ampacity and temperature derate before and after junction box

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I have a scenario where I will be running a number of conductors from an electrical room, up through the building, penetrating the roof, and then over a roof for a while. I'm hoping to only have one penetration and will have 12 CCC's running through the single conduit within the building. On the roof, the conductors will be going multiple directions in many separate conduits, splitting off in a junction box only a few feet away from the penetration.

I know I need to use the temperature adder for exposed conduit on a roof for my temperature derate, and that I'll need to use a 50% derate for having 12 conductors in the same conduit inside, but I'm trying to determine if I can justify not needing to use both the 50% conduit fill derate and temperature derate at the same time for all the conductors. Basically - can I use the temperature adder only for the conductors on the roof because they will be in separate conduits and no fill factors will apply, and then do a separate calculation for the conductors in the building with the fill factor applied but no temperature adder because the length of conduit on the roof with all the conductors in it is less than 10', then just take the result of the two separate calculations that leads me to needing the larger wire size as my result?

Is it ok to make these two calculations separately with the conductors being continuous, or if I had some sort of a splice in the junction box where the conductors all separate so that technically the conductor going through the building is different from the conductor on the roof would that make it ok?

I hope I've described this well enough to understand, but please ask for more details if I haven't.
 

beanland

Senior Member
Location
Vancouver, WA
You answered your own question

You answered your own question

"I have a scenario where I will be running a number of conductors from an electrical room, up through the building, penetrating the roof, and then over a roof for a while."

If all 12 conductors are in one conduit above the roof, you need to apply both the 50% and solar exposure derate to that section of conduit. Below the roof the solar derate does not apply. After you split the conductors into different conduits, the 50% derate does not apply.
 

Fliz

Member
Location
San Francisco
"I have a scenario where I will be running a number of conductors from an electrical room, up through the building, penetrating the roof, and then over a roof for a while."

If all 12 conductors are in one conduit above the roof, you need to apply both the 50% and solar exposure derate to that section of conduit. Below the roof the solar derate does not apply. After you split the conductors into different conduits, the 50% derate does not apply.


Agree. The reason being that two factors are impinging on the conductors (fill and roof temperature) both derating rules apply simultaneously.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Agree. The reason being that two factors are impinging on the conductors (fill and roof temperature) both derating rules apply simultaneously.

It would be nice if the code contained an exception for the roof temperature derating for short lengths of conduit, but it does not. Is there a reason that you cannot mount the junction right above the penetration?
You might instead have to use a larger size conduit or two conduits for the short distance from penetration to junction box.
 

kwired

Electron manager
Location
NE Nebraska
Before some of you say no, read the exception after 310.15(A)(2).

OP said there is less than 10, feet from roof penetration to this J box where it splits up. If the amount of the raceway beyond the room penetration is also less than 10 percent of the higher ampacity portion of the conductor length then it need not be derated for this short distance. So if there were 5 feet of raceway exposed above the roof there would need to be at least 50 feet of conductor inside that was not derated for same temperature as what is on the rooftop. The less conductor on the rooftop, the less the length inside needs to be to be able to do this.
 

Smart $

Esteemed Member
Location
Ohio
Before some of you say no, read the exception after 310.15(A)(2).

OP said there is less than 10, feet from roof penetration to this J box where it splits up. If the amount of the raceway beyond the room penetration is also less than 10 percent of the higher ampacity portion of the conductor length then it need not be derated for this short distance. So if there were 5 feet of raceway exposed above the roof there would need to be at least 50 feet of conductor inside that was not derated for same temperature as what is on the rooftop. The less conductor on the rooftop, the less the length inside needs to be to be able to do this.
I'll second that. :thumbsup:
 

Smart $

Esteemed Member
Location
Ohio
I'll second that. :thumbsup:
Ohh! Forgot to add that the conductor's ampacity on the roof's underside is not the only portion that can apply for the exception. The ampacity to the load side can too. The exception says adjacent portions of a circuit.
 
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