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#446365 - 03/29/08 05:36 PM Running THHN Wire Through Underground Conduit
bob621 Offline
newbie

Registered: 03/14/08
Posts: 17
Loc: St. Peters, Missouri
Rigid conduit seems to be recommended over UF cable for underground runs involved in wiring a spa, although I believe UF is still up to code. In any case, I have a question about feeding THHN wires through PVC, Schedule 40 conduit that is to run underground. I have never actually done it. It seems that it is recommended that one fit and glue the conduit sections together for the entire underground "run," and then use a fishtape to pull wires through the entire length of conduit, including possible bends. It would seem easier to me to pull the wires through each section, bend, or segment of conduit as one goes along and then glue the conduit sections together afterwards. Is there some reason why this should not be done?

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#446366 - 03/29/08 06:14 PM Re: Running THHN Wire Through Underground Conduit [Re: bob621]
dora Offline
Search and Rescue
Carpal Tunnel

Registered: 02/16/02
Posts: 21240
Loc: Somewhere under the sun
Quote:

although I believe UF is still up to code




No, UF cable isn't allowed in outdoor runs to a spa. The ground wire isn't insulated. You must use THHN/THWN individual wires installed inside conduit.

Quote:

It would seem easier to me to pull the wires through each section, bend, or segment of conduit as one goes along and then glue the conduit sections together afterwards. Is there some reason why this should not be done?





The conduit is required to be built totally first BEFORE fishing wires through it. This insurs the bend radius isn't exceeded.

"NEC 346-11: “There shall be no more than the equivalent of four quarter bends (360 degrees total) between pull points, e.g., conduit bodies and boxes.”
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#446367 - 03/29/08 10:06 PM Re: Running THHN Wire Through Underground Conduit [Re: dora]
ront02769 Offline
Carpal Tunnel

Registered: 03/03/04
Posts: 10115
Loc: New England
it is nice to say that the conduit must be all in place before fishing the wires through it. if you wer to ever ask how that might be enforced, there would be no answer. to the op, as long as all bends are withih limits, I'd pull thought partialy to where you need to get to and go from there. sorry about the typing. laptop whilst watching UFC

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#446368 - 03/30/08 08:59 AM Re: Running THHN Wire Through Underground Conduit [Re: dora]
bob621 Offline
newbie

Registered: 03/14/08
Posts: 17
Loc: St. Peters, Missouri
So, out of curiosity, what is the point of allowing only 360 degrees of bending in any one conduit run? I thought it was because it would make the wires too difficult to pull through the run, and thereby make it more likely to damage wires or insulation on the wires when doing the pulling. Yet, if this is so, then requiring conduit to be fastened together before pulling wire through it, seems to be precisely what creates the need for minimizing the number of allowable bends or turns in the first place. In other words, it would seem much safer (in terms of potential damage to wire, or scraping off of insulation from the wire) to thread the wire through as one goes, before connecting segments of conduit.

I have to assume the reasoning here isn't circular. It therefore seems there must be some other valid reason (other than for minimizing damage to the conductors by pulling them through the conduit) for requiring that the conduit segments be fastened together first, before pulling the wire through. What (again, out of curiosity) would that reason be? I had considered that, with respect to PVC conduit there might be the danger that the "glue" which holds segments together, a "glue" which is actually a plastic solvent, could potentially dissolve insulation if splashed onto wires. But, to the best of my knowledge, this consideration would not apply, for example, to EMT, where, to the best of my knowledge, the same rules apply concerning pulling wires versus number of bends.

BTW, someone who stated he was a former working electrician and who currently is working for a major home supply chain, told me that UF cable satisfied NEC code for running underground to spas. He was prepared to sell it to me. Actually, I don't intend to use it in any case. I intend to use conduit. But, as always, perhaps because I am a retired scientist/researcher, I am curious to know the facts. Where in the NEC 2005 does it prohibit UF cable for this purpose or specify that a ground wire used here must be insulated? And why doesn't the rubber or plastic sheathing in UF cable qualify as insulation for the ground wire in that cable?

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#446369 - 03/30/08 10:39 AM Re: Running THHN Wire Through Underground Conduit [Re: bob621]
reorange Offline
Super Handyman

Registered: 02/19/05
Posts: 2016
Loc: CENTRAL NEW YORK
Quote:

So, out of curiosity, what is the point of allowing only 360 degrees of bending in any one conduit run?




The conduit is supposed to be installed so that the wires can be pulled in or pulled back out at any time.
Install the wire into the conduit as you go, because you have to go around several obstacles, put in the equivalent of 8 or 9 90 degree bends, bury the conduit. After you have all that done, you realize that you made a mistake and forgot to pull in enough wires or you put in the wrong size wires or whatever else, try pulling the wire back out so you can redo the installation. Good luck getting it back out.

Quote:

that UF cable satisfied NEC code for running underground to spas




Wrong. Maybe that's one of the reasons he isn't an electrician anymore.

