Short answer: on any circuit over 250 volts to ground, a metal raceway through an oversized, concentric or eccentric knockout needs a bond that does not depend on the stamped ring — a bonding bushing with its jumper, or a bonding-type hub with a lug for a bonding conductor. The relaxation reaches you only through two doors: a box listed to bond its own knockouts, or a plain full-size hole. At 120/208 V and 120/240 V the threshold never arrives, so a fully punched hole with a standard hub is enough. The worked example below runs on a 1 Ω fault-loop resistance, an illustrative assumption, and the voltage scaling it shows holds at any resistance you substitute.
When does a concentric knockout need a bonded connection?
The NEC's bonding rule for raceways starts at 250 volts to ground: above that line, a metal raceway or metal-sheathed cable passing through an oversized, concentric or eccentric knockout must be bonded by a listed method.
A 277/480 V system sits at 277 volts to ground, so the rule reaches nearly every circuit in the building. A 347/600 V system, at 347 volts to ground, is in just the same. A 120/208 V system touches ground at 120 volts and a 120/240 V split at 120 volts, so ordinary terminations cover them. Service raceways sit outside the voltage test: the conduit feeding a service bonds under the service rules at any voltage, which listed conduit hubs for bonding at service equipment covers in detail. The fault-path fundamentals behind the rule are in the grounding and bonding guide.
What does the NEC require above 250 volts to ground?
Bond the raceway with a fitting that does not lean on the knockout itself: a bonding bushing with its bonding jumper, or a bonding-type hub whose lug carries a conductor to the ground bar.
NEC 250.96(A) states the duty underneath: metal parts serving as equipment grounding conductors, raceways and enclosures included, are bonded together. The bond has to carry safely any fault current likely to be imposed on it. A lone locknut across the stamped rings makes contact only at a few slot edges, so it is not on the code's list.
The relaxation has exactly two doors: the opening is a plain full-size hole, or the box is listed to give a reliable bonding connection at its concentric or eccentric knockouts. Behind either door, two locknuts on rigid metal conduit or IMC, one inside and one outside, or a threadless connector on metal-sheathed cable, are permitted. The bushing decision itself is laid out in when a bonding bushing is required at a concentric knockout.
Does a standard Myers hub make the bond?
Not at a concentric above 250 volts to ground: a Myers-style hub is listed as a weatherproof fitting, not as a bonding device, so its ring spans the knockout slots the way a locknut does.
In a plain full-size hole the answer flips, because the hub's thread grips solid metal all the way round, and a threaded connection listed for grounding is one of the code's continuity methods. Below the 250 volt line, a standard hub or locknut is an ordinary termination even over a stamped ring, outside service work. A bonding-type hub is the different animal: its lug and clamp screw give the fault current a ride on a conductor instead of on a questionable metal-to-metal contact. Sealing and material questions for hubs are separate; Myers hub with PVC conduit and conduit hub vs sealing locknut for outdoor top entries cover them.
Do reducing washers fix a concentric knockout?
Yes, when you drive the ring out completely first: a steel reducing washer closes the opening back to a plain hole, so the locknut or hub clamps solid metal instead of bridging slots.
The washer is a spacer, not a conductor; the bond comes from the termination now bearing on full metal around the whole circumference. Leave any part of the ring standing and the washer only shrinks the gap, so treat that joint as unbonded on a 277/480 V panel. Punching out the ring on an existing stamped box raises a listing question of its own, which adding a knockout to a panel walks through.
How do you know if a box is listed to bond its knockouts?
Read the marking on the box or its carton: an enclosure listed to provide a reliable bonding connection at its concentric or eccentric knockouts says so, often as a note that no extra grounding is needed above 250 volts.
Stamped 4 in and 5 in square steel device boxes commonly carry that listing on the packaging. Painted-over stamps, older boxes and untraceable catalogs get the worst-case answer: fit a bonding bushing with a jumper, and the question closes for a few dollars of hardware. That habit also settles the joints an inspector asks about first, the ones nobody can document any more.
Why does 277 volts get stricter treatment than 120 volts?
