# conductor derating on rooftops



## sunpower (Jul 15, 2011)

Can a 15amp cot be derated to #6wire for a roof top. Seems crazy


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## oldtimer (Jun 10, 2010)

sunpower said:


> Can a 15amp cot be derated to #6wire for a roof top. Seems crazy


 Please Explain This!!!

15 amp cot???

Someone may be able to answer, if we know what you are asking! :blink:


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## sunpower (Jul 15, 2011)

Sorry ckt


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## oldtimer (Jun 10, 2010)

sunpower said:


> Sorry ckt


 O.K. 

Are you asking about an R T U ?

Voltage?

Single phase?

3 phase?

And as much other pertinent info. Please.


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## sunpower (Jul 15, 2011)

Threephase
110 volt
100' run
Rtu?


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## Wirenuting (Sep 12, 2010)

sunpower said:


> Threephase
> 110 volt
> 100' run
> Rtu?


3 phase 110? :/


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## sunpower (Jul 15, 2011)

One phase to ground is 110. Ps voltage does not affect ampacity as long as its under 600 volts


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## macmikeman (Jan 23, 2007)

sunpower said:


> One phase to ground is 110. Ps voltage does not affect ampacity as long as its under 600 volts


I'm curious to hear more about this. Can you please do some further elaboration on it so I can fill in some gaps in my knowledge?

Thanks in advance.


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## Wirenuting (Sep 12, 2010)

sunpower said:


> One phase to ground is 110. Ps voltage does not affect ampacity as long as its under 600 volts


Ok, if you say so.


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## sunpower (Jul 15, 2011)

What else do we need here. 110/208v 3 phase
One inch emt
22 conductors no neutrals
145 f ambient temp
Thwn-2 wire
75 c terminals on equipment


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## sunpower (Jul 15, 2011)

Its a tricky one right?


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## Wirenuting (Sep 12, 2010)

sunpower said:


> Its a tricky one right?


I just took a stab at it and came up with #6 also. 
Your #14 came up with a 5 amp capacity before the 45% derate. 
I'll wait for a better answer.


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## sunpower (Jul 15, 2011)

Now if I put j box right off roof. I can splice non temperature derated wires to the number 6 awe wire? As long as I derated conduit fill. Right


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## Wirenuting (Sep 12, 2010)

sunpower said:


> Now if I put j box right off roof. I can splice non temperature derated wires to the number 6 awe wire? As long as I derated conduit fill. Right


Don't go with my answer. 
I'll bet a buck I'm wrong.


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## sunpower (Jul 15, 2011)

Thanks for actually trying to help. 
I appreciate the calcs


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## ilikepez (Mar 24, 2011)

All the conductors on your roof should be derated for temperature. I assume you are doing a PV system? Why are you inverting right at the cells? You should put the cells in series until you have like 500 volts to lower your amperage then take it down through the conduit to an inverter right next to your main panel.


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## sunpower (Jul 15, 2011)

Micro inverters,


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## macmikeman (Jan 23, 2007)

sunpower said:


> Micro inverters,


Do you have an attic on this structure? With micro inverters you could put a j-box at the roof / attic connection point, then inside the attic, while the temps may still be high, they are not subject to the derate rules about conduit on rooftops so you can drop to an appropriate wire size based on the attic ambient, and you can also use romex if allowed in your area, since you have ac power after the micro inverters. Then from the attic , convert again to thwn and bring it down the exterior of the wall in conduit to the disco. Without calculating this my swag will still have you using #10 all the way if you do it this way. swag.


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## sunpower (Jul 15, 2011)

No attic, can't use romex . But love the idea about switching wire size once indoors. Thanks.


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## Dennis Alwon (May 9, 2009)

I would like to know how you will get 22 #6 thwn-2 wires in a 1" EMT?


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## sunpower (Jul 15, 2011)

Typo sorry. Two inch


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## Dennis Alwon (May 9, 2009)

sorry .......... calc was incorrect


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## sunpower (Jul 15, 2011)

Ok. Not sorry


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## Wirenuting (Sep 12, 2010)

Dennis Alwon said:


> sorry ..........


LoL, I thought that also, then figured he ment 22 #14's. That gave him a 27% fill.


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## Dennis Alwon (May 9, 2009)

Wirenuting said:


> LoL, I thought that also, then figured he ment 22 #14's. That gave him a 27% fill.


I mistakenly used the area of #6 thw-2 instead of #6 thwn-2. I didn't realize there was a thw-2


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## ilikepez (Mar 24, 2011)

Can you combine all the circuits together at a combiner box and then make the 100' run? At least that would save you from having to derate for conduit fill.


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