Conductor Correction for a THHN/THWN

[elmundogrande]

I had some one ask me why I was using a #8 THHN/THWN instead of using #10 wire after corrections, this is outdoors so the insulation and temp. correction would be at 75* for a THWN wet location not at 90* THHN dry. I am a Instructor (retired electrician 35 years) this is the way I was taught and the way I teach and my understanding of the NEC. So the question is am I right or wrong? Any way like I like to say make it bigger and let it flow.

 

[GoldDigger]

I had some one ask me why I was using a #8 THHN/THWN instead of using #10 wire after corrections, this is outdoors so the insulation and temp. correction would be at 75* for a THWN wet location not at 90* THHN dry. I am a Instructor (retired electrician 35 years) this is the way I was taught and the way I teach and my understanding of the NEC. So the question is am I right or wrong? Any way like I like to say make it bigger and let it flow.

Most of the readily available THHN wire (think big box stores anyway) is actually cross listed as both THWN and THWN-2. The latter lets you use the higher temp rating for wet areas also.

But I have to ask what the terminations will be. Without explicitly rated 90C terminations at both ends you cannot use that temperature anyway.

If your wire is really just THHN/THWN, then it seems to me that you are correct.

 

[bob]

What corrections are you making? Load amps?

 

[Dennis Alwon]

I had some one ask me why I was using a #8 THHN/THWN instead of using #10 wire after corrections, this is outdoors so the insulation and temp. correction would be at 75* for a THWN wet location not at 90* THHN dry. I am a Instructor (retired electrician 35 years) this is the way I was taught and the way I teach and my understanding of the NEC. So the question is am I right or wrong? Any way like I like to say make it bigger and let it flow.

I agree with you in the hypothetical example however , as stated above, all the thwn wire is dual rated as thwn-2 which is rated 90C wet and dry. Can't answer what wire size without knowing more particulars

 

[Dennis Alwon]

But I have to ask what the terminations will be. Without explicitly rated 90C terminations at both ends you cannot use that temperature anyway.

I am sure you know this but for the op's sake the equipment must also be rated for 90C to use the wire at 90C. You can, however use 90C for de-rating if the wire is rated 90C.

[QUOTE110.14(C)(1)]Conductors with higher temperature ratings, provided
the ampacity of such conductors does not exceed the
75°C (167°F) ampacity of the conductor size used, or
up to their ampacity if the equipment is listed and identified
for use with such conductors
[/QUOTE]

 

[NorthwestPV]

You will need more data including number of conductors in conduit, highest recorded temperature for your area, and distance of conduit from roof. You then will use tables 310.15(B)(16), 310.15(B)(3)(c), 310.15(3)(a), and 310.15(B)(2)(a) to calculate the wire size.

Sorry, I just reread your post and you mentioned wet location. In my mind I assumed you were talking about a roof install but that was not mentioned.

 

[Carultch]

Most of the readily available THHN wire (think big box stores anyway) is actually cross listed as both THWN and THWN-2. The latter lets you use the higher temp rating for wet areas also.

But I have to ask what the terminations will be. Without explicitly rated 90C terminations at both ends you cannot use that temperature anyway.

If your wire is really just THHN/THWN, then it seems to me that you are correct.

Is there any THHN/THWN that you would buy today, that isn't also dual rated as THWN-2? I see THHN on the quotes from supply houses, and without being in the know, I have to ask for a datasheet to confirm if it is also THWN-2. Most of the time it is. Why they don't just write THWN-2 on the quotes, I don't know. THWN-2 covers all applications, and THHN does not.

I can understand if it is old wire, that isn't also dual rated.

 

[johnmeto]

A large portion of the promptly accessible THHN wire (prepare to stun the world box stores at any rate) is really cross recorded as both THWN and THWN-2. The recent gives you a chance to utilize the higher temp rating for wet zones moreover.

Be that as it may, I need to ask what the terminations will be. Without expressly appraised 90C terminations at both closures you can't utilize that temperature in any case.

 

[Carultch]

A large portion of the promptly accessible THHN wire (prepare to stun the world box stores at any rate) is really cross recorded as both THWN and THWN-2. The recent gives you a chance to utilize the higher temp rating for wet zones moreover.

Be that as it may, I need to ask what the terminations will be. Without expressly appraised 90C terminations at both closures you can't utilize that temperature in any case.

You can take credit for the 90C rating, when you need to make a bundling adjustment, temperature correction, or both. In any case, the termination ampacity before any derate factors is limited to the equipment manufacturer's requirement. 60C by default for 100A and less unless listed/marked otherwise (a mostly academic rule), or 75C by default for greater than 100A. In a practical sense, it is usually 75C.

Even if lugs are marked AL9CU, very common, that doesn't necessarily mean that it is 90C rated. The equipment as a listed manufactured product, needs to be rated for 90C (rare, if ever) before you take credit for that rating at the termination. Unless you are making a separately installed connection, such as a splice.

 

[elmundogrande]

16-250w micros on a roof 1amp output current

16-250w micros on a roof 1amp output current

this is my math 16x1A=16A, 16x125%=20A The wires in question were marked THWN , THHN and that was for the #10 and #8 because it did not say THWN-2 I de-rated at 75* for a ambient 0f 104*+40*= 144*in a wet location, so I used a correction factor of 0.47 35x.47=16.45A and 50x.47=23.5A. And my understanding of the code you use wire insolation type for de-rating, so it does not say THWN-2 I cant de-rate at the value

 

[Carultch]

this is my math 16x1A=16A, 16x125%=20A The wires in question were marked THWN , THHN and that was for the #10 and #8 because it did not say THWN-2 I de-rated at 75* for a ambient 0f 104*+40*= 144*in a wet location, so I used a correction factor of 0.47 35x.47=16.45A and 50x.47=23.5A. And my understanding of the code you use wire insolation type for de-rating, so it does not say THWN-2 I cant de-rate at the value

If it only says THHN, you can only use it in a dry location period. The two H's indicate that you can use the 90C rating.
If it only says THWN, you can use it anywhere inside a raceway, but only with the 75C rating. Single H = 75C, W = wet location.
If it says THHN/THWN and not THWN-2, you can use it in a dry location with the 90C rating, or in a wet location with the 75C rating.
If it says THHN/THWN/THWN-2, you can use it anywhere inside a raceway with the 90C rating. The -2 means dual rated.

Ask for a datasheet if you receive a quote that only indicates one of the identifiers. Most wire in this family that is sold today as an individual conductor, is THWN-2.

 

[MasterTheNEC]

Is there any THHN/THWN that you would buy today, that isn't also dual rated as THWN-2? I see THHN on the quotes from supply houses, and without being in the know, I have to ask for a datasheet to confirm if it is also THWN-2. Most of the time it is. Why they don't just write THWN-2 on the quotes, I don't know. THWN-2 covers all applications, and THHN does not.

I can understand if it is old wire, that isn't also dual rated.

To be honest with you....it is the Engineers who have to answer to this issue. I do a lot of specification writing and updating of specifications for various engineering firms and without a doubt their specs always say THHN/THWN. This makes a huge difference in terms of adjustments and corrections based on the condition of use. To assume all THHN/THWN is also THWN-2 is not totally accurate. While most all of the major manufacturers in the Wire and Cable industry do provide dual or even tripled ratings on their insulation....they don't have to under UL 83 or 44.

This also goes for XHHW as well...which is still only rated at 75C in wet locations without that -2 added.