3" Core Hole

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mkgrady

Senior Member
Location
Massachusetts
I'm installing some floor outlets in an existing concrete floor in an office. This Poke-thru floor outlet made by Wiremold is advertised as fitting a 3" hole turns out to actually need a hole that is between 3-1/16 and 3-1/8". I thought just using a 3" core bit would do it. I don't want to just use a 3" bit and hope the actual hole meets the above limits. So my question is, do I need to get a special core bit for this? Or do 3" core bits generally make a hole a little larger. Seems crazy that the tolerances are so limited.
 

lordofthisworld

Senior Member
Location
Massachusetts
I'm installing some floor outlets in an existing concrete floor in an office. This Poke-thru floor outlet made by Wiremold is advertised as fitting a 3" hole turns out to actually need a hole that is between 3-1/16 and 3-1/8". I thought just using a 3" core bit would do it. I don't want to just use a 3" bit and hope the actual hole meets the above limits. So my question is, do I need to get a special core bit for this? Or do 3" core bits generally make a hole a little larger. Seems crazy that the tolerances are so limited.

You should be good with a 3” hole.
 

mkgrady

Senior Member
Location
Massachusetts
That's my first thought, but, Man! you'd think it's GOT to be for the standard 3" saw, right ?!?!?! right?!?!?!

That's what I'm thinking. Why would the manufacturer design a requirement that seems so out of the norm. I'm stumped because core drill bits at 3" are very common when compared to 3-1/16 to 3-1/8". I don't understand why they would design the outlet box that way. It makes me think that a 3" core bit probably makes a hole that falls within the tolerance. I just wish I could get a consensus on that thought.
 

MAC702

Senior Member
Location
Clark County, NV
Wait, this is one of those sites where we read the instructions? :huh:

Oddly, we need a 6-3/8" hole saw to install 6" recessed lights. Hence why I seconded the hesitation for confirmation.
 

JFletcher

Senior Member
Location
Williamsburg, VA
That's my first thought, but, Man! you'd think it's GOT to be for the standard 3" saw, right ?!?!?! right?!?!?!

Nope. Round simplex outlet floor boxes we set in wood floors often have an odd size, and going even an 1/8" over makes for an extremely sloppy fit. An 1/8" under, well, it doesn't fit at all.

This is definitely a case of "measure twice, cut once". You may even want to drill a scrap cinder block or the like with the bit you intend to use and dry fit the receptacle before you drill the real thing.
 
+1 on not just drilling this and hoping it fits ok.

After much hair pulling, I got a whole set of hole saws from about 1" up to 4" by eighths. When I install something, I don't ever trust nominal dimensions. I use a set of calipers to measure the part, then the same calipers to measure across the TEETH of the holesaw, which will determine the finished hole size.

For a core drill, you may want to do a test drill instead. The teeth don't have such a regular set and the cut swarf probably makes for a larger hole as it rubs around during drilling.

Another hole saw (not core drill) trick for when you goof too small: The larger hole saw threads are a standard size, I think maybe 5/8x18 TPI (fine thread). If you buy a bolt and a couple jam nuts, you can cut the head off and thread both holesaws onto it spaced by a jam nut. Then the goof holesaw will center the larger one for the re-cut.

If 5/8 doesn't fit your drill, you can turn it down in diameter by chucking the threaded end into a drill press, turning it on, and taking an angle grinder to the shank until it fits. Or use a lathe, but that would be a whole nother forum :D
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Often people in the trade will call the core bit size based on the size of the conduit being installed. Typically a 3" conduit is installed using a core bit that is actually 3.5".
 
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