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Old 06-07-2019, 11:25 AM   #1
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THHN/THWN in attic


I'm in the middle of pouring a slab for a new freestanding garage and I'm running 4 wires 100' of #3 AWG THHN/THWN in 2.5" sch 40 from the main to a new sub in the new garage. So far I've run about 30' underground from the new garage and up the side of the single level existing house about 10' and I'm now preparing to run diagonally about 25' in the attic to the other side of the house in the attic space.

What's the best way to run the wiring in the attic? I'll have an LB coming in through the rafter blocking. Should I find some flex 2.5" to connect to the outside LB? Run flex all the way to the other side of the attic? Or use a foot of flex and switch to straight PVC? Do I use single sided clamps to attach the conduit to the ceiling joists?

Last edited by FenBat; 06-07-2019 at 11:39 AM.
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Old 06-07-2019, 01:26 PM   #2
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Re: THHN/THWN in attic


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Originally Posted by FenBat View Post
I'm in the middle of pouring a slab for a new freestanding garage and I'm running 4 wires 100' of #3 AWG THHN/THWN in 2.5" sch 40 from the main to a new sub in the new garage. So far I've run about 30' underground from the new garage and up the side of the single level existing house about 10' and I'm now preparing to run diagonally about 25' in the attic to the other side of the house in the attic space.

What's the best way to run the wiring in the attic? I'll have an LB coming in through the rafter blocking. Should I find some flex 2.5" to connect to the outside LB? Run flex all the way to the other side of the attic? Or use a foot of flex and switch to straight PVC? Do I use single sided clamps to attach the conduit to the ceiling joists?
I think you oversized your conduit too much. Code book says you can use 1 1/4". 1 1/2" if you wanted to oversize would have been fine. I don't think 2.5" plastic flex is made. Metal flex would have to be grounded.

I would use conduit reducers on your LB and downsize. Why do you need flex for the attic connection? Can't you align the PVC to connect with the LB. If you need to you can heat up PVC to bend it. If you don't have a PVC heat blanket or toaster cut a piece and stick in your oven at around 225 degrees until it gets rubbery. Keep the windows open. Use a wet rag to cool it once you have it shaped. Use gloves.

There are also premade bent PVC elbow fittings that you can use. Maybe all you need is two 45 degree elbows to make an offset.

One hole rigid metal straps are fine.

You may need one expansion coupling up the side of the house and definitely one inside the attic if you use PVC conduit. I install them in the ground as well.

Can you post some pictures?
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Last edited by MrElectricianTV; 06-07-2019 at 01:46 PM.
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Old 06-07-2019, 03:59 PM   #3
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Re: THHN/THWN in attic


Thanks for the reply!

You must be much stronger than me. Pulling four #3 THHN wires through 2.5" is difficult enough, especially with a couple of 90's. I can't imagine pulling through half that size! :-) Besides, I left some poly line in there in case I need something added in the future.

Thanks for the idea on using expansion couplings! Hadn't thought of that.

Looks like Platt has some 2.5" flex at $3.60 / ft. Not too bad. Can a heat gun be used for bending?
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Old 06-07-2019, 04:44 PM   #4
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Re: THHN/THWN in attic


Oops, forgot what I was using! Yes, it's 1.5", not 2.5"! 1.5" is spec'ed for eight #3 wires in Sched 40, so I thought it would be an easy pull. It wasn't.
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Old 06-07-2019, 08:00 PM   #5
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Re: THHN/THWN in attic


You need an equipment grounding conducor as well.
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Old 06-07-2019, 08:10 PM   #6
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Re: THHN/THWN in attic


Leave the conduit the same size. Kinda late to reduce anything. Besides, some people always think bigger is better.

Since you're right in the middle of the pour, you better get back to work.....lol
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Old 06-08-2019, 10:38 AM   #7
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Re: THHN/THWN in attic


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Originally Posted by FenBat View Post
Thanks for the reply!

You must be much stronger than me. Pulling four #3 THHN wires through 2.5" is difficult enough, especially with a couple of 90's. I can't imagine pulling through half that size! :-) Besides, I left some poly line in there in case I need something added in the future.

Thanks for the idea on using expansion couplings! Hadn't thought of that.

Looks like Platt has some 2.5" flex at $3.60 / ft. Not too bad. Can a heat gun be used for bending?
Usually wire is not pulled until the entire conduit run is installed. With PVC a lot of pulling lubricant is needed. With a pull such as yours one person would pull and another person would push the wire into the conduit.

You should not have more than four 90 degree elbows in one pull run. Three elbows is easier to pull through.

Expansion couplings are required by code due to the characteristics of PVC.

You can probably heat the conduit with a heat gun, but it will take a long time to get it evenly heated over an 18" - 24" length. Easier to use prefab elbows if possible. An oven works well if you don't have a heat blanket or a PVC toaster.
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Old 06-08-2019, 04:41 PM   #8
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Re: THHN/THWN in attic


I'll have to get some pulling lube. That should help a lot.

I've got 2 90's up to the LB at the entrance to the attic. Then as straight as possible across the attic, a little less than 45 degrees.

I'll add expansion couplings on the run in the attic and the vertical run up the side of the house. I really don't want to see the underground PVC again, but the temperature should be really stable down there! :-)

Thanks!
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