Cable Connectors and Knockout Covers

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  #1  
Old 11-08-15, 01:31 PM
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Cable Connectors and Knockout Covers

I am rewiring a 1941 house with "knob and tube" wiring. I am currently rewiring ceiling lights, and I would like to reuse the existing junction boxes because they are damn near impossible to remove (nailed under the lath to the underside of the joists ... don't ask). In order to meet code, I need cable connectors (where I run the NM-B into the existing open holes) and knockout covers (to cover open holes I'm not using). I measured the holes, and they are just a hair over 1/2" in diameter. So I went to the hardware store and was directed to buy a package of cable connectors labeled <3/8" (1/2" KO)>. I took them home and tried them -- they are too large for the KOs. So I measured the connectors. I have no idea what 3/8" refers to, because there's nothing 3/8" about this cable connector. I measured the inside diameter (where it enters the KO), and it looks to me to be about 11/16" (which is larger than either the 3/8" or 1/2" marked on the package). I went back to the store, and they said that was the smallest cable connector they sold, and that they "should work" (how I hate those words) for a 1/2" KO. So I picked up one of their junction boxes that said it had 1/2" KOs, and I measured the knockout hole ... well over 3/4". Subsquently, I searched online for cable connectors and knockout covers smaller than 1/2" (or should I say, nominal 1/2") ... no luck.

So here are my questions. (1) Am I stuck in some sort of <a 2 by 4 isn't really 2"x4"> kind of scenario? Are so-called 1/2" KOs not really 1/2"? (2) Does anyone have any suggestions for how I find knockout covers and cable connectors that are ACTUALLY 1/2" to fit in the holes in my ancient junction boxes? Some of the boxes have KOs, not removed, which actually look even smaller than the ones I'm using. Where can I find cable connectors and KO covers smaller than the "nominal" 1/2" (that aren't really 1/2" at all)?
 
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  #2  
Old 11-08-15, 02:26 PM
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You are dealing with trade sizes. They differ from actual dimensions.

The bars can be cut with a hacksaw or sawzall blade through the plaster and only leave a small slot to repair.
 
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Old 11-08-15, 02:28 PM
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3/8" is trade size which as you have found out is not the same as actual size. Try a plastic push in cable connector. It probably won't work but at least you have eliminated it. I'd suggest drilling just a bit with a step drill. If you over drill you can use conduit washers on the connectors. If you just want to plug a hole use a a nut and bolt with a either a washer or fender washer on either side.

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Last edited by ray2047; 11-08-15 at 04:17 PM.
  #4  
Old 11-08-15, 03:35 PM
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If you don't have room for the step drill, and you have access to both inside and outside of box, you can use a greenlee punch like this: Greenlee 77U-1/2 Slug-Buster Self Centering Knockout Punch Unit for 1/2-Inch Conduit - Hand Tool Knockout Punches - Amazon.com

That may not be the size you need, but they come in many sizes.
 
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Old 11-08-15, 03:41 PM
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Also, places like grainger and McMaster sell hole plugs in many sizes, including 1/2" actual.
 
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Old 11-09-15, 03:33 PM
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>3/8" is trade size which as you have found out is not the same as actual size.

Why must the world be so complicated? <grin> ANyway, there are lots of good suggestions on this thread now ... thanks, everyone.
 
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Old 11-09-15, 03:38 PM
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Most of the problems with trade sizes to actual sizes is that trade sizes are inside diameters.
 
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