Boxes vs enclosures for bonding over 250 volts

Short answer: at 480 or 277 volts to ground, a metal outlet, junction or pull box with concentric knockouts takes a standard locknut and bushing, because its listing covers those knockouts for bonding. A cabinet, disconnect or panelboard enclosure earns the same shortcut only when its label says the knockouts are listed for it. Absent that marking, land a bonding jumper sized from NEC Table 250.122 or fit a threaded hub, and the connection passes at any voltage. This page sorts the listed boxes from the unmarked enclosures, works a 480Y/277 V feeder end to end, and ends with the entry lines to put on a custom stainless order.

What does bonding over 250 volts to ground require?

Above 250 volts to ground, the NEC requires continuity of the metal raceway to the enclosure through a threaded joint or hub, a threadless fitting pulled up wrenchtight, or a bonding jumper landed on a pressure connection; a jumper merely run around the knockout rings fails the rule.

The threshold matters. A 120/240 V or 120/208 V circuit never crosses 250 volts to ground, so plain locknuts pass on any metal enclosure. A 480Y/277 V system puts 277 volts to ground on every feeder and lighting circuit, and the rule follows them into every box, gutter and cabinet.

The rule borrows the NEC's service-entrance bonding methods and strips out one: at a service, a jumper around a concentric or eccentric knockout is a permitted method, and above 250 volts to ground it is not. The exception hands the locknut methods back when the box's listing covers the knockouts, which the next two sections sort out. NEC 250.96(A) is why any of it matters: metal parts must be bonded to carry the fault current likely to be imposed on them, and the grounding and bonding guide covers that path end to end.

Which boxes are listed for bonding over 250 volts?

Outlet boxes, junction and pull boxes, cabinets and cutout boxes are investigated for bonding around concentric and eccentric knockouts under UL 50, so a standard locknut and bushing on those knockouts counts as reliable bonding at any voltage to ground.

UL 50, the enclosure standard, also sets knockout diameters and fitting dimensions in its Table 71-1A, so the ring geometry is part of the tested design. Boxes with concentric or eccentric knockouts are evaluated for bonding without any additional bonding means, on circuits above or below 250 volts, whether or not the packaging carries extra marking. The listing, not the printing, makes the knockout a bonding path.

Put numbers on it: a 4 in square box fed through concentric 1 in [25 mm] EMT knockouts on a 277 V lighting circuit passes on standard locknuts and insulating bushings. No jumper, no label hunt, no argument on the walkthrough.

Do panelboard and disconnect enclosures carry the same listing?

Panelboards, disconnect switches, switchboards, control-panel enclosures and wireways are listed under other product standards, and a knockout there is not automatically a listed bonding path, so read the label and treat an unmarked enclosure as one that needs a jumper.

One large enclosure maker's published answer to a code authority states that its load-center and metering knockouts carry no UL test for this bonding at all. The investigation differs by product line, and the label is the only thing that tells you which side your enclosure falls on. When the marking is silent, treat the rings as ordinary steel and use the jumper route below. The UL 50E versus NEMA 250 guide explains why a manufacturer rating and a listing are different animals.

When do you still need a bonding jumper at 480 volts?

Fit a bonding jumper, or make a threaded joint with a hub, whenever the raceway lands on an enclosure whose marking does not cover the knockouts; size the jumper from NEC Table 250.122, which asks for 10 AWG copper at a 60 A device.

Two fittings do the work. A bonding bushing carries a lug for the jumper, so the path lands on the ground bus and stops leaning on the knockout ring. A threaded hub makes a threaded joint to the raceway, a method that passes at any voltage; the hub question at service equipment is its own subject in which conduit hub passes for bonding.

Sizing is one table. NEC Table 250.122 follows the overcurrent device ahead of the conductor: 10 AWG copper at 60 A, 8 AWG at 100 A, 6 AWG at 200 A. Insulated jumpers of 6 AWG and smaller wear green under NEC 250.119. For the ring-versus-bushing decision in detail, see when a bonding bushing is required at a concentric knockout and the bonding jumper through an eccentric knockout.

How does a 480Y/277 V feeder to a 4 in square box work out?

A 480Y/277 V feeder carries 277 volts to ground, so the over-250-volt rule applies, and on a 4 in square junction box with concentric knockouts a standard locknut and bushing each end passes with no jumper when the box carries the UL 50 listing.

Take a 60 A feeder: 4 AWG copper phase conductors, 25 ft of 1 in [25 mm] EMT, one panel and two landings.

• 277 V to ground clears 250 V, so the over-250-volt bonding rule governs both landings.
• NEC Table 250.122 at 60 A sets the EGC and any jumper at 10 AWG copper.
• Landing 1, the 4 in square box: the UL 50 listing covers its concentric knockouts, so standard locknuts and insulating bushings pass at both ends, no jumper.
• Landing 2, the unmarked disconnect enclosure: treat its knockouts as unlisted. Fit bonding bushings and land a 10 AWG copper jumper on the ground lug, or make a threaded joint with a hub on RMC. The jumper stays 10 AWG because the feeder's 60 A breaker, not the 100 A disconnect frame, is the device ahead of the conductor.
• Drive the disconnect's rings fully out and stack reducing washers up to the 1 in conduit, and the entry is a plain hole again, where standard locknut methods pass.

What changes on a custom stainless enclosure?

A custom stainless box arrives with holes cut to your drawing instead of concentric rings, and it is rated NEMA 4X / IP66 by the manufacturer; not UL listed, so the bond has to stand on the fitting you order, not on a knockout listing.

That is easier than it sounds. With no oversized, concentric or eccentric knockout on the body, the exception's first branch already applies and standard locknut methods pass at 480 volts. Order each conduit entry as a plain hole sized to the hub, or to the bonding bushing, and the connection is decided by the drawing. The custom stainless cutout checklist covers hole sizes, and knockouts and fittings on a NEMA 4X drive enclosure covers the fitting choices.

Watch the coating, not the steel. NEC 250.12 requires paint, lacquer and enamel to come off threads and contact surfaces at bonding connections, so a powder-coated carbon steel box needs the window cleared back to bare metal. 304 and 316L stainless arrive bare and bond through clean metal as delivered. Where the entry is a cable gland rather than conduit, cable gland bonding requirements carry the same story.

Does a nonmetallic box with metal conduit change anything?

Yes: NEC 314.3 allows a nonmetallic box with metal raceways only where internal bonding means join all the entries, or where the box is listed with integral bonding and a provision for an equipment bonding jumper, so continuity moves inside the box.

Without one of those two provisions, the plastic box is off the table for metal raceways. With one, the knockout question disappears: the internal bond or the jumper carries the fault path, sized from NEC Table 250.122 as before. On a 480 volt circuit that jumper is still 10 AWG copper at a 60 A device.

What goes on the drawing and the order?

Write four lines on the drawing: the system voltage to ground, the overcurrent device rating, the EGC size from NEC Table 250.122, and the entry style — plain hole for a threaded hub, or knockout where a listed box takes over.

• System: 480Y/277 V, 277 volts to ground, over-250-volt rule applies.
• Circuit: 60 A device, 10 AWG copper EGC.
• Entries: 1 in [25 mm] plain holes, stainless threaded hubs, no concentric knockouts.
• Landings: locknut and bushing on UL 50 listed boxes; bonding bushing with a 10 AWG jumper on unmarked enclosures.

Send those lines with the custom enclosure layout and the cutouts arrive cut and deburred to match, delivered in 10 business days counted from your approval of the drawing. The bond the inspector then traces rests on fittings you chose, not on rings nobody tested.

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.

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