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Old 08-11-2008, 03:29 PM   #1
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THNN/THWN-2 Question


Hello:

I'll be running conductors through 1" underground PVC pipe from our house to a detached garage/studio I'm building. There's a subpanel in the garage, fed from a 60 amp breaker in our main panel in the basement of the house. I'll be using #6 wire, two hots and a neutral. The subpanel will be grounded to a rebar ufer ground embedded in the concrete foundation of the garage.

My city inspector said to use THHW or THWN wire, suitable for wet locations like my underground pipe. At this point I haven't seen any wire labeled just THWN though. What I have seen is wire with multiple designations. A nearby electrical store has #6 conductor labeled THHN, THWN-2, MTW, AWM. Am I correct in assuming that this wire meets the specifications of all four of these applications, and thus would be suitable for my feeder?

Also, is it vital that I use red and black and white conductors? Can I use blue if they're out of black?

Thank you.

Shawn Marshall
Portland, Oregon

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Old 08-11-2008, 05:05 PM   #2
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THNN/THWN-2 Question


Use the THWN-2, using blue instead of black does not matter.

THHN is for use in dry and damp locations, couldnt find anything on AWM.

MTW is listed for use in wet locations but not given as an option in the ampacities table 310-16. It seems like its made specifically for wiring machinery? It may cost more or less than THWN?

310-13 Conductor application and insulation points you towards article 670.


Last edited by Steven Jackson; 08-11-2008 at 06:13 PM.
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Old 08-11-2008, 06:08 PM   #3
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Originally Posted by WShawn View Post
Hello:


My city inspector said to use THHW or THWN wire, suitable for wet locations like my underground pipe. At this point I haven't seen any wire labeled just THWN though. What I have seen is wire with multiple designations. A nearby electrical store has #6 conductor labeled THHN, THWN-2, MTW, AWM. Am I correct in assuming that this wire meets the specifications of all four of these applications, and thus would be suitable for my feeder?
The wire is dual rated thats all, as long as it has THWN-2 your good to go... Most wire today is dual rated, meaning its good for multiple location use.


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Originally Posted by WShawn View Post
Also, is it vital that I use red and black and white conductors? Can I use blue if they're out of black?

Thank you.

Shawn Marshall
Portland, Oregon

Grounded conductor (aka neutral) must be either White or Gray for #6 AWG and smaller. I also highly recommend pulling an equipment ground as well, its required for 2008 NEC.

Last edited by chris75; 08-11-2008 at 06:11 PM.
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Old 08-11-2008, 06:56 PM   #4
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THNN/THWN-2 Question


Oregon is using the 2008. Like stated above, you'll need a #10 equipment grounding conductor also, so 4 wires total.
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Old 08-11-2008, 07:22 PM   #5
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Hi:

Thanks for your replies, I appreciate it. I figured I was good to go as long as it listed some variation of THWN.

I'm going to double check with the Portland inspectors regarding the ground. I ordered a consultation inspection a few weeks ago to go over a long list of questions I had, and that inspector specifically said that since I was grounding the subpanel in the garage to a rebar ufer ground in the garage foundation I didn't have to run a ground from the main panel in the house. The reason this was a bit of an issue was because I unwittingly ran regular plumbing PVC pipe with regular plumbing turns and then poured the foundation over that last year. He said it'd be difficult to run the conductors through that sharp turn, so having to run only three instead of four would be easier.

I was able to dig up one end and replace the 90 with a long sweep turn and a second city inspector (reluctantly) approved it.

Thanks.

Shawn Marshall
Portland, Oregon
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Old 08-11-2008, 10:17 PM   #6
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THNN/THWN-2 Question


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Originally Posted by WShawn View Post
Hi:

Thanks for your replies, I appreciate it. I figured I was good to go as long as it listed some variation of THWN.

I'm going to double check with the Portland inspectors regarding the ground. I ordered a consultation inspection a few weeks ago to go over a long list of questions I had, and that inspector specifically said that since I was grounding the subpanel in the garage to a rebar ufer ground in the garage foundation I didn't have to run a ground from the main panel in the house.
The rebar in the garage is for lighting, and surges, its called a Grounding Electrode System, its not the same thing as a equipment ground from the main service.


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Originally Posted by WShawn View Post
The reason this was a bit of an issue was because I unwittingly ran regular plumbing PVC pipe with regular plumbing turns and then poured the foundation over that last year. He said it'd be difficult to run the conductors through that sharp turn, so having to run only three instead of four would be easier.

I was able to dig up one end and replace the 90 with a long sweep turn and a second city inspector (reluctantly) approved it.

Thanks.

Shawn Marshall
Portland, Oregon

Good luck with that, personally I would not accept it.
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Old 08-12-2008, 10:42 AM   #7
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THNN/THWN-2 Question


I agree with chris75.

Plumbing PVC is not approved for electrical circuits, so the sharp turns don't even matter. The inspector should have stopped this at the beginning.
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Old 08-12-2008, 11:57 AM   #8
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Thanks for your replies.