Quote:

specify that a ground wire used here must be insulated




1999 NEC
Art. 680. Part D. Spas and Hot Tubs. 680-40 Outdoor Installations. A spa or hot tub installed outdoors shall comply with the provisions of Parts A and B of this article.............
Art. 680. Part B Permanently Installed Pools. 680-25 (c) Motors. .............motors shall be connected to an equipment grounding conductor sized in accordance with Table 250-122..............It shall be an insulated copper conductor..............


Edited by reorange (03/30/08 10:42 AM)
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#446370 - 03/30/08 12:41 PM Re: Running THHN Wire Through Underground Conduit [Re: dora]
monocline Offline
Handyman

Registered: 02/06/03
Posts: 656
Quote:

The conduit is required to be built totally first BEFORE fishing wires through it. This insurs the bend radius isn't exceeded.

"NEC 346-11: “There shall be no more than the equivalent of four quarter bends (360 degrees total) between pull points, e.g., conduit bodies and boxes.”





Section 346-11 was from the 90’s and it pertained to the number of bends for Rigid Metal Conduit (RMC). It had nothing to do with the bend radius of Rigid PVC (RNC prior to 2008).

Bend radius and the number of bends two different things. The assembly of raceway sections prior to installing conductors does nothing to insure compliance with the bend radius or number of bends provisions.

The reasons for all of these provisions are to protect the conductors. Shoving a 10 ft stick of pipe down 50 ft of wire will likely damage the insulation and the conductor.

Section 300.18 covers raceway installation.

Article 352 covers Rigid Nonmetallic Conduit (RNC). For 2008 article 352 specifically covers Rigid PVC.

Section 352.24 covers bending PVC.

For bend radius see Table 2 of chapter 9.

Section 352.26 covers the number of bends for PVC.

Section 680.21 covers the insulated grounding conductor.

Section 680.25(A) covers feeders for pool equipment. Generally, the feeder must be in a conduit.

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#446371 - 03/30/08 01:12 PM Re: Running THHN Wire Through Underground Conduit [Re: reorange]
bob621 Offline
newbie

Registered: 03/14/08
Posts: 17
Loc: St. Peters, Missouri
Thanks for your input. I see your point about being able to pull wires back out of buried conduit if necessary, and it is doubtless a good reason for following the rules about the number of allowable bends in a run.

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#446372 - 03/30/08 03:14 PM Re: Running THHN Wire Through Underground Conduit [Re: monocline]
bob621 Offline
newbie

Registered: 03/14/08
Posts: 17
Loc: St. Peters, Missouri
Thanks for your input, Monocline. I reread the sections of Aricle 680, NEC 2005, which you specified. But I think I need an interpretation please. Based on 680.21, (A), (1) or,for that matter, on any other pertinent sections of the NEC 2005 code, am I going to be unable to use NM cable (Romex) from the service entrance panel in my basement along the mud sill inside the basement and then out through a rim joist to the disconnect box that comes with the spa (Caldera), a box which will be mounted on the outside of the house? Does that part of the run inside the house also require an insulated ground whereas the 6-3G that I thought I would be able to use inside the house has only an uninsulated ground wire? Must I use all insulated single conductors protected by conduit *inside the house* also as part of the run?


Here is, in general, how I thought the spa would be wired. What part of this is wrong? The NM cable on the inside of the house? Other parts of the run?

1. Establish a new, dedicated 220V circuit with a new 50 Amp double pole breaker added to the main bus of the service panel. Actually, adding the new breaker/circuit to the service panel would be the last step performed after the rest of the wiring was completed.
2. Use Romex, 6 AWG, 3 wires, with an (uninsulated) ground, to start the new circuit and make about a 20 foot run along a mud sill (stapling, without the use of conduit) to the point of exit of the line from the basement.
3. Go through the rim joist and use rigid PVC conduit to protect the 6-3G Romex going up to the Caldera disconnect box (which would be mounted on the outside wall of the house). The spa wiring diagram suggests only 8 gauge supply wire coming into the disconnect box, but I thought I would use the larger wire for possible future upgrades. The supply wires coming into the disconnect box would be connected per the manufacturer's wiring diagram.
4. The Caldera disconnect box (subpanel) splits this 220V line into two lines, one protected by a 30A double pole breaker and one by a 20A double pole breaker, and also introduces GFCI protection into these lines.
5. Come off the disconnect box with a couple of feet of watertight flexible conduit against the house wall, leading to buried, rigid PVC conduit (Scedule 40), which leads to the spa located on a concrete patio. Not counting the flexible piece of conduit to start the run, or a piece of flexible conduit to end the conduit run, there would be three bends in the rigid conduit, and I would use the pre-formed wide right angle PVC pieces which one can buy to make these bends. The conduit would be 18 inches down, 1" in diameter (for ease of pulling wires), a total of about 30 feet in length, and would come up near the edge of the patio concrete slab, near the spa, where another short piece of watertight flexible conduit would be used to enter the spa shell.
6. Running through the conduit would be six insulated wires, those called for by the wiring diagram from the spa manufacturer. They would all be THHN, stranded copper. One would be #10 gauge green for the equipment ground. One would be #10 gauge white/neutral and come off of a terminal on the 30A breaker as directed by the manufacturer's wiring diagram. And there would be two #12 gauge hot wires from the 20A breaker, and two #10 gauge hot wires from the 30A breaker. These 6 wires/insulated conductors would run through the conduit to the control panel of the spa and be attached to terminals in the spa control panel as per the manufacturer's wiring diagram.
7. The disconnect subpanel would be more than 5 feet from the spa and in line of sight.

