Underground Feeder Insulation Conflict

[greg_wirenut]

I am involved in a situation where an engineering firm is telling me our underground feeders, which are in conduit, have to have XHHW-2 insulation. We pulled THHN, like we have forever. He says THHN is not rated for that installation because once you get above the frost line (in Atlanta GA mind you) THHN's insulation will deteriorate due to cold temperatures. The feeders terminate in pad mount transformers keeping them nice and toasty. According to Table 310.13 there is no minimum temperature only maximum. Anybody got any suggestions or clarifications?

 

[raider1]

Welcome to the forum.

Read the markings on the wire that you used. Most THHN that is sold will be multi rated such as THHN-2/THWN-2.

He says THHN is not rated for that installation because once you get above the frost line (in Atlanta GA mind you) THHN's insulation will deteriorate due to cold temperatures.

I have never heard that THHN's insulation will deteriorate in cold tempatures.

Chris

 

[walkerj]

If this is a spec, you got caught.
If not, ask for proof.

 

[infinity]

Underground raceways need conductors rated for wet locations. THHN isn't one of them. But as Raider said almost all THHN is dual rated as THHN/THWN which is permitted in wet locations.

 

[iwire]

If this is a spec, you got caught.

That was what I was thinking as well.

 

[wallyworld]

Thhn works pretty well in MAine, its just a hair cooler than Atlanta

 

[ron]

As mentioned, the frost line isn't the issue. If it is underground, THHN is no good as it is not rated for wet locations like THWN. It is likely that your THHN is dual rated, so maybe you lucked out per the code. If the project specs require XHHW-2 insulation, then it is not a code issue but a Contractual issue.

 

[jim dungar]

He says THHN is not rated for that installation because once you get above the frost line (in Atlanta GA mind you) THHN's insulation will deteriorate due to cold temperatures.

Insulation deteriorate in cold. I hope not, it has been below 0?F at least once a day for at least nineteen of the last thirty days. i don't even want to guess what the frost depth is.

As others have said, if the conductor is rated at least THWN it is rated for underground which is a wet location. Very few of the supply houses, in Wisconsin, even stock copper wire with XHHW insulation

 

[ohmhead]

Thhn-2

Thhn-2

Well we dont know much but raider is correct on the dual rated wire if thhn -2 your ok if it doesnt have a W its no good underground . but if thhn has a 2 next to thhn its good .this was a note in older code books hardback under table 310 16 ? best to ya

 

[btharmy]

XHHW is the typical insulation found on the compact aluminum wire I pull for feeders. Can't recall ever seeing it on copper however.

 

[curt swartz]

Well we dont know much but raider is correct on the dual rated wire if thhn -2 your ok if it doesnt have a W its no good underground . but if thhn has a 2 next to thhn its good .this was a note in older code books hardback under table 310 16 ? best to ya

There is no such thing as THHN-2. THHN is a 90C insulation for suitable for dry locations. THWN is a 75C conductor suitable for wet locations. THWN-2 is a 90C rated version of THWN.

 

[woodduder]

I don't have a codebook with me to look up XHHW, but why do these engineers often specify this requirement?

 

[ron]

Randy,
Southwire has a good article comparing "Waste-Water Applications Go for Thicker Insulations"
http://www.southwire.com/processGetArticle.do?commonId=750d8f09738d1010VgnVCM1000002702a8c0________

 

[tom baker]

I don't have a codebook with me to look up XHHW, but why do these engineers often specify this requirement?

XHHW-2 is a much better insulation than THHW for wet locations. I have seen several failures of THHW in underground conduits, which around here, are water filled.
Plus XHHW-2 is 90 C rated for wet locations and THHN in a wet location is not.

 

[iwire]

XHHW-2 is a much better insulation than THHW for wet locations.

I agree 100%:smile:

I have seen several failures of THHW in underground conduits, which around here, are water filled.

Same here.:smile:

Plus XHHW-2 is 90 C rated for wet locations and THHN in a wet location is not.

I think all the THHN/THWN we get is type "-2" so it also has a 90 C rating in wet locations.

 

[JohnJ0906]

I think all the THHN/THWN we get is type "-2" so it also has a 90 C rating in wet locations.

For Southwire, the "-2" is for sizes #8 AWG and larger.
FWIW, their "THHN" has multiple ratings.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet1

 

[greg_wirenut]

Thanks for the reply. I did find a FPN at the end of table 310.13 that says thermoplastic insulation may stiffen at temps below -10C, +14F. Nothing about deterioration though. I know what you mean about no supply houses carrying copper with XHHW insulation, it's a special order item here also. I don't know how you guys do it at temps like you referred to. Brrrr!