# Derating on Roof Top



## Dash Dingo (Mar 3, 2012)

I got 6 current carrying conductors in a conduit running on a roof top to feed two RTU's.
Do I have to derate my conductors at 80% based on 310.15 B2a and then also derate them again based on 310.15B2c?

Or can I just derate them one time using 310.15 B2c ( adjustments for conduits exposed to sun light on roof tops).

Also I have a roof top feed that pokes through the deck, right under the disconnect and runs 3 feet right into the disconnect for the RTU (it's out side).
Does the nec require where it poked through the roof to be derated within a 1/2" off the roof?

And if i am running conduit for the RTU's from the panel inside the building, set a box inside the building under the RTU, feed it from the panel in 10 awg. 
Can I feed from that box up through the witches hat or roof jack in #8 to meet the requirement?

or does it have to be in #8 the whole way? There would only be 3' exposed seal right on roof, straight up to the disconnect.


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## FaultCurrent (May 13, 2014)

First derating factor will be 80% based on the wire ampacity dependent on insulation class. For THHN it would be 90 deg column times 80%. 

The circular raceway exposed to sunlight on roof factor would then be applied.

In actual practice the 90 degree column times 80% ampacity will usually leave you with room at 75 degree termination ampacity using the next higher size rule. Assuming it's not over 800 amps and the load doesn't exceed the corrected value.

I would never apply the solar correction to the stub through the roof directly into the bottom of the enclosure. The conduit doesn't really "cross" the roof.


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## Pete m. (Nov 19, 2011)

IIRC, there was a proposal to remove the derating requirements for rooftops. Guess we will see when the 2017 comes out. Unfortunately, until the jurisdiction you are working in adopts the 2017 (if the proposal survives) you may not be able to take advantage of it.

Pete


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## Dash Dingo (Mar 3, 2012)

Pete m. said:


> IIRC, there was a proposal to remove the derating requirements for rooftops. Guess we will see when the 2017 comes out. Unfortunately, until the jurisdiction you are working in adopts the 2017 (if the proposal survives) you may not be able to take advantage of it.
> 
> 
> 
> Pete



Can I splice a larger wire onto a smaller one before it penetrates up through the roof to meet the requirement.
Ex. Run 10's from the panel and then splice 8's at a j box below and then run 8's on the rooftop?


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## Pete m. (Nov 19, 2011)

Dash Dingo said:


> Can I splice a larger wire onto a smaller one before it penetrates up through the roof to meet the requirement.
> Ex. Run 10's from the panel and then splice 8's at a j box below and then run 8's on the rooftop?
> 
> 
> Sent from my iPhone using Tapatalk


I can think of no reason that would not be permitted.

Pete


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## luckylerado (Mar 19, 2010)

Dash Dingo said:


> Can I splice a larger wire onto a smaller one before it penetrates up through the roof to meet the requirement.
> Ex. Run 10's from the panel and then splice 8's at a j box below and then run 8's on the rooftop?
> 
> 
> Sent from my iPhone using Tapatalk


This only works if you meet the requirement of the 310.15(A)(2) exception 

(2) Selection of Ampacity. Where more than one ampacity 
applies for a given circuit length, the lowest value shall be used.

_*Exception:* Where two different ampacities apply to adjacent 
portions of a circuit, the higher ampacity shall be permitted 
to be used beyond the point of transition, a distance equal to 
3.0 m (10 ft) or 10 percent of the circuit length figured at the 
higher ampacity, whichever is less._


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## Grab (Nov 3, 2012)

Take a look at 310.15(3)(c) Type XHHW-2 insulated conductors shall not
be subject to this ampacity adjustment


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