And another 2008 question

[ryan_618]

Does a metal raceway that penetrates a rooftop vertically and enters directly into the bottom of a disconnect (cutout box) have to comply with the ampacity adjustment rules of (new) section 310.15(B)(2)(c)? If yes, is there any way around derating the conductors?

 

[Pierre C Belarge]

Does a metal raceway that penetrates a rooftop vertically and enters directly into the bottom of a disconnect (cutout box) have to comply with the ampacity adjustment rules of (new) section 310.15(B)(2)(c)? If yes, is there any way around derating the conductors?

After a quick peak at the change, here is my take.

Yes, the raceway penetrating the roof and installed in a vertical position will require derating - "installed in conduits exposed to direct sunlight ON or ABOVE rooftops."


Notice this says CONDUITS and EXPOSED TO DIRECT SUNLIGHT

I would think they were very specific in this wording.
If so, then raceways may not be the proper term, but conduits which include...RMC, IMC and RNC. I wonder, by the wording they chose, if they meant to not include EMT-electrical metallic TUBING ?
What about a trough (wireway) installed in rooftop locations?

 

[celtic]

I wonder, by the wording they chose, if they meant to not include EMT-electrical metallic TUBING ?
What about a trough (wireway) installed in rooftop locations?

You might want to re-read 100~ Raceway

 

[Pierre C Belarge]

Conduits are raceways, but not all raceways are conduits...

 

[celtic]

Maybe I oughta read your question again

I see what you mean...CONDUITS is the question, not RACEWAYS

 

[Dennis Alwon]

if they meant to not include EMT-electrical metallic TUBING ?
What about a trough (wireway) installed in rooftop locations?

I cannot imagine why EMT would be excluded from this derating. I would assume that the derating has to do with the heat that is produced from the rooftops thus incresing the ambient temp.

I would guess that masts for services would also be affected.

 

[ryan_618]

What about the part that says "bottom of conduit". Does that mean it only applies to horizontal raceways?

 

[Pierre C Belarge]

Ryan
I read the wording out of the analysis book. I do not see where it says the "bottom" of the conduit. It the wording different in the actual code book?

 

[ryan_618]

I don't have my code book here, but I am pretty sure that the table is measured to the "bottom" of the conduit.

 

[tallguy]

I don't have my code book here, but I am pretty sure that the table is measured to the "bottom" of the conduit.

That is indeed what it says. "Distance above roof to bottom of conduit"

How would that make it only apply to horizontal though? Call me a simpleton, but no matter how you orient it, the conduit will have a "bottom".

 

[Pierre C Belarge]

I see it in the table, not in the general body of language as per the section number. That makes it very interesting.

I remember reading about the study performed, I think it was in Las Vegas...I thought what happens in Las Vegas, stayed in Las Vegas

Maybe this wording should still be there...that is until they can get it to make more sense...such as does this apply to Conduits installed vertically through the roof?

 

[M. D.]

2008 NEC

310.15(B)(2)(c)
Conduits Exposed to sunlight on rooftops. Where conductors or cables are installed in conduits exposed to direct sunlight on or above rooftops, the adjustments shown in table 310.15(B)(2)(C) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in table 310.16 and 310.18

My two pennies;

If the conduit through the roof runs directly into the disconnect and it is in direct sunlight there does not seem to be an adjustment in the table to apply , if there is no conduit there is no adjustment to apply , if it is not in direct sunlight (shady side ) no adjustment to apply

 

[ryan_618]

How would that make it only apply to horizontal though? Call me a simpleton, but no matter how you orient it, the conduit will have a "bottom".

Thats the question, isn't it?

So, if I have one foot of conduit on the roof, do I have to take a 50% (or so) hit?

 

[M. D.]

What if the cable is not in a conduit???

http://www.afcweb.com/pdfs/afc_pdeck_flyer.pdf

Would this cable need to have the adhustment made.