Fit a bonding bushing — or a threaded hub — where a raceway lands on a concentric, eccentric or reducing knockout at a service, whatever the voltage. Everywhere else the rule starts above 250 volts to ground, and it carries an exception that covers most boxes on the shelf: a 4 in square or 4-11/16 in square steel box listed for its concentric knockouts takes a standard connector even on a 277 V lighting circuit. Here are the two NEC rules, the systems each one reaches, and the hole specification that keeps a custom stainless box out of the argument.
The two places the NEC calls for a bond
At the service entrance, the NEC treats every concentric, eccentric or reducing knockout as a bond that is not yet proven. Where a metal raceway enters service equipment over one of those knockouts, you fit one of the methods the code names. That list is short: a bonding bushing, a bonding-type pressure connector, or a threaded hub. The voltage of the service makes no difference, so a 120/240 V residential service and a 480 V commercial service follow the same rule. A reducing washer set into an oversize hole counts the same way: it lifts the locknut off the wall, and the entry takes one of the named methods.
Away from the service, the trigger is voltage to ground. A raceway that carries more than 250 volts to ground and terminates at an enclosure through a concentric or eccentric knockout must be bonded to that enclosure. The exception: an enclosure listed for use with its knockouts. That is why the 4 in and 4-11/16 in square steel boxes you pull off the shelf rarely need a bushing — the maker had the box and its factory knockouts evaluated, so a standard fitting does the bonding job.
Underneath both rules sits NEC 250.96(A): raceways, cables, equipment and enclosures are bonded together so they can safely carry any fault current likely to be imposed on them. NEC 250.4(A)(5) adds that the earth is not part of that fault path, so the current returns through metal all the way to the source. The grounding and bonding guide walks that whole system.
For a grounded system, voltage to ground is the reading from any ungrounded conductor to ground, so the line-to-neutral value of a 480Y/277 V system is 277 V. On an ungrounded 480 V delta you take the line-to-line value, 480 V. The table below is the decision:
| System | Highest voltage to ground | Concentric knockout at a feeder or branch box | The same knockout at the service |
|---|---|---|---|
| 120/240 V split-phase | 120 V | standard connector | bond the entry |
| 208Y/120 V | 120 V | standard connector | bond the entry |
| 240 V delta, center-tapped | 240 V | standard connector | bond the entry |
| 480Y/277 V | 277 V | listed box: connector; otherwise bond | bond the entry |
| 480 V delta, ungrounded | 480 V | listed box: connector; otherwise bond | bond the entry |
"Bond the entry" means a bonding bushing, a bonding-type connector or a threaded hub, at any voltage.
Why a concentric knockout can break the bond
A concentric knockout is two or three rings stamped in one hole so you can punch out the size you need. Punch only the inner ring and the locknut lands on a reduced, raised edge; the connector can bite through mill scale or powder coat instead of bare steel. The joint looks solid on installation day and can corrode away through years of condensation cycles. Eccentric knockouts do the same with an off-centre ring, and a reducing washer lifts the locknut clear of the wall.
A bonding bushing closes that gap on purpose. It seats on the raceway end like an ordinary bushing, and its lug takes a jumper to the enclosure wall or the ground bar, so fault current has a path that never depends on the locknut's bite. NEC 250.12 already tells you to strip paint, lacquer and enamel from threads and contact surfaces at grounding and bonding connections. A concentric knockout is exactly the surface where that paint survives.
When a standard connector is enough
Three cases need no bonding bushing:
- The enclosure is listed for use with its concentric or eccentric knockouts. Most 4 in square and 4-11/16 in square steel boxes are; the marking sits on the carton and in the maker's data. The listing covers the box with its factory punchings, so a standard set-screw or compression connector makes the bond.
- The hole is a clean, single-size knockout, punched or machined. No rings and no reducing washer means the locknut bears on a full, flat wall, and the connector does its ordinary job.
- The circuits run at 250 volts to ground or below and the entry is not a service. A 120 V or 208Y/120 V feeder through any listed fitting is finished; the equipment grounding conductor does the rest.
