Conduit Sizing - Electrician Talk - Professional Electrical Contractors Forum

23027 · 03-19-2009, 08:42 PM

So I was in class today (Currently finishing my pre-app course for 1st year Apprenticeship) and we were doing table D3 calculations.

We began talking about how the new Canadian "Trade Size" measurement of conduit sizing is more accurate than ½" and ¾"... Because technically, a ½" piece of EMT is actually larger, I think almost ¾", although still called ½"?

What's the story behind these inaccurate names becoming standard terminology? My teacher has been trying to figure it out, now he's given me the bug.

Thanks in advance
Jeff

brian john · 03-19-2009, 08:45 PM

Quote:

Originally Posted by 23027

So I was in class today (Currently finishing my pre-app course for 1st year Apprenticeship) and we were doing table D3 calculations.

We began talking about how the new Canadian "Trade Size" measurement of conduit sizing is more accurate than ½" and ¾"... Because technically, a ½" piece of EMT is actually larger, I think almost ¾", although still called ½"?

What's the story behind these inaccurate names becoming standard terminology? My teacher has been trying to figure it out, now he's given me the bug.

Thanks in advance
Jeff

I'd like to tell you but it is an American thing, we are swore to secrecy to NEVER tell our neighbors to the north the how's and why's of this!

23027 · 03-19-2009, 08:57 PM

Don't take this the wrong way, but we sort of figured it originated from the US.

Retarded how they can't just let a good thing be, instead they're trying to enforce the use of Metric measurement into the trade.. It's completely taken over the code book, which it too bad because well.. Imperial measurement is like, the Latin of science.

Pretty sure if I went to get some EMT and said I needed "Trade Size 63 please!", I'd get a lot of blank stares.

william1978 · 03-19-2009, 09:26 PM

Quote:

Originally Posted by 23027

Pretty sure if I went to get some EMT and said I needed "Trade Size 63 please!", I'd get a lot of blank stares.

Your damn right, and welcome to the forum.

JohnJ0906 · 03-19-2009, 09:33 PM

Quote:

Originally Posted by 23027

Because technically, a ½" piece of EMT is actually larger, I think almost ¾", although still called ½"?

What's the story behind these inaccurate names becoming standard terminology? My teacher has been trying to figure it out, now he's given me the bug.

Thanks in advance
Jeff

Possibly it is based on the inside diameter of rigid conduit. Although even those sizes aren't exactly ½", ¾", 1", etc., they are fairly close.

Just a guess.

23027 · 03-19-2009, 09:35 PM

Quote:

Originally Posted by JohnJ0906

Possibly it is based on the inside diameter of rigid conduit. Although even those sizes aren't exactly ½", ¾", 1", etc., they are fairly close.

Just a guess.

Yeah there is nothing that measures to ½", ¾", 1", etc..

There MUST be some explanation to this.

480sparky · 03-19-2009, 09:49 PM

Quote:

Originally Posted by 23027

Yeah there is nothing that measures to ½", ¾", 1", etc..

There MUST be some explanation to this.

They're called trades sizes, not exact measurements.

william1978 · 03-19-2009, 10:00 PM

Google it,And let us know what you find.

480sparky · 03-19-2009, 10:04 PM

Quote:

Originally Posted by JohnJ0906

Possibly it is based on the inside diameter of rigid conduit. Although even those sizes aren't exactly ½", ¾", 1", etc., they are fairly close.

Just a guess.

Table 4, Ch. 9 has 1/2" rigid as 0.632" diameter. Not what I would call 'close' to 0.5". PVC is the closest at 0.526"

23027 · 03-19-2009, 10:12 PM

Just looking for an explanation or story behind all this

JohnJ0906 · 03-19-2009, 10:13 PM

Quote:

Originally Posted by 480sparky

Table 4, Ch. 9 has 1/2" rigid as 0.632" diameter. Not what I would call 'close' to 0.5". PVC is the closest at 0.526"

True.

I also wonder what those dimensions were 100 years ago - which is when this probably originated.

23027 · 03-19-2009, 10:17 PM

Quote:

Originally Posted by JohnJ0906

True.

I also wonder what those dimensions were 100 years ago - which is when this probably originated.

That's what I'm putting my money on, something that's just never really been technically corrected for the sake of simplicity.

Humans really are just creatures of habit.

brian john · 03-19-2009, 10:21 PM

I removed some 1930 1/2" rigid and a new lock nut fit, not to tight not to loose.

480sparky · 03-19-2009, 10:26 PM

Quote:

Originally Posted by JohnJ0906

True.

I also wonder what those dimensions were 100 years ago - which is when this probably originated.

The numbers in Table 4 only go back to the '96. Before that all trades sizes were considered the same diameter. I'll have to dig out some old engineering books for actual numbers from years gone by.

Edit to add: my 1953 American Electrician's Handbook lists ½" trade size conduit and tubing at 0.622". Same number in the 1941 Standard Handbook for Electrical Engineers.

waco · 03-19-2009, 10:27 PM

I'm confused....

Conduit sizes are based on their inside diameter, just like the pipe sizes they're based on. Last time I looked, they were pretty darned close to right.

480sparky · 03-19-2009, 10:36 PM

My guess is they didn't originate in the electrical trade. Most likely from the plumbing trade, as many homes & businesses used gas for lighting.