Short answer: yes. A metal reducing washer is a listed bonding method at a knockout whose ring has been beaten out clean: UL's White Book lists reducing washers as suitable for grounding in circuits over and under 250 volts, on walls of at least 0.053 in [1.3 mm], for non-service conductors only. Where a bonding bushing with its jumper is already installed, that bushing is the bond, and sanding and washers stacked on top of it add nothing the NEC requires. That washer recipe is one compliant way around a concentric or eccentric knockout; a bonding bushing or a listed hub is another. Which method holds on your job turns on three checks: the circuit voltage to ground, the wall thickness at the knockout, and bare metal at every contact.
What a reducing washer does at a knockout
A reducing washer is a flat plated steel ring with a large outside diameter and a smaller hole. It sits under a connector's locknut when the connector is smaller than the hole behind it: a 3/4 in connector landing in a 1 in knockout. Without it, the locknut spans the empty ring and the connector rocks; with it, the locknut clamps connector, washer and wall into one metal-to-metal stack. That clamped stack is the bond path.
UL 50, the enclosure standard, sets knockout diameters and fitting dimensions; the common trade sizes run 0.875 in [22 mm] for 1/2 in, 1.115 in [28 mm] for 3/4 in and 1.375 in [35 mm] for 1 in, so a washer steps a hole down one size. The same fitting logic applies on a stainless drive panel: knockouts and fittings on a NEMA 4X drive enclosure walks that case.
What the listing covers
UL's White Book, the directory behind fittings marked suitable for grounding, lists metal reducing washers for grounding in circuits over and under 250 volts, installed per NFPA 70, the National Electrical Code. The listing carries three conditions, and each one can quietly fail on a real panel:
- Wall of at least 0.053 in [1.3 mm]. Read the wall at the hole with calipers: 1.5 mm stainless reads 0.059 in and passes; 1.0 mm steel reads 0.039 in and sits outside the listing.
- Non-service conductors only. Fine downstream of the service point; at a meter socket or service equipment the washer listing stops.
- Every concentric and eccentric ring beaten out first. The listing covers washers installed in enclosures with those knockouts only after the rings are completely removed.
UL tested the washer itself, under those conditions. A half-cut ring, a painted wall or a 1.0 mm wall sits outside the tested case, so the washer leaves its listing there. Cutting one fresh hole at the connector's own size skips the whole question; adding a knockout to a panel covers what a new opening does to a listing.
Bare metal at the contact: NEC 250.12
NEC 250.12 requires paint, lacquer and enamel on equipment to be grounded to be removed from threads and other contact surfaces, unless the fitting is designed so the removal is unnecessary. NEC 250.96(A) repeats the rule for enclosures and other metal parts serving as grounding conductors. A typical powder coat runs 2 to 4 mil [0.05 to 0.10 mm] thick, which is plenty to keep a locknut from reaching metal.
On painted steel, dress a bare band around the hole before the washer and locknut go on, or use a fitting with teeth designed to bite through the coating. Bare stainless arrives with no coating at all: the washer, connector and locknut land on bare metal as delivered, so the clean-surface work is done before the box reaches your bench. Where a panel carries bond paths at several openings, that is a reason on its own to spec 304 or 316L. The grounding and bonding guide covers the rest of the path, from jumpers to the ground bus.
When the bond is required: over 250 volts to ground
For metal raceways carrying circuits over 250 volts to ground, other than service conductors, the NEC requires positive electrical continuity around any concentric or eccentric knockout ring left in the wall. A typical string-inverter DC run sits at 600 V to ground, so the rule applies across most of a solar DC side. The NEC accepts several ways to make that bond: a bonding bushing with its jumper, a threaded hub or threadless connector listed for bonding, or the reducing washer on bare metal with the ring fully out. Below 250 volts to ground, a fitting listed for bonding still covers the path, and service equipment carries its own stricter bonding rules at raceway openings. Boxes versus enclosures for bonding over 250 volts takes the voltage split further, when a bonding bushing is required at a concentric knockout covers the ring case, and a bonding jumper for rigid conduit through an eccentric knockout covers the jumper.
Worked example: a DC disconnect and a meter socket
Take a common solar layout. A 600 V DC string lands on a disconnect through 3/4 in EMT at an eccentric knockout; a 1 in service conduit lands on a meter socket at a concentric knockout; the AC side runs 240/120 V split phase.
DC disconnect: over 250 V to ground, bond required. A bonding bushing is already fitted, with its jumper sized from the upstream 20 A string fuse: NEC Table 250.122 calls for 12 AWG copper. That bushing is the bond, and nothing more is owed; washers and sanding on top of it are belt-and-braces, not a second requirement. To build the washer route instead, run three checks:
- Calipers read the wall at 1.5 mm = 0.059 in, clearing the 0.053 in [1.3 mm] listing floor by 0.006 in [0.15 mm].
