Hey y'all. Been a while since I was here. Hope someone can un-confuse me.
Problem: Currently using #10 THWN, protected by a 20-amp breaker, for a run from the house to a residential greenhouse. I'm using the #10 in this case for Voltage drop. Once at the greenhouse, however, I was planning on switching to #12 THWN for branch circuit. My question is about derating the wire once it is inside the higher ambient temperature of the greenhouse. Assuming the temperature will be higher enough to warrant derating, would I have to derate to the point of having the circuit on a 15-amp circuit? If so, what are the ramifications, if any, of having my #10 on a 20-amp breaker?
How hot is the greenhouse? Bear in mind you derate from the 75c column so until it gets up in the 130f range you should be OK with 3 wires in the pipe/cable. If this is an agricultural greenhouse I doubt it really gets over 100 before the plants burn up. If it is a solar collector greenhouse you need the design temp.
Thanks for the quick reply, George. My mistake, I guess, was in using the 60 degree column. But now I see from the examples in NEC2008 Handbook for derating per article 310.15 that they actually use the 90 degree column (though this is for number of conductors in conduit). I can't find which column to use for temperature correction. Can you help me out with a code reference for using the 75 degree column? Much obliged.
The principle holds ... assuming the wire is a 90 degree rated type, you do ALL your derating starting from the 90 degree column. Looking at the temperature limitations of your terminations is the last step in the process.
The derating info is in the table at the bottom of 310.16. The only reason I said 75c was if you are using THHN/THWN in a wet location you should use the THWN rating to be totally code compliant. I thought the greenhouse might be wet. If this is single rated THWN you have to use 75c
I was looking at THWN[size:8pt]-2, (90 degree column), but I can see that the temperature in the g'house will not be a continuous 90 degreesC. Nevertheless, "Reno" sez always begin derating from the 90 degree column.
"Reno": how come? Code reference please. Thanks.[/size]
I think since you are using THWN-2 you are starting at the 75° column. If you were using THHN-2 then you'd be starting from the 90° column. The ambient temperature apparently is not an issue. By the way, don't call Greg, George, he might get insulted:)
[size:11pt]THWN -2 is clearly listed under the column for 90 degree. [size:8pt] Still no one is giving me a code reference.
The only thing I "know" about which column to use for what situation is that for NM you use the 60 degree column if the amperage is 100a or less; and the 75 degree column for NM over 100a. And the only reason I "know" this is that it is written in my 5th -yr. codebook, per my instructor at the time, in preparation for the big exam.. I have no idea where he got this.
Besides, none of that helps me with the question of where in the code does it specify which column to use for derating. You may be forcing me to dig out old study aids from the musty bins of my garage.[/[/size]size]