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:)

"By the way, don't call Greg, George, he might get insulted:) "

Holy Toledo! Humble apologies to all parties involved!!

- Tripp

No problem here, I am honored to be confused with George L.

[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]

I didn't say 'always start in the 90 degree column' ... I said "assuming the wire is a 90 degree type ...."

Where does the code say so? What the code says is that the table lists the ampacities of the different wire types. That is, for example, that #12 THHN can handle 25 amps.
That's your starting point.

THEN the code lists various derating factors .... that is, circumstances where the wire is no longer allowed to carry 25 amps ..... things like ambient temperature being higher than usual, conduit fill, voltage drop, etc. These things are used to reduce from that (in our example) 25 amp figure. Finally, the table has a note that says "don't use #12 with a breaker larger than 20 amps unless it's a special case."

Finally, you look at how many of those 25 amps are left after all your calculations. If that figure is higher than the figure in the 75 degree column, you need a larger wire ... so the wire won't get hotter (from carrying current) than the termination at the device can handle.

Look at it this way: you can't often start at the 75 degree column, then 'derate' your way up to the 90 degree column.

The key is to istall the wire in a manner that will ensure that it never gets hot enough to damage the insulation. Code reference? You'd have to go back to 110, and the 'follow instructions' requirements. If the wire is listed for only 75 degrees (for example, THWN actually used in a wet location), you would have to start your calculations from the 75 degree values.

I agree Reno, and as for which column you go to the code reference associated with the tables is 310.15(B)(2) for starters. If you have a THW conductor go to the 75° to start with and derate as needed. If the wire is XHHW start with the 90° column and derate as needed. There are a number of rules pertaining to derating found in 310.15(B)(1) through (6). The NM Article 334 is no exception.

One more code reference is 110.14(C). This code reference gives us permission to use the higher temperature rating as a starting point and then derate for raceway fill (ampacity) and ambient temperature (correction). Why you would not start with the highest temperature rating is a mystery to me. If I'm bidding a job use the smallest wire possible and that's going to be the one with the highest rated insulation.

As for NMB cable (Romex) the same thing applies. Start derating from the 90° column. See 334.80. NEC 2005 edition.

O-kay. Got it through my thick skull.

One addendum to 334.80 and NM (which I know I didn't start out asking about, but which has since been brought up). And that is that, yes, you start in the 90 degree column for NM....but only for derating purposes. Otherwise, as pointed out by 334.80, you would use the 60 degree column, regardless of the fact that 334.112 rates NM at 90 degrees. Furthermore, you can use the new derated ampacity(and conductor) from the 90 degree column only if that ampacity does not exceed what is given in the 60 degree column for that conductor size.

As for 110.14(C), I know this is where my instructor got the "under 100amps" and "over "100amps" thing, as per 110.14(C)(1)((a) and (b). But by this far removed date, I have no idea how he applied it, and for the life of me I cannot interpret it on my own.

AND I have another kink to throw in when I add voltage drop to the situation. But that will have to wait until at least tomorrow. You've been warned.

The short answer is voltage drop will usually make all the other computations useless since you will generally end up with a wire far exceeding anything you could possibly get to from the derating. Basically when you spread that heat out along a long conductor, to get to a temperature capable of hurting the insulation, there won't be a useful amount of power getting to the load.

110.14(A) basically says in equipment less than 100a assume it is 60c termination (1 & 2) unless it says so (3) or it is a design B, C or D motor(4) but still limited to the rating of the conductor.

Hey guys, I have an application with 63C ambient temperatures and cabling supplying a transformer (also in 63C ambient) with 80C temperature rise. What derating factor do I use?



























(OK, it's a trick question. The answer is: whatever the manufacturer of the 150C cable says!)