What the listing never covers is work done after the box left the factory: a concentric-pattern hole you punch yourself, an oversize hole closed with a reducing washer, or an enclosure with no listing at all. Those take a bonding bushing, a bonding connector or a threaded hub. At service gear, which hub passes at service equipment sorts the hub choice.
The 277 V lighting feeder, worked end to end
Take the case the rule was written for: a 480Y/277 V panel feeds a lighting circuit through 1 in EMT to a steel pull box on the wall.
- Voltage check: 277 V to ground, and the line starts above 250 V — 277 − 250 = 27 V past it, so the knockout bonding rule applies.
- Box check: the box is a 4-11/16 in square steel box, 2-1/8 in deep, listed for use with its concentric knockouts the way nearly every box of that size is. The exception takes over, and the standard set-screw connector is enough. Fitting a bonding bushing on top is cheap enough that some crews do it anyway; the code does not ask for it there.
- Unlisted box: the same circuit onto a concentric knockout in an enclosure with no listing takes a bonding bushing on the raceway end, with a jumper from the lug to the box wall.
- Conductor check: the circuit runs 12 AWG copper on a 20 A breaker. NEC Table 250.122 asks for a 12 AWG copper equipment grounding conductor at 20 A, so a bushing jumper here, had one been needed, is 12 AWG copper.
- The stainless alternative: the same 277 V circuit into a 316L wall-mount enclosure with the hole machined full size for the connector. A single-size machined hole has no concentric rings, so the knockout rule never starts, and you bond the enclosure itself to the ground bar through its ground lug.
De-energize the circuit before you open the box to fit a bushing or land a jumper.
What to specify on a custom stainless enclosure
Our wall-mount and HMI enclosures come cut to your drawing: you set material, size and every hole, built from one piece and delivered in 10 business days — counted from your approval of the drawing for custom work. Specify each raceway entry as a single-size hole matched to the fitting you chose: a threaded hub where you want threads into bare metal, a standard connector elsewhere. Hole diameters by connector are worked out in the cord grip connector hole size guide, and knockouts and fittings on a NEMA 4X drive enclosure shows the same entries on a real box.
Two things to carry onto the order. Our enclosures are rated NEMA 4X / IP66 by the manufacturer; they are not UL listed, so the listed-box exception is not available to you. The clean single-size hole, or an explicit bonding bushing, is how you meet the rule. Where the finished panel itself has to carry a listing, the approval path for an unlisted enclosure in a listed control panel sets out what an inspector will take. And bond the box itself: a ground lug tied to the ground bar, plus a jumper across the door hinge — bonding an enclosure to a ground bus and door bonding jumpers when a hinge is not enough cover both. Cable entries made with glands instead of raceways follow the gland bonding rules for stainless junction boxes.
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
Does a standard EMT connector pass at a concentric knockout on a 277 V circuit?
Yes, when the box is listed for use with its concentric knockouts, as most 4 in and 4-11/16 in square steel boxes are. 277 volts to ground sits 27 volts above the NEC's 250-volt-to-ground line, so the bonding rule applies, but the listing makes a standard connector enough. On an unlisted enclosure, fit a bonding bushing with a jumper or a threaded hub.
How big is the jumper at a bonding bushing?
Size it from the overcurrent device ahead of the raceway, using NEC Table 250.122: a 60 A breaker takes a 10 AWG copper jumper, 200 A takes 6 AWG, and 400 A takes 3 AWG. The jumper runs from the bushing lug to the enclosure wall or the ground bar.
Is a bonding bushing needed at a 240 V service entrance?
Yes, where the raceway lands over a concentric, eccentric or reducing knockout: the NEC's service bonding rule applies at any voltage. A threaded hub or a bonding bushing does the job, with a jumper sized from NEC Table 250.122. At a clean, single-size hole, a standard listed connector makes the bond.
Do machined holes in a custom stainless enclosure need bonding bushings?
Not when the hole is cut full size for one connector: a clean single-size hole has no concentric rings and no reducing washer, so the knockout bonding rule never starts, even at 480 V to ground. The enclosure is manufacturer-rated NEMA 4X / IP66 and not UL listed, so bond the enclosure itself to the ground bar and use the listing approval path when the panel must be listed.
