a 480 V machine panel, and the closing table gives you the decision for every case.
Does an MC connector bond a concentric knockout by itself?
Yes, when the knockout is punched fully open and the connector is listed as a bonding means. Three cases still take a jumper around the ring: a service entrance, anything above the 250 volt line in a box that is not listed for it, and a ring left floating in the hole.
The connector and the knockout are two separate halves of the answer. The connector's listing says what the fitting itself can carry. The enclosure's listing and the circuit voltage say what the installation accepts at that hole. MC cable brings its own equipment grounding conductor, a bare bond wire that runs the length of the cable, and a bonding-marked connector keeps that path continuous into your wall.
NEC 250.96(A) sets the duty: metal parts intended to serve as equipment grounding conductors are bonded together so they can safely conduct any fault current likely to be imposed on them. That covers raceways, cables, equipment and enclosures. A marked connector in a clean hole meets it. The rest of this article is about the hole, the box and the voltage — the three things that turn a yes into a jumper.
What does the NEC require around concentric and eccentric knockouts?
The NEC treats a leftover ring as a broken link. At service equipment the bonding jumper goes around any concentric or eccentric knockout whose rings remain, and a standard locknut or plain bushing is never the sole bonding means there.
Three regimes cover everything you will meet. At service equipment, UL's published code guidance reads the rule strictly: bonding jumpers are required around the knockouts unless every ring is completely removed, and a plain locknut or bushing is rejected as the sole means. The standard locknut question at services and the concentric-knockout bonding rules cover this ground in detail. Above the voltage line, the jumper goes in unless the box or enclosure is itself listed for the purpose; where it is, the marking permits the listed bonding methods. At or below the line, the NEC 250.96(A) duty governs, and a marked connector in an effective connection carries it.
The grounding fundamentals behind all three regimes — fault path, bonding versus grounding, jumper sizing — are in the enclosure grounding and bonding guide.
When does a knockout ring impair the bonding connection?
A stamped ring impairs the connection while it hangs on its ties. The stamping leaves each outer ring attached at two or three narrow points, so a fitting that bites only the ring bonds through metal that can rock, score and corrode.
Anatomy, in trade sizes. A concentric knockout stacks a 1/2 in, 3/4 in and 1 in ring set around one center disc; Table 71-1A of UL 50 sets the knockout diameters behind those trade sizes. An eccentric knockout offsets the ring instead of stacking it, which is why it is the classic impaired connection. The fitting lands on metal held by ties rather than on the wall itself. You can check a suspect hole in seconds: push the ring with a screwdriver, and if it rotates or flexes, treat the connection as impaired.
Washdown and weather make it worse. Ties score, gasket crevices hold moisture against stainless, and the joint you cannot see degrades every season. If the hole is still on the drawing, the fix is free: ask for the rings fully punched. For fitting-to-hole fits, cord grip connector hole size covers which hole each fitting family takes.
Does the connector's listing change anything over 250 volts?
Above the threshold the jumper goes in unless the enclosure itself is listed for the purpose. A 480 V system runs 277 V to ground, so most industrial feeders land in this case, and the connector's own marking does not take the enclosure's place.
The threshold arithmetic is quick. A 480 Y/277 V system is 480 V ÷ 1.732 = 277 V to ground, over the line. A 600 V system is 347 V to ground, over it as well. A 208 V three-phase panel is 120 V to ground, under it, and so is a 240/120 V single-phase service. If your feeder crosses the line, the enclosure decides. Metallic outlet boxes carry listings that cover bonding at their own concentric knockouts; panelboards and enclosures carry it only where their listing and marking say so, as boxes vs enclosures for bonding over 250 volts explains. Where the enclosure is listed for the purpose, a threaded hub is the common listed method, and which hub passes at service equipment sorts the hub types. Where the entry is raceway rather than cable, when a bonding bushing is required covers the bushing fix.
An enclosure rated only by its manufacturer gets no exception here, which is why the jumper appears on almost every custom-panel drawing. See an unlisted enclosure for a listed control panel for the approval logic around one.
What is a snap-in MC connector actually listed and tested for?
A snap-in connector is listed as a cable fitting. The versions meant for bonding carry a marking and instructions that say so; one low-profile design covers 14/2 through 10/3 MC cable in copper and aluminum.
The listing covers the fitting: grip on the jacket and, on the bonding versions, the grounding path through the fitting's own contact elements. Read the marking and the instructions packed with your fitting. They state the cable range and the installation conditions, and they are what separate a cable grip from a bonding means. What the fitting's listing does not cover is your wall: the knockout's rings, the enclosure's coating, and the fault current your feeder can impose, which NEC Table 250.122 sizes the conductor for.
Three habits keep you out of trouble: buy the marked version even where you plan to punch the rings out, land the cable's bond wire on the fitting as its instructions show, and keep to one fitting family per panel so your bond points look the same. Where a gland carries the same duty on a sealed entry, why cable gland earthing is required explains the fault-path physics.
How does a 480 V machine panel feeder get bonded?
Run a jumper equal to the equipment grounding conductor. This 30 A feeder takes 10 AWG copper, from the connector's bonding screw to a bare point on the wall, routed around the loose ring.
Worked example. A machine panel takes a 480 Y/277 V, 30 A feeder through a 3/4 in concentric knockout in the bottom of a custom 316L enclosure rated NEMA 4X / IP66 by the manufacturer; not UL listed. The cable is 10/3 MC on a snap-in connector listed for bonding, feeding a drive starter.