Because fault heat scales with the square of the voltage: on the same 1 Ω fault loop, a 277 volt-to-ground fault releases about 77 kW at the weak joint, 5.3 times the 14.4 kW of a 120 volt fault.
Here is the arithmetic, with the loop resistance an illustrative assumption of 1 Ω. At 120 volts to ground, fault current is 120 V ÷ 1 Ω = 120 A, and heat at the joint is 120² × 1 Ω = 14,400 W. At 277 volts to ground, current rises to 277 A and heat to 277² × 1 Ω = 76,729 W, about 77 kW. The ratio is (277 ÷ 120)² = 5.3, and it stays 5.3 at any loop resistance because resistance cancels out. Two slot edges of contact do not carry 77 kW; they weld, arc or burn loose, which is the failure these bonding methods exist to prevent.
What about MC cable through a concentric knockout?
The same 250 volt line governs metal-sheathed cable: above it, the armor's continuity rides on a threadless connector listed for the sheath, and the code allows that fitting only where the opening is plain or the box is listed to bond its knockouts.
MC and similar armored cables use the sheath as their equipment ground path, so the entry fitting sits in that path directly. Below 250 volts to ground, a standard connector over the stamped ring is an ordinary termination. On drawings that reach us for a stainless box, we cut the opening plain so the connector lands on full metal.
What should you specify to keep knockouts out of the job?
Order plain round cutouts at each hub's cutout diameter instead of stamped knockouts: on 304 or 316L stainless, the termination then bears on solid metal around the whole hole and no concentric ring enters the job.
Stainless also arrives bare, so there is no coating to clean; NEC 250.12 requires paint, lacquer and enamel to be removed from threads and contact surfaces at grounding and bonding connections, which is real scraping on a painted steel box and none at all on ours. Send the hub maker's hole diameter with the drawing through the custom stainless cutouts page or the cutouts, glands and windows guide, and keep the gasket seat clean as you fit the hub. The decision by case then reads like this:
| Case | What passes |
|---|---|
| Over 250 V to ground, concentric entry, unlisted box | Bonding bushing with jumper, or bonding-type hub with lug |
| Over 250 V to ground, box listed to bond its knockouts | Two locknuts on rigid or IMC, or a listed threadless connector |
| Over 250 V to ground, plain full-size hole | Standard fittings, two locknuts included |
| 250 V to ground or below, no service conductor | Full punch, reducing washer, standard hub or locknut |
| Service raceway, any voltage | Threaded hub or bonding bushing under the service rules |
Pick the row the circuit lands in and the knockout argument ends before the inspector starts it.
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
What size bonding conductor does a bonding bushing take?
Size it from the rating of the overcurrent device ahead of the raceway, read against NEC Table 250.122, which the grounding rules use for exactly this sizing. The jumper runs from the bushing's lug to the enclosure's ground bar. A 60 A branch and a 400 A feeder land on different rows, so take the ampere rating from the breaker or fuse protecting that raceway rather than copying the feeder next to it.
Do service raceways follow the same 250 volt threshold?
No. The raceway carrying service conductors bonds under the service bonding rules at any voltage, so a 120/240 V service still needs continuity through the opening by a listed method: a threaded hub or a bonding bushing with its jumper. A lone locknut over a stamped ring is the weak joint at a service whatever the voltage, and it is the first thing to replace.
How much more fault heat does 277 volts to ground release than 120 volts?
About 5.3 times, because heat rises with the square of the voltage: (277 ÷ 120)² = 5.3. On the illustrative 1 Ω loop used above, the joint sees 14,400 W at 120 volts to ground and 76,729 W at 277 volts to ground. Substitute a real loop impedance and both wattages move, but the 5.3 ratio between the two voltages stays fixed.
Can a stainless enclosure be ordered without stamped knockouts for hub entries?
Yes. Every opening in our 304 and 316L boxes is cut to your drawing, so you specify plain round holes at each hub's cutout diameter and no concentric ring ever appears. Custom bodies are delivered in 10 business days counted from your approval of the drawing; catalog sizes from the order date.
