Derating Factor

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rl2

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We have 12 conduits running on a roof top with 3-500kcm + gnd.

Do we need to apply derating factors on the wires?

Please direct me to the code for reference.

Thank you friends!:D
 
Depending on the Code cycle and the insulation type, you may or may not have to do temperature corrections that will be significant.
Fill-based adjustments, no, since you have at most 3 CCCs.

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As Digger stated it depends on the code cycle. Under the 2017 if your raceway is elevated above the roof (I believe) 7/8" no derating required.
 
rl2 said:
We have 12 conduits running on a roof top with 3-500kcm + gnd.

Do we need to apply derating factors on the wires?

Please direct me to the code for reference.

Thank you friends!:D
Click to expand...

what code is applicable in your area?

NEC 2017 B3:

(c) Raceways and Cables Exposed to Sunlight on Rooftops.
Where raceways or cables are exposed to direct sunlight
on or above rooftops, raceways or cables shall be installed a
minimum distance above the roof to the bottom of the raceway
or cable of 23 mm (7⁄8 in.). Where the distance above the roof
to the bottom of the raceway is less than 23 mm (7⁄8 in.), a temperature
adder of 33°C (60°F) shall be added to the outdoor
temperature to determine the applicable ambient temperature for
application of the correction factors in Table 310.15(B)(2)(a) or
Table 310.15(B)(2)(b).
 
its 2014. are there any changes since then? thanks for the inputs.

Jim1959 said:
what code is applicable in your area?

NEC 2017 B3:

(c) Raceways and Cables Exposed to Sunlight on Rooftops.
Where raceways or cables are exposed to direct sunlight
on or above rooftops, raceways or cables shall be installed a
minimum distance above the roof to the bottom of the raceway
or cable of 23 mm (7⁄8 in.). Where the distance above the roof
to the bottom of the raceway is less than 23 mm (7⁄8 in.), a temperature
adder of 33°C (60°F) shall be added to the outdoor
temperature to determine the applicable ambient temperature for
application of the correction factors in Table 310.15(B)(2)(a) or
Table 310.15(B)(2)(b).
Click to expand...
 
rl2 said:
its 2014. are there any changes since then? thanks for the inputs.
Click to expand...


2014 eliminated the derating requirement if you use XHHW conductors, for other insulation types you still need to use table 310.15(B)(3)(c). The 2017 requires the minimum of 7/8" above the surface of the roof with no need to derate for all insulation types.
 
infinity said:
2014 eliminated the derating requirement if you use XHHW conductors, for other insulation types you still need to use table 310.15(B)(3)(c). The 2017 requires the minimum of 7/8" above the surface of the roof with no need to derate for all insulation types.
Click to expand...

Do you have the '17 ROPs or ROCs ? I'd like to know the reasoning. Just surprising to go from a fairly massive derating in '14 to practically none in '17/
 
augie47 said:
Do you have the '17 ROPs or ROCs ? I'd like to know the reasoning. Just surprising to go from a fairly massive derating in '14 to practically none in '17/
Click to expand...

I do not have the ROP, but IIRC it was a lack of hard evidence for the first changes. There was no proof of roof top fires for the adder in the first place.

ETA:

There a bunch of inputs when I just looked, so I just snagged this:

This proposal seeks to remove the requirement of temperature correction for wiring methods on rooftops. There has still not been a documented incident of this ever being a problem in the 120 or so years of wiring methods on rooftops. One would think that if this were a real issue there would have been at least one incident.
Click to expand...

Public Input 1947, which correctly points out that no documented conductor failures due to exposure to direct sunlight have been reported to the panel.
Click to expand...
 
Last edited:
infinity said:
2014 eliminated the derating requirement if you use XHHW conductors, for other insulation types you still need to use table 310.15(B)(3)(c). The 2017 requires the minimum of 7/8" above the surface of the roof with no need to derate for all insulation types.
Click to expand...

Well, then this solves our problem. we are using type XHHW conductors and the conduits will be at least 1 1/2" above the roof surface.

Thanks for all the help guys!
 
augie47 said:
Do you have the '17 ROPs or ROCs ? I'd like to know the reasoning. Just surprising to go from a fairly massive derating in '14 to practically none in '17/
Click to expand...

I also heard that the test that brought the rooftop derating to the code was done by putting a piece of conduit on a rooftop in the sun and plugging both ends. Not a real world installation if you ask me.
 
augie47 said:
Do you have the '17 ROPs or ROCs ? I'd like to know the reasoning. Just surprising to go from a fairly massive derating in '14 to practically none in '17/
Click to expand...
There are no more ROPs or ROCs and no PDF of the code change documentation. You have to look at everything online now.
Part of it was the original testing was done with the conduits sealed preventing air flow and that resulted in higher temperatures. When they repeated the testing with unsealed conduits there was not a significant temperature rise within the raceway. With the 2017 change to 225.27, this issue may have to be revisited.
 
The code change for this section goes back to a FPN in 2005, then a code rule.
Seems like the copper development association was behind the changes.
It was a poor rule from the beginning, stating with "conduits exposed to sunlight", and EMT is not conduit, so that got fixed in 2011, and didn't apply to cable, so that got fixed in 2014...
 
tom baker said:
The code change for this section goes back to a FPN in 2005, then a code rule.
Seems like the copper development association was behind the changes.
It was a poor rule from the beginning, stating with "conduits exposed to sunlight", and EMT is not conduit, so that got fixed in 2011, and didn't apply to cable, so that got fixed in 2014...
Click to expand...

EMT is not conduit?
 
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