Here's my question - If the ambient temperature of the conductors installed on the roof is 129 degrees and the run is 100 foot long with 10 feet on the roof and 90 feet installed inside the building at an ambient temperature of 86 degrees what if any adjustment has to be made in the way of correction factor to the conductor ampacity?

George:

May this be the answer to your inquiry:

2008 NEC 310.15 (A) (2)

"(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply 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."

I bet that is why he picked 10 feet

I do think it is funny that they give you a break on ampacity for a 10 foot section in this article but bust you for 1.5" if it is going through insulation in 334.80

You think George is giving a test??

The logic behind this is the roof "heat' will disapate in the remaining 90'????

Me, I would derate the run! Not that I would have to, but....

I believe the example in the handbook was a pipe running through a warmer room than the rest. I am with you, on the roof I would derate. If for no other reason, the heat in the pipe will rise to the highest point and that is probably the roof.

I think that George is going to lurk around and see what (hopefully) the lot of us say.

John- How in the world could you even think that I would ever do such a devious thing as that?.

This roof top heat thing is all over the map. What if the roof is white what if the roof is black or maybe a metal roof. What if the conductors are on the roof but in a shaded part of the roof due to structural features?

Yes the code reference is 310.15(A)(2) and I don't think it solves the problem.

In the mean time I hope John will humble himself and apologize for thinking poor little ole George was trying to be crafty.

This roof thing is still in it's infancy. I assume there will be adjustments to the language as we go along but I am happy that they are referencing it. Down here in sun country, walking around on a roof with an IR thermometer will really open your eyes. Metal is usually too hot to touch and 140-150f is not unusual. A built up "tar and gravel" roof will be gooey.

RANT ON Heat does not rise! Heat travels from high temperature to low temperature. Gravity is not involved! Hot air rises because the denser cold air is pulled down by gravity. If the roof was colder than the building, would the cold fall to breaker panel? RANT OFF

Sorry, thermodynamics and nuclear power are triggers for over explinations.

Larry

OK at night time the cold air from the roof would be pulled down by gravity displacing the warmer air in the building. Happy now
Air is still going to be the working fluid and it will be affected by gravity. The warmer air will be at the top.

When the sun is shining the roof will be the hottest spot unless this is a smelting plant.

George:

Humble pie in hand....I sorry! Let's see where this goes......

Okay we won't settle the argument about what happens when the temperature changes, or what came first the chicken or the egg or whether it is hole flow or electron flow when current flow happens.

Next question: Given the temperatures mentioned in the original post, what size wire would we use if the load were 34 amperes? Plan on THWN.

Oh, John I accept your apology.

How high is the conduit above the roof?
(Table 310.15(B)(2)(c))

Figure 129 degrees F. as the temperature on the conductors inside the conduit and that I've already done the math required for 310.15(B)(2)(c).

If you added the height factor in 310.15(B)(2)(c) then I think you derate to 67% of the appropriate column (THWN vs THWN-2)

Greg- you and I and probably John are on the "same page" as they say and now I need someone to tell me what gauge THWN wire would be required for this branch circuit.

If you could find some THWN-2 I would say #8 cu.

You did make this a little test tho. If you say #8 THWN you only get 33.5a

Considering the 10' thing, I would probably take the #8 either way.

I agree Greg. I'd wink at the .5 amp. THWN-2 is something I've not seen out there and don't know how popular it really is.

The elusive THWN-2. This was left over from my wife's 700' pump circuit job. The wire came From City Electric.

Interesting, You have a conduit that would absord the sun energy in free air, add in a roof abosrbing heat and creating thermal effects then reflection. Does the white roof now cause double the efect on the conduit when it refects. Isn't there requirements based on spacing off the roof? What if the conduit is painted with some type of refective paint?

Ob

Ob- The original post was evolved and it was an attempt to see if the rules for roof top temperature could be followed and if the rule for 310.15(A)(2) could be also incorporated.

Picture this: Ambient temperature based on ASHRAE is 89 degrees F in Detroit. Add to that 40 degrees F for the adder mentioned in T. 310.15(B)(2)(c) for a conduit installed 4" above the roof. Now we have 129 degrees and we need a THWN conductor that will be adjusted to that temperature and still be rated for 34 amperes. Since 10' of wiring in the 100' run example exceeds the limit we can not ignore the temperature correction factor.

I imagine this will still be surprising to the guys who slept through that part of their code update and "never did it that way before".
I suspect the code language was really just trying to recognize you needed an adjustment and they did not want to get into the detail of raceway color, roof color etc. This is complicated enough as it is. How many guys know where to find the ASHRAE data?

I see this biting the guys who should understand, the hardest, the solar installers. I wonder if they know they should derate the conductors in their DC raceways? 690.8 doesn't seem to mention it.

I can also see this getting the HVAC installations and the guys who are installing to the label on the condenser.

Is this really being enforced?

Hello, first post here, been a lurker for awhile now.

This is being enforced in Calif, has been for a few years now in the PV installs at the very least (DC and AC conductors).

Whether by NEC 2005 as 690.31(C) or NEC 2008 as 310.15(B)(2)c and/or 690.31(C).

Each jurisdiction is different and some even have their own correction factor regardless of what the code says.

Oh, the temp in conduit is higher above a white roof than a black roof.

Vindeceptor:

Welcome to ECN forums from one of the 'Jersey Guys'!!

Thanks for your input!

Vindiceptor,

Glad to have you here, don't be afraid to speak up, we won't bite. This is a great bunch of guys here and there are many knowledgeable people with great advice they are willing to share.