derating # 12 copper outdoors

[Stevenfyeager]

I would like to run 3 20 amp gfci circuits in a 1/2" pvc conduit outdoors on the side of a brick wall, using total of 7 #12 conductors. (3 hots, 3 neutrals, one ground) My derating would derate from 30 amps for #12 copper to 21 amps, correct? Or does being outside change this? thank you.

 

[Ponchik]

Being outside does not change it. Unless the heat of the sun will have an effect on it. Even then I don't think that comes into effect because you are not on the roof.

That would be 6 CCC at 80%. So your conductor ampacity after the derating will be 24A.

 

[Jraef]

I would like to run 3 20 amp gfci circuits in a 1/2" pvc conduit outdoors on the side of a brick wall, using total of 7 #12 conductors. (3 hots, 3 neutrals, one ground) My derating would derate from 30 amps for #12 copper to 21 amps, correct? Or does being outside change this? thank you.

Wait, what? Where are you getting 30A on #12 to begin with? Are you reading 31O.15(B)(17)? Because that's only for SINGLE conductors in FREE AIR. Conductors in a raceway is 31O.15(B)(16), which means 25A max, then you de-rate for more than 3 in a raceway per 310.15(B)(3)(a). So you are really at 80% of 25A, so 20A max. That still gets you where you wanted to go, it's just that I think you were starting off wrong.

 

[Ponchik]

Wait, what? Where are you getting 30A on #12 to begin with? Are you reading 31O.15(B)(17)? Because that's only for SINGLE conductors in FREE AIR. Conductors in a raceway is 31O.15(B)(16), which means 25A max, then you de-rate for more than 3 in a raceway per 310.15(B)(3)(a). So you are really at 80% of 25A, so 20A max. That still gets you where you wanted to go, it's just that I think you were starting off wrong.

Free air is 40A.

 

[Jraef]

Free air is 40A.

Only if using the 90C column, and you can only use the 90C rating if every device and termination in the circuit is rated 90C, which you will not find on anything that you can connect with 12awg wire to a GFCI...

 

[Ponchik]

Only if using the 90C column, and you can only use the 90C rating if every device and termination in the circuit is rated 90C, which you will not find on anything that you can connect with 12awg wire to a GFCI...

For derating I have been using the 90 degree column all this time. Am I missing something here??

 

[Little Bill]

Only if using the 90C column, and you can only use the 90C rating if every device and termination in the circuit is rated 90C, which you will not find on anything that you can connect with 12awg wire to a GFCI...

For derating I have been using the 90 degree column all this time. Am I missing something here??

Jraef, the 90° column can be used for derating purposes. The devices/terminations don't have to be rated 90° because you're only using the column for derating.
Now this all depends on the wire type. So if you're using THWN (as an example) then you can use the 90° column for derating.

Edward, you have been correct.

 

[JFletcher]

You derate #12 from 90* column, which is 30A. 240.4(D) prohibits using #12 with more than a 20A OCPD, except in cases for things like motors.

7 conductors present is for figuring out conduit fill, but as mentioned before, only 6 of those conductors are current carrying, so you would be at 80%, or 24A.

Ambient temperature comes into play even outside of attics, tho you'd have to be at 40*C+ for it to cause an ampacity adjustment. What you have is fine.

If you have a long run of PVC, you will most likely need an expansion coupling due to ambient temperature changes, and if it's in direct sunlight, you may want to add 20*F to the upper end, tho ambient temperature extreme highs rarely affect ampacity of the conductors within the conduit. Here, I figure 100*F change for purposes of outdoor, unburied conduit expansion (10*-110*), and the same for attic conduits (40-140*).

In summary, 7 #12s, 6 of them being CCC, is fine for conduit fill, and at 24A derated ampacity before ambient temps come into play, your install is fine. At 114*F+, 46-50*C, your ampacity adjustment factor is .82, which 24A x .82 = 19.68A, which to me is still fine on a 20A breaker.

eta: here is an older but still good page that describes derating

http://mjobee.com/projects&news/nec art 310.15.B.2.C.pdf

 

[david luchini]

Now this all depends on the wire type. So if you're using THWN (as an example) then you can use the 90° column for derating.

If you are using THWN, you have to use the 75° column for derating, not the 90° column.

If you are using THWN-2, you can use the 90° column.

 

[Jraef]

OK, OK, I have been schooled... again.:dunce:
Somewhere in the deep dark crevasses of my memory, I now realize I knew that... I just got in a habit years ago of never using the 90C column because the rules on when and how you can use it got me in trouble once and I had to re-pull conductors. So from there on out I just considered it more trouble than it's worth. Then when I got into this side of the biz, where people are always trying to use it inappropriately to keep costs down on equipment, I've had to keep citing that issue over and over. Totally forgot that the subject in this case was the derating factor of the conductors themselves.

Mea culpa... :ashamed:

 

[iwire]

If you are using THWN, you have to use the 75° column for derating, not the 90° column.

If you are using THWN-2, you can use the 90° column.

Good point to bring up.

 

[Ponchik]

OK, OK, I have been schooled... again.:dunce:
Somewhere in the deep dark crevasses of my memory, I now realize I knew that... I just got in a habit years ago of never using the 90C column because the rules on when and how you can use it got me in trouble once and I had to re-pull conductors. So from there on out I just considered it more trouble than it's worth. Then when I got into this side of the biz, where people are always trying to use it inappropriately to keep costs down on equipment, I've had to keep citing that issue over and over. Totally forgot that the subject in this case was the derating factor of the conductors themselves.

Mea culpa... :ashamed:

I think you owe me a beer for making me doubt my self.

Welllll, make that beer, pizza, and ice cream.

 

[Little Bill]

If you are using THWN, you have to use the 75° column for derating, not the 90° column.

If you are using THWN-2, you can use the 90° column.

That is the "generic/normal" term we use for THWN-2 here as that is all they sell.
They use to call everything THHN, then when they stopped stocking that it was changed to THWN. They/we are just to lazy to add the "2".:happyyes:

 

[Jraef]

I think you owe me a beer for making me doubt my self.

Welllll, make that beer, pizza, and ice cream.

 

[Ponchik]

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