Design temperatures for PV circuits in attic space

Status
Not open for further replies.

SunFish

NABCEP Certified
Location
ID
Occupation
Sr. PV Systems Design Engineer
What is an acceptable design temperature for derating conductor ampacities of PV output circuits when they are running through attic spaces? Attics can get much hotter than ambient outdoor temperatures in the summer. Is there anything in the NEC that addresses this? Recommendations?
 

Carultch

Senior Member
Location
Massachusetts
What is an acceptable design temperature for derating conductor ampacities of PV output circuits when they are running through attic spaces? Attics can get much hotter than ambient outdoor temperatures in the summer. Is there anything in the NEC that addresses this? Recommendations?

I've never thought about that before. I sure would like to know the answer too.

The way that I see the "rooftop temperature adder", is that it is in regards to a raceway in direct sunlight.

I'd also like to know what to do about PV circuits that are immediately beneath the PV array. Are they in direct sunlight? Well not really, because they are in the array shadow. Are they hotter than ambient for other reasons? Of course.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Measure the attic temperature over the course of some hot summer days?

Do this in a few different attics and develop a 'rule of thumb' for a temperature adder over ambient outside temp?

I'm just trying to make some suggestions. I'm not aware of anything in the NEC that addresses this.
 

shortcircuit2

Senior Member
Location
South of Bawstin
I think you're missing the point of the question. The question is, how much higher above the outside air ambient temperature would we need to consider for wiring in an attic?

That is...if any at all.

It doesn't have anything to do with the outside air ambient temperature. Derate for the temperature of the location. There couldn't be any standard design temperature for all attics, as it would depend on the location.

So lets take Massachusetts for an example...lets say the temperature of a given attic is 140 degrees F. Wiring in that location would need to be adjusted according to table 310.15(B)(2)(a)...so a 90 degree conductor would have a correction factor of 0.71%

So with #10 THWN-2, I would multiply 40amps X 0.71 = 28.4 amps
 

Carultch

Senior Member
Location
Massachusetts
It doesn't have anything to do with the outside air ambient temperature. Derate for the temperature of the location. There couldn't be any standard design temperature for all attics, as it would depend on the location.

So lets take Massachusetts for an example...lets say the temperature of a given attic is 140 degrees F. Wiring in that location would need to be adjusted according to table 310.15(B)(2)(a)...so a 90 degree conductor would have a correction factor of 0.71%

So with #10 THWN-2, I would multiply 40amps X 0.71 = 28.4 amps


The question is, suppose the outdoor air is 90F, and you have no data for the particular attic in which you are interested. Is there a practical method for figuring out what the air temperature inside of an attic is, when all you know is the outdoor air temperature?
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
The question is, suppose the outdoor air is 90F, and you have no data for the particular attic in which you are interested. Is there a practical method for figuring out what the air temperature inside of an attic is, when all you know is the outdoor air temperature?

No.

If the house has no shade, the attic will get hotter than one that is always shaded.

Ventilation is also a factor.

Even something like the color of the shingles can affect the attic temperature.

There are too many variables to try to come up with an answer based simply on outdoor temp.
 

Smart $

Esteemed Member
Location
Ohio
Great reference.
Considering how many variables there are, I thought it to be quite straightforward and not overly complex.

I ran across an ASHRAE document that covered running ducts through attic spaces, and there were a bunch of calculations involved in determining attic temperature, heat loss/gain, etc., etc. yada yada yada. We electricians want something a lot more concise and easy to implement. :happyyes:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Considering how many variables there are, I thought it to be quite straightforward and not overly complex.

I ran across an ASHRAE document that covered running ducts through attic spaces, and there were a bunch of calculations involved in determining attic temperature, heat loss/gain, etc., etc. yada yada yada. We electricians want something a lot more concise and easy to implement. :happyyes:

I've got a boatload of #10 in the truck. Will that work? :D
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I find #10 to work best for most individual PV source circuit applications. It gives you flexibility for most derate applications and common circuit length/voltage drop.
That's why most PV installers have a boatload of it in the truck. :D
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I've got a boatload of #10 in the truck. Will that work? :D

In all seriousness, that's a good approach. If anyone asks, you say:

"Well, I used #10 THWN-2 wire, which is good up to about 167 degrees Fahrenheit (for 3 conductors in a conduit).
Do you think the attic actually gets that hot?" :cool:

Personally I have yet to run into a situation where 90C rated #10 wasn't good enough for a PV source circuit in a residential install. (It can happen much more easily in commercial if you don't separate your source circuits to avoid having 10 CCCs in a conduit.)
 
I guess I'm about a year late on this, just thought I'd pitch in with my 2 cents. This handout from City of LA lists the ambient temps for attics and behind modules. Wish there was a more standardized guideline based on ambient temp, even if it were conservative.

Also once had an AHJ require using a 70 deg C temperature behind the modules per the informational note at 690.31(A). Enphase trunk cable is #12AWG - so much for stocking up on #10.
 

Attachments

  • LADBS Ambient Temps.pdf
    85.1 KB · Views: 2

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I guess I'm about a year late on this, just thought I'd pitch in with my 2 cents. This handout from City of LA lists the ambient temps for attics and behind modules. Wish there was a more standardized guideline based on ambient temp, even if it were conservative.

Also once had an AHJ require using a 70 deg C temperature behind the modules per the informational note at 690.31(A). Enphase trunk cable is #12AWG - so much for stocking up on #10.
You do know that #10 is bigger than #12, right? Just checking.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Well that'd be the issue wouldn't it. Got to use the #12 trunk cable for the Enphase. Using a 70 degree design temp definitely limits that branch circuit size.
Puts a high premium on keeping the trunk cable well above the roof surface and keeping it in free air.
It would be great for 2014 users if trunk cable with X... type conductors were available. You could still combine two trunk cables into one feeder circuit using better/larger wire.
 
Status
Not open for further replies.
Top