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Old 03-19-2010, 01:36 AM   #1
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Default Knockout size vs. fitting size

A knockout for 1/2" conduit is 7/8", but the outside diameter of a 1/2" EMT fitting is about 3/4". 3/4" knockout is 1-3/32" (close enough to 1-1/8), and O.D. of 3/4" fitting is roughly 1". So basically 1/8" for fudging around? Otherwise your bends/etc would have to be ultra precise?
Would it be unwise to cut my hole for 1/2" conduit to exactly 3/4" with a unibit?

Are the O.D. of other fittings (PVC, GRC, etc) the same size?

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Old 03-19-2010, 01:47 AM   #2
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The outside diameter for emt is different than the rest. Metal rigid, pvc, flex etc..is all the same outside diameter. They reduce the innner wall on those to make it thicker.

PVC coated rigid I think has a bigger outside diameter but I don't know for sure since I have never worked with it.

If you drill your holes that tight you probably will almost have to thread them into knockout, by the way why do you ask such oddball questions about conduit all the time ? are you in school or something ?


Last edited by user438; 03-19-2010 at 01:50 AM.
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Old 03-19-2010, 01:49 AM   #3
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Quote:
Originally Posted by Rich R View Post
The outside diameter for emt is different than the rest. Metal rigid, pvc, flex etc..is all the same outside diameter. They reduce the innner wall on those space to make it thicker.

PVC coated rigid I think has a bigger outside diameter but I don't know for sure since I have never worked with it
I know the OD of the actual pipe differs, but the fittings would have to be pretty similar in OD, otherwise they wouldn't fit in the same knockouts...
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Old 03-19-2010, 01:52 AM   #4
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ok I misunderstood, you mean the outside diametr of the threads, yeah they are the same no matter what type conduit, they have to meet the NPT standard
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Old 03-19-2010, 01:53 AM   #5
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Well the fact that all the different fittings can use the same steel locknuts just shows that all the fittings would have to have the same OD.. so I kinda just answered my own question
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Old 03-19-2010, 01:54 AM   #6
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Well the fact that all the different fittings can use the same steel locknuts just shows that all the fittings would have to have the same OD.. so I kinda just answered my own question

yep, threads are all the same diameter no matter what conduit type
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Old 03-19-2010, 02:00 AM   #7
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by the way why do you ask such oddball questions about conduit all the time ? are you in school or something ?
Not in school right now but I always like to know the reason behind these things rather than just doing them.
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Old 03-19-2010, 02:06 AM   #8
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Not in school right now but I always like to know the reason behind these things rather than just doing them.

Good questions, just don't normally see them, I'm used to helpers/apprentices more concerned about texting on cell phone than outside asking about diameter of conduit threads.

That is the key to learning this trade, I always tell new guys "don't think you will memorize this stuff, you have to know the reason behind doing what you doing" Once you realize that, there will be no problem or job that you can't figure out
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Old 03-19-2010, 02:14 AM   #9
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Don't forget those thin-wall connectors from China that fall though the 7/8 hole..
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Old 03-19-2010, 05:10 AM   #10
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Quote:
Originally Posted by Conduit Phil View Post
A knockout for 1/2" conduit is 7/8", but the outside diameter of a 1/2" EMT fitting is about 3/4". 3/4" knockout is 1-3/32" (close enough to 1-1/8), and O.D. of 3/4" fitting is roughly 1". So basically 1/8" for fudging around? Otherwise your bends/etc would have to be ultra precise?
Would it be unwise to cut my hole for 1/2" conduit to exactly 3/4" with a unibit?

Are the O.D. of other fittings (PVC, GRC, etc) the same size?

If I'm drilling just a few holes I'll unibit em in a second. I get alot of use out of my unibit!
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Old 03-19-2010, 07:01 AM   #11
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Originally Posted by jwjrw View Post
If I'm drilling just a few holes I'll unibit em in a second. I get alot of use out of my unibit!
Me too.
And please DRILL ON LOW WITH MY UNIBIT!
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Old 03-19-2010, 02:18 PM   #12
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a hole for a half inch emt connector is 7/8 inches. a hole for a 3/4 emt connector is 1 1/8 inches. that is the same for all connectors.
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Old 03-19-2010, 06:20 PM   #13
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Quote:
Originally Posted by Rich R View Post
The outside diameter for emt is different than the rest. ...why do you ask such oddball questions about conduit all the time?
He is Conduit Phil. Why wouldn't he ask questions about conduit? That's one of the ways that some of us learn.

Why call someone 'oddball'?

Your answers are very informative, by the way.
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Old 03-19-2010, 06:27 PM   #14
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Quote:
So basically 1/8" for fudging around? Otherwise your bends/etc would have to be ultra precise?
Nothing to do with the bends. If a connector fit exactly into the hole it would be a PITA to install. We need wiggle room. The more the better.


Quote:
Would it be unwise to cut my hole for 1/2" conduit to exactly 3/4" with a unibit?
First, a unibit has to take out too much material. A hole saw only cuts a small portion therefore takes less energy to do so.

Second, like I said, you need wiggle room.
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Old 03-19-2010, 06:48 PM   #15
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When I was a boot,we had to carry a 1/2"-1 1/4" knockout set. Life before hole saws.
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Old 03-19-2010, 06:53 PM   #16
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On a related note, here's a handy chart put out by the US Iron and Steel Institute a good many years ago. It's a knockout spacing chart, which takes into account the locknut and the bushing. It's the distance, center to center, of adjacent knockouts. You look up the one pipe size down the left hand side, and the adjacent pipe size along the top. Follow both into the chart where they intersect, and that's the spacing between their centers.

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Old 03-19-2010, 07:31 PM   #17
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On a related note, here's a handy chart put out by the US Iron and Steel Institute a good many years ago. It's a knockout spacing chart, which takes into account the locknut and the bushing. It's the distance, center to center, of adjacent knockouts. You look up the one pipe size down the left hand side, and the adjacent pipe size along the top. Follow both into the chart where they intersect, and that's the spacing between their centers.
That's good stuff!!

I am gonna print that out and keep it in my tool box!
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Old 03-19-2010, 07:48 PM   #18
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Quote:
Originally Posted by Rich R View Post
The outside diameter for emt is different than the rest. Metal rigid, pvc, flex etc..is all the same outside diameter. They reduce the innner wall on those to make it thicker.

PVC coated rigid I think has a bigger outside diameter but I don't know for sure since I have never worked with it.

If you drill your holes that tight you probably will almost have to thread them into knockout, by the way why do you ask such oddball questions about conduit all the time ? are you in school or something ?
It's been a while but as I remember, the PVC coated rigid is only different in size by the thickness of the coating.When you have to thread it you just trim the PVC back the length of the thread. The couplings are PVC coated with the coating extending to both sides of the coupling in a form of a boot.
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Old 03-19-2010, 10:14 PM   #19
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If I'm drilling just a few holes I'll unibit em in a second. I get alot of use out of my unibit!
Great tip I learned on here years ago.... dip the unibit in cold water after using it. Make it last longer.
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Old 03-19-2010, 10:22 PM   #20
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Great tip I learned on here years ago.... dip the unibit in cold water after using it. Make it last longer.
Wont that change the temper?

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