Derating conductors in conduit

[dhalleron]

Do I understand this right?

Can someone explain it better to me?

I'm sure I've been guilty if not derating many times in the past. I was just now trying to figure the right way to do it for a small resi job I have coming up.

I will be installing about 20' to 30' of 1" PVC from an outdoor panel to a junction box in the attic.
I will have the following circuits pulled in using conductors that should be dual rated THHN/THWN:
6 - 20 amp multiwire circuits - these are mostly small appliance circuits, dishwasher, disposal, bath GFCI etc.
1- grounding conductor

I assume my 6 multiwire circuits will be considered 9 current carrying conductors.

2008 Table 310.15(B)(2)(a) tells me I need to derate to 70%
Table 310.16 tells me #12 can handle 30 amps in the 90 degree C column.
This would allow me to use #12 wire on a 20 amp breaker I think because it would be derated down to 21 amps.
If I have to use the 75 degree column I believe I would need to go to #10 wire on a 20 amp breaker.

Thanks.

 

[Dennis Alwon]

If they are MWBC then the neutral do not count as a current carrying conductor. I think when you saw you have 6 MWBC you mean you have 3 MWBC-- 6 circuits all together-- NO.

 

[milemaker13]

Another consideration is that for building wiring, #12 is only allowed to be considered 20 amp wire. I have always derated from there. If this isn't correct, I hope someone will chime in and set me straight:dunce:

 

[Dennis Alwon]

You derate from the max in Table 310.16 but the final ampacity must not be more than 20 amps. Thus #12 is rated 30 amps at 90C and with 8 CCC you derate 70%. 30 X 70% = 21 amps. Good for a 20 amp breaker.

 

[augie47]

Another consideration is that for building wiring, #12 is only allowed to be considered 20 amp wire. I have always derated from there. If this isn't correct, I hope someone will chime in and set me straight:dunce:

It's a little involved.
You can derate based on the insulation rating. THHN being a 90? conductor you can use that rating to derate.
BUT
110.14 limits you to the termination ratings. So, using '08 Code, #12 THHN is 30 amps in the 90? column so you can use that, however, io your termination terminals are 75? such as most breakers, you can not exceed the 75? rating of 25 amps.
(of course with #12 you have to also keep 240.4(D)(2) and 249.4(D) in mind )

 

[jumper]

Since we using dual rated THHN/THWN in a wet location, shoudn't we be derating using the 75C column?

 

[augie47]

Since we using dual rated THHN/THWN in a wet location, shoudn't we be derating using the 75C column?

Correct.
My answer was meant to be more generic.
If the conduit the OP mentions is outside, then you would need to derate using the 75? column for the THWN.
I wonder how often that is taken into account.

 

[Dennis Alwon]

Since we using dual rated THHN/THWN in a wet location, shoudn't we be derating using the 75C column?

I always assume thhn/thwn-2. This is pretty standard stuff and it is rated 90C in wet

 

[jumper]

I always assume thhn/thwn-2. This is pretty standard stuff and it is rated 90C in wet

Looks like you are correct, thhn/thwn-2 it is.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet276

 

[Dennis Alwon]

Looks like you are correct, thhn/thwn-2 it is.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet276

Just go to T.310.104(A) in the NEC 2011 and 310.13(A) in the 2008

 

[jumper]

Just go to T.310.104(A) in the NEC 2011 and 310.13(A) in the 2008

I understand the -2 stuff, I just never paid attention that the dual rated wire had -2 on it.

 

[milemaker13]

I have (almost) always used THHN. Is this differant from the THHN/THWN-2? I never heard of derating to 75* due to wet conditions, or outdoor use... Is this true?

Anyway, seems I can load up a bit more in the stuffed runs if I can derate from actual ampacity instead of going from building wire limits. Thanks for the info.:thumbsup:

 

[infinity]

I have (almost) always used THHN. Is this differant from the THHN/THWN-2? I never heard of derating to 75* due to wet conditions, or outdoor use... Is this true?

Anyway, seems I can load up a bit more in the stuffed runs if I can derate from actual ampacity instead of going from building wire limits. Thanks for the info.:thumbsup:

THHN/THWN is dual rate for wet and dry locations. In a wet location it's a 75 degree C conductor, in a dry location it's a 90 degree C conductor. If it's rated THWN-2 then it's a 90 degree conductor installed in wet locations too.

 

[Dennis Alwon]

I have (almost) always used THHN. Is this differant from the THHN/THWN-2? I never heard of derating to 75* due to wet conditions, or outdoor use... Is this true?

Anyway, seems I can load up a bit more in the stuffed runs if I can derate from actual ampacity instead of going from building wire limits. Thanks for the info.:thumbsup:

I would almost bet that the wire you use is not just THHN. Almost all of it today is dual rated, at least around here anyway.

 

[dhalleron]

I did mean 3 MWBC or 6 circuits. And the conduit is outside for most of the run.

I'm not near my code book now and will look later. Maybe I counted the ccc wrong. Hopefully I can still get by with #12.

With everybody's help I think I understand it better now. I did forget about the termination temp rating.

 

[ritelec]

I did mean 3 MWBC or 6 circuits. And the conduit is outside for most of the run.

I'm not near my code book now and will look later. Maybe I counted the ccc wrong. Hopefully I can still get by with #12.

With everybody's help I think I understand it better now. I did forget about the termination temp rating.

6 conductors = 80% of 30 = 24 even 9 conductors =70% of 30 =21 (thhn-2)

6 conductors = 80% of 25 =20 thhn wet..............(although is it really going to get wet)

should be good..............no?

60 degree (2011) = 20


Thanks for the heads up on the thhn and thhn-2 wet and dry.