- Voltage to ground: 480 V ÷ 1.732 = 277 V. That is over 250 V, so the over-threshold rule applies.
- Enclosure listing: the box is manufacturer-rated, not listed for the purpose, so the connector's marking alone is not the answer.
- Jumper size: NEC Table 250.122 at 30 A gives 10 AWG copper, the same gauge as the cable's own bond wire.
- Route: a 6 in length of 10 AWG, one ring terminal on a bare stainless point of the wall, the other end under the connector's bonding screw, clearing the loose 3/4 in ring.
- Conductor check: at 75 °C, 10 AWG copper carries 35 A in the NEC copper ampacity table, so it carries the 30 A feeder with margin.
The whole fix costs one ring terminal and 6 in of wire. Without it, the fault path rides on two stamped ties at the knockout — the exact connection the jumper rule exists to remove.
What changes when the knockout lands in a custom stainless enclosure?
Two things change. A factory-punched hole arrives with the rings fully removed, which ends the jumper question at services. And an enclosure rated only by its manufacturer is never listed for the purpose, so above the threshold you run the jumper every time.
Ordering the hole from the factory is the clean route. The stainless cutout service punches the exact trade size you list, a single 3/4 in hole instead of a concentric stack, and the cutouts, glands and windows guide walks the drawing details. Bare stainless helps here: there is no coating to strip, so the jumper lands straight on the wall. On powder-coated carbon steel, NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces at grounding and bonding connections, so clean the landing point to bare metal.
Field drilling is the other route, and adding a knockout to a panel covers what a new hole does to a listing. Deburr the hole, take every ring out, and land the jumper on bare metal. Where the entry must hold a washdown rating, the weakest part sets it: a snap-in fitting is a cable grip, so pair it with a rated gland or hub and bond that too. The wet-location flex connector piece covers the outdoor version of that entry problem.
What should the drawing and the order say?
Put three things on the drawing: the knockout as fully punched (a single 3/4 in hole, not a concentric stack), the connector specified as listed for bonding, and the jumper gauge picked from NEC Table 250.122 for the breaker ahead.
| Situation | Connector listed for bonding? | What you install |
|---|---|---|
| Service entrance, rings remain | yes or no | Bonding jumper around the ring |
| Service entrance, rings fully removed | yes or no | Connector on solid metal, no jumper |
| Above the line, e.g. 480 Y/277 V, enclosure listed for the purpose | yes | The listed method the enclosure's marking permits |
| Above the line, enclosure not listed | any | Bonding jumper around the ring |
| At or below the line, e.g. 120/240 V, hole clean-punched | yes | Connector alone, its bonding path carries the fault |
| At or below the line, ring floating | yes | Jumper wherever the fitting cannot bite solid metal |
Jumper sizing, copper, from NEC Table 250.122:
| Breaker ahead | Jumper size |
|---|---|
| 20 A | 12 AWG |
| 30 A | 10 AWG |
| 60 A | 10 AWG |
| 100 A | 8 AWG |
| 200 A | 6 AWG |
The same table drives jumper sizing inside a box, as bonding jumper size inside a metal junction box shows. For the common industrial case — a 480 V feeder of 30 A to 60 A in a manufacturer-rated stainless box — the answer is one line: jumper around the ring at 10 AWG copper, a bonding-marked connector carrying the cable, and the rings punched out wherever the drawing still lets you. Send the knockout sizes with your custom enclosure order and you get the drawing back for your approval, delivered in 10 business days anywhere in the world, counted from that approval.
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
How do you size a bonding jumper around a knockout ring?
NEC Table 250.122 sizes the jumper from the rating of the breaker ahead of the run: 20 A takes 12 AWG copper, 30 A and 60 A take 10 AWG, 100 A takes 8 AWG, and 200 A takes 6 AWG. Keep the run short; a 6 in jumper with two ring terminals lands one end on the wall and the other under the connector's bonding screw.
Can punching the knockout rings fully out replace the jumper?
Yes. UL's published code guidance reads the service rule that way: bonding jumpers are required around concentric or eccentric knockouts unless all the knockouts are completely removed. Punch the rings out and the jumper leaves the parts list: a clean 3/4 in hole seats the connector on solid wall metal, with no floating ring in the path.
Does a concentric knockout change the NEMA 4X rating of the enclosure?
The entry decides, and the weakest part sets the assembly rating: a snap-in MC connector is a cable grip, not a watertight seal, so it caps a NEMA 4X / IP66 enclosure below its rating where washdown or rain hits the wall. Use a rated hub or gland at that entry; our enclosures are manufacturer-rated, not UL listed.
Is a bonding bushing a better fix than a jumper at an MC connector?
They serve different entries: a bonding bushing lands on a raceway end, and where the system runs above the same 277 V-to-ground line, its jumper is sized from NEC Table 250.122 like any other. An MC connector has no raceway thread to accept a bushing, so a jumper around the knockout ring is the standard fix there.
What does 'listed for the purpose' mean at a knockout?
It means the enclosure's own listing covers bonding at its concentric knockouts, so standard locknuts and locknuts with bonding flash are permitted there without a separate jumper. A listed metallic outlet box carries that marking; an enclosure rated only by its manufacturer does not, which is why our boxes take the jumper on those higher-voltage feeders.