I had also considered the possibility that, to accomodate a possible future sunroom to be built around the spa, one that would require a frost wall to be poured near the edge of the patio, near where the flexible conduit would be coming off the disconnect box, that it would be nice to be able to take the line from the basement to the disconnect box and, having made the necessary connections there, go right back through the wall into the basement and run about three feet back along the wall in the basement, so as to be able to come out of the house again on the other side (outside) of where a sunroom foundation/wall might someday be located. I thought that could be done with two NM cables, 12-3G and 10-3G coming off the appropriate breakers/terminals in the disconnect box and running to a junction box inside the basement from which the THHN wires in conduit would begin. I wondered if one could do this and just not use the unneeded neutral and ground wires in the 12 gauge cable. Is that possible? Must everything in the entire circuit be single conductor wire protected by conduit? Even inside the basement?

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#446373 - 03/31/08 07:35 AM Re: Running THHN Wire Through Underground Conduit [Re: bob621]
3phase Offline
Carpal Tunnel

Registered: 09/09/02
Posts: 6849
Loc: Licensed Electrical Contractor...
In the interior section of the wiring to the outdoor tub you may use the 6-3/G NM cable. But you must change over to the THHN/THWN wire before leaving the inside of the structure, i.e. before it goes through the mud sill. Depending on where the NM is located outside it is most likely in a wet area and cannot be used in a wet area. The inside of the conduit is a wet area as it will condense water in it, especially since it is exposed to outside temperature changes. I read that in the 2008 NEC that a buried conduit is now clearly defined as a wet enviroment.
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#446374 - 03/31/08 08:43 AM Re: Running THHN Wire Through Underground Conduit [Re: 3phase]
bob621 Offline
newbie

Registered: 03/14/08
Posts: 17
Loc: St. Peters, Missouri
Thanks very much for your response. My city still uses NEC 2005, without amendments, as law.

I am unclear about the THHN/THWN designation versus simply THHN. I believe that THHN stands for Thermoplastic High Heat-resistant Nylon insulation. And that THWN stands for Thermoplastic High Water-resistant Nylon insulation. Are they one and the same thing, as a practical matter? Must one be careful to insure that one buys a type of single wire conductor that is BOTH heat and water resistant? In other words, is there such a thing as THHN that is not THWN or vice versa?

If I understand you correctly, I could not run the 6-3/G cable through the house wall and through the conduit on the wall outside the house to get to the disconnect box. I assume I would have to place a junction box inside the basement where the 6-3/G cable would end, and, in that junction box, splice single conductor, 6 gauge THHN wires to the 4 cable wires, including the ground wire. The 4 THHN wires would then leave the house in the conduit to go to the disconnect box. Is that correct/OK?

I note that my outside A/C compressor seems to have NM cable running through outside metal conduit to it, and that outside receptacles on my house utilize ordinary plastic outlet boxes mounted to studs within the outside wall of the house, with the Romex going into those outlet boxes and ending on ordinary receptacles, which are then simply covered with an outdoor, water-resisitant type of wall plate which uses a gasket and receptacle covers.

Could the disconnect box be legally mounted low on the outside wall, at the level of the rim joist (maybe only 6-8 inches off the ground), to be able to go through the wall (rim joist) with the 6-3/G cable *directly into* the disconnect box and thereby eliminating the need for conduit on the outside of the house to get to the disconnect box? I don't suppose mounting the box only 6 to 8 inches off the ground is a good idea though, right? And, not up to code? And, I would still not be legally able to bring the NM wire through the wall, even *directly into* the outside box? What if the 6-3G Romex stays inside the stud space of the outside wall of the house, goes up to an appropriate height along a stud, and then goes directly into the disconnect box without outside conduit?

If I wanted to go back into the basement with the line running out of the disconnect box (and heading towards the spa), then I assume I could not use NM cables here either. Could I use the single conductor THHN wires in conduit and just continue that conduit inside the basement for about 2.5 to 3 feet and back out through the wall again at a point that would wind up being outside the sunroom wall? The idea here is to have the disconnect box ultimately wind up inside the sunroom with the spa, but NOT to have the conduit/wiring that is between the disconnect box and the spa going through the wall of the sunroom or through concrete. As long as this run of conduit is less than 6 feet long and not buried, but running into and back out of the basement, this could legally be watertight flexible conduit? Would I have to/could I instead use rigid PVC and employ multiple condulets to make tight turns?

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