The backstory. We had our service upgraded last year by licensed electricians in anticipation of adding this garage to the load. They had to move our service entrance and upgraded the box. I was also going to have them do the wiring for the new garage. To prepare for that they told me to run PVC pipe through the garage foundation 18" underground and up to the house. Unfortunately they didn't specify that I had to use grey electrical pipe with long sweeps and that it had to be inspected before burial. Perhaps that was obvious to them, but not so much to me. The foundation was formed and poured almost a year ago. I decided this year (months after the pipe had been buried) to do the garage wiring myself, relying on books and inspections to make sure things were right.

Well, I had two inspectors who looked at the situation and determined that the usage of PVC plumbing pipe would be acceptable.

Just to improve my understanding, what is it about white PVC plumbing pipe that's unacceptable for electrical conductors? Besides the grey PVC electrical pipe being suitable for above ground exposure (none of the white PVC is above ground in my situation) how is the grey PVC different than the white PVC? What's the physical difference?

Thanks.

Shawn Marshall
Portland, Oregon
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Old 08-12-2008, 03:28 PM   #9
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Originally Posted by WShawn View Post

Just to improve my understanding, what is it about white PVC plumbing pipe that's unacceptable for electrical conductors? Besides the grey PVC electrical pipe being suitable for above ground exposure (none of the white PVC is above ground in my situation) how is the grey PVC different than the white PVC? What's the physical difference?

Thanks.

Shawn Marshall
Portland, Oregon

All raceways have to be listed for electrical use, Plumbing pipe is not one of them.
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Old 08-12-2008, 03:35 PM   #10
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THNN/THWN-2 Question


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All raceways have to be listed for electrical use, Plumbing pipe is not one of them.
That wasn't the question, the question is: what is different about the Grey PVC that makes it "listed" while white is not. Does it have something added to it to make it better suited for electrical purposes or is it simply for identification purposes (so Bubba knows there are wires inside that pipe and not water).
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Old 08-12-2008, 03:47 PM   #11
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Originally Posted by daxinarian View Post
That wasn't the question, the question is: what is different about the Grey PVC that makes it "listed" while white is not. Does it have something added to it to make it better suited for electrical purposes or is it simply for identification purposes (so Bubba knows there are wires inside that pipe and not water).
What makes it listed is that its in the NEC, I dont know what the actual differences are with plumbing pvc and electrical pvc. Probably nothing, I dont really care either, the raceway that you choose to use must be a Chapter 3 wiring method of the NEC.
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Old 08-12-2008, 05:54 PM   #12
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That wasn't the question, the question is: what is different about the Grey PVC that makes it "listed" while white is not. Does it have something added to it to make it better suited for electrical purposes or is it simply for identification purposes (so Bubba knows there are wires inside that pipe and not water).
No, that WAS the question. More specifically, "...what is it about white PVC plumbing pipe that's unacceptable for electrical conductors?...".

Simple answer is exactly as chris75 answered. The rest does not matter. Period.

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Old 08-12-2008, 06:04 PM   #13
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No, that WAS the question. More specifically, "...what is it about white PVC plumbing pipe that's unacceptable for electrical conductors?...".

Simple answer is exactly as chris75 answered. The rest does not matter. Period.

willis

Yep, wondering why you cant use it is pretty useless, because you just cant use it, end of story...
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Old 08-12-2008, 06:29 PM   #14
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THNN/THWN-2 Question


What does matter is the fact white pvc pipe matches the gray pvc pipe sizes, so the couplings still fit.

Farmers are known to run wire in water pipe, a few times I've had to hook irrigation pivots back up and found water pipe with direct burial wire installed. I think this is still better protection than actually installing the wire in the dirt, so I'll just sleeve the last 10' or so in gray electrical conduit where it exits the ground. These are straight pipe runs with no plumbing elbows though. Runs are usually 1000' plus, so retrenching to install the correct listed conduit is not really an option.
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Old 08-12-2008, 07:58 PM   #15
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Yep, wondering why you cant use it is pretty useless, because you just cant use it, end of story...
Well, I've always thought the best way to learn is to understand the underlying premise or concept, not just rote memorization.

Here's a question: Why do I need circuit breakers in my wiring?

Answer 1 - Because the NEC says so. Period.

Answer 2 - Because circuit breakers can prevent your wiring from overheating and catching on fire in the case of shorts or overcurrents or other mishaps.

Which of those answers is most likely to promote understanding?

I'm not trying to diminish the importance of the NEC; its rules are there for a reason. But I don't think those rules exist in a vacuum. I think, as with many things in life, that rules can be subject to interpretation within the context of the situation.

I'm driving down I-5; there are no cars within a mile of me. The posted speed limit is 65mph; I'm going 68mph. A cop pulls me over and writes me a ticket for $80. Technically, I broke the law, but in the context of the situation is it reasonable to get a ticket?

I'm glad that the inspectors I've been working with were flexible, looking at the big picture and letting me use white pipe in my situation. I didn't try to hide my error. I pointed it out and asked what I had to do to satisfy them. As far as I know, the NEC doesn't employ NEC cops to fine people when they don't follow all the NEC rules. Is it not up to the local inspectors to act as judges on a case by case basis? In my case they decided that the effort it would take to dig up the white pipe wasn't worth whatever benefit I'd receive by installing the grey pipe underground.

Thanks for the replies.

Shawn Marshall
Portland, Oregon

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