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Re: Temperature Derating [Re: Tripp] #186227 04/27/09 10:53 PM
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renosteinke Offline
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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.

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Re: Temperature Derating [Re: renosteinke] #186228 04/27/09 11:02 PM
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George Little Offline
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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.


George Little
Re: Temperature Derating [Re: George Little] #186232 04/27/09 11:26 PM
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George Little Offline
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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.


George Little
Re: Temperature Derating [Re: George Little] #186233 04/28/09 12:33 AM
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George Little Offline
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As for NMB cable (Romex) the same thing applies. Start derating from the 90 column. See 334.80. NEC 2005 edition.


George Little
Re: Temperature Derating [Re: George Little] #186237 04/28/09 03:24 AM
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Tripp Offline OP
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O-kay. Got it through my thick skull.


Re: Temperature Derating [Re: Tripp] #186238 04/28/09 03:34 AM
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Tripp Offline OP
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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.

Re: Temperature Derating [Re: George Little] #186239 04/28/09 03:45 AM
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Tripp Offline OP
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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. confused

Re: Temperature Derating [Re: Tripp] #186240 04/28/09 04:06 AM
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Tripp Offline OP
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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.

Re: Temperature Derating [Re: Tripp] #186244 04/28/09 12:20 PM
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gfretwell Offline
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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.


Greg Fretwell
Re: Temperature Derating [Re: gfretwell] #186248 04/28/09 02:10 PM
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SteveFehr Offline
Member
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!)

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