I got my new gas range installed and want to remove the 50 amp breaker for the electric range and replace it with a dual pole 20 amp one for a workshop in the basement. I was going to cap off the electric range wire in the panel, but when I opened it up, I see (I think) 6 gauge aluminum (looks like aluminum and it says 6AL-3 on the cable). So, I'm not sure I can get an appropriate wire nut for that. So, instead, I'm thinking of just removing the cable and use that knockout for the new 12 gauge wire. I'm pretty sure that cable clamp is too big for the 12 gauge, and my 3/8 trade size cable clamps won't fit inside that large knockout. I think the diameter of the knockout is 1 1/8 inches, judging by the knockouts around that one. I see there are reducing washers for things like this...

So, my question is, what size reducing washer do I need to get? Obviously the outside diameter must be bigger than the knockout hole. However, I don't know how they are "sized" (physical, or trade size like that 3/8 inch trade size clamp for 1/2 inch knockout) & I don't know how much larger the outside diameter of a reducing washer needs to be over the knockout hole. Do I get a 1 inch one, a 1 1/4 inch, or what? I'm assuming the size for the reduced hole is 1/2 inch (the physical diameter of the reduced knockout--for example, a 1 1/4 x 1/2 reducing washer). The local big-box stores have a small selection, so I may need to order them, which means I may not have the luxury of being able to just measure them with a ruler before purchase.



  • Measure with a pair of calipers or vernier.
    – Solar Mike
    Mar 25, 2020 at 19:41
  • I used a stainless steel rule. I would think I would need to get at the hole to use a caliper correctly. Mar 25, 2020 at 20:18
  • UPDATE: I was able to get a digital caliper around the outside of the raised edge of the "ring". It was tough to make sure I measured the true diameter though. It measured 1.13 inches. Mar 25, 2020 at 20:25
  • If it's 6-3 with ground (4-wire plug, if there was a plug, or 4 wires if hard-wired) and you have a workshop under the location, You'd do better to leave the breaker and put in a subpanel for the workshop fed by the cable.
    – Ecnerwal
    Mar 25, 2020 at 21:53

1 Answer 1


They do make reducing washers if your original hole is ~1-1/8 it is 1” using 12 awg Romex will take a 1/2” clamp down you can use 2 1’ to 1/2 “ washers one outside one inside the box or plug that hole and find an unused 1/2” hole , I don’t remember for sure but think a large blue or blue gray wirenut will cover a single #6 wire , I would not cut them in case they are wanted at a later time.

  • 1
    Thanks. I just measured it again. The knockout has two sizes, depending on what you knock out--the inside hole or that & the outside "ring". The inside hole is 7/8 inches diameter and the outside is 1 1/8. Is this some "trade size" thing? Do you know what can I expect the outside diameter of a 1 inch reducer to be? The wire nuts that are for sale around here specifically say they are not rated for aluminum. And yes, I don't want to cut the wire in case the next homeowner likes an electric range. Mar 25, 2020 at 20:14
  • The sizes for conduit (the hole size) 1/2” conduit 7/8 hole. 3/4 conduit 1-1/8 hole , 1” conduit 1-3/8” hole so yes trade sizes I expected 1” clamp to be used with 4 wire #6 that 3/4 clamp must have been tight but now you have the correct size(s). The sizes are based on the conduit so when you say you need a 3/4 to 1/2 reducing washer the od will be over 1-1/8 with the I’d of 7/8” , this will allow you to use a standard 1/2” Romex clamp in that hole. I usually plug holes with a snap in plug on residential and use an unused knockout hole to me that is a bit quicker, but both ways work.
    – Ed Beal
    Mar 25, 2020 at 20:36
  • Thanks Ed. I think I understand. I need a 3/4 x 1/2 reducing washer. I NEVER would've guessed that. They have those (and the 1 inch as well), so I'll go tomorrow and pick them up. As for the plug, that was the first thing I searched for, but couldn't find any that large. Having said that (and armed with the info you supplied), I think I want a 3/4 inch plug (NOT one larger than 1 inch which is what I searched for). They have metal 3/4 inch ones, but descriptions says they are for "weatherproof boxes". Does it matter? Mar 25, 2020 at 20:48
  • UPDATE: I kept searching and found knockout SEALS (instead of enclosure plugs). These seem to be more applicable. That's what I'll probably get and just open another knockout for the 12 gauge. Mar 25, 2020 at 21:20
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    I want to summarize what I learned from this answer (for others that find this via a search). You do NOT order reducing washers like you would order regular ones (physical outer and inner diameters). Instead, you order them using the trade size knockout hole you want reduced and the trade size knockout hole you want to reduce to. It does make sense now that I understand. Mar 26, 2020 at 22:43

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