- The 3 mil [0.08 mm] powder coat comes off in a band from the 1.375 in [35 mm] hole out to a 1.75 in [44 mm] circle: (1.75 - 1.375) / 2 = 0.19 in [5 mm] of bare width.
- The eccentric ring is beaten fully out, with no saw kerf or bent-back tab left in the wall.
Meter socket: service conductors, no washer. The listing covers non-service conductors only, so the reducing washer is the wrong tool at the socket regardless of voltage. The bond comes from the bonding bushing or a listed conduit hub, sized like any service bonding conductor.
Shut the DC disconnect and verify the terminals dead before beating a ring out of a panel that sees sun.
The decision, case by case
| Situation | The bond comes from | What you do |
|---|---|---|
| Over 250 V to ground, ring beaten out, wall 0.053 in [1.3 mm] or more, bare metal | The reducing washer itself | Fit the washer and connector; the listing makes the stack the bond |
| Over 250 V to ground, bonding bushing fitted with its jumper | The bonding bushing | Nothing more owed; extra washers become mechanical, not electrical |
| Ring half-cut, or wall under 0.053 in [1.3 mm] | Anything but the washer | Beat the ring fully out or fit the bushing |
| Service conductors at a meter socket or service equipment | A listed bonding means | Bonding bushing or listed hub; the washer listing excludes service |
| Under 250 V to ground, connector listed for bonding | The connector's own listing | Install per the fitting's instructions |
| Painted wall, plain washer and locknut | Bare metal at the contacts | Dress the band bare per NEC 250.12, or use a toothed fitting |
Pick by voltage first, then wall thickness, then coating. On a stainless box with factory cutouts, the ring and paint checks pass before you open the parts bag; the caliper check on wall thickness stays yours.
What to put on the order
Order the holes you need and the ring question disappears. The cutout designer lays out every hole, gland and operator on the face you choose and sends the layout with the quote; custom stainless enclosures start from one piece of 304 or 316L with your cutouts, delivered in 10 business days, counted from your approval of the drawing. Bare stainless means bare metal at every opening, so NEC 250.12 has nothing to remove.
Where the job's specification wants a listed enclosure, note the difference once: our boxes are rated NEMA 4X / IP66 by the manufacturer and are not UL listed, and the listed versus manufacturer-rated guide explains when an inspector will want the listed part. For the bond hardware itself, the kit is short: listed reducing washers one trade size down from each hole, bonding bushings with jumpers for anything at service or over 250 V, and jumper wire sized from the upstream overcurrent device, 12 AWG copper behind a 20 A fuse per NEC Table 250.122.
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
If a bonding bushing is already installed, do I still need reducing washers?
No. The bushing and its jumper are the bond the NEC accepts around a ringed knockout, and no second method stacks on top of it. Size the jumper from the upstream overcurrent device using NEC Table 250.122: 20 A calls for 12 AWG copper, 60 A for 10 AWG. The sanding-and-washer recipe is an alternative method, not an extra layer owed when a bushing is already in place.
What wall thickness does a reducing washer need?
The UL listing sets 0.053 in [1.3 mm] as the minimum enclosure wall, for non-service conductors only. Calipers decide it: 1.5 mm stainless reads 0.059 in and passes with 0.006 in to spare, while 1.0 mm steel reads 0.039 in and misses by 0.014 in [0.36 mm]. On a wall that thin, take the bond from a bonding bushing instead of a washer.
Does the concentric or eccentric ring have to come out completely?
Yes. The listing covers washers in enclosures with those knockouts only after all the rings are completely removed, so beat them out rather than sawing a slot and leaving the rest in place. For a 1 in knockout that is a full 1.375 in [35 mm] hole. Check the inside face of the wall too: a ring remnant hanging on the back breaks the metal-to-metal path just as a front-side fragment does.
Is paint removal required on a stainless steel enclosure?
No. Bare stainless carries no coating, so the clean-surface work of NEC 250.12 is already done at the knockout: washer, connector and locknut land on bare metal as delivered. Painted steel is the case that needs work, because a typical powder coat is 2 to 4 mil [0.05 to 0.10 mm] thick. Watch retrofits too: touch-up paint brushed over an old opening is the usual culprit on an otherwise bare panel.
Can a reducing washer bond a meter socket?
No. The listing covers non-service conductors only, so at a meter socket or any service equipment the bond comes from a bonding bushing with its jumper, or from a threaded hub or threadless connector listed for bonding. The over-and-under-250-volt flexibility applies downstream of the service point, on walls of at least 0.053 in [1.3 mm] with every ring beaten out.
