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 BigJimmy 08-21-2007 12:09 PM

Wire sizes based on actual connected load

Here's another one that I'd like some feedback on (although this one wasn't prompted by arguements with the AHJ!):

Let's say you've designed a residential lighting system and the total connected load on the branch circuit is known, say 8A @ 120V. Further, there are no other outlet devices present on the circuit which is being wired with 14awg. THHN. Moving forward, this circuit is routed through 3/4" EMT along with 6 other current carrying conductors. According to 310.15.(B)(2)(a), the conductor ampacities must be derated by 70%. So in reality, the 14awg. wire is only good for 10+A. However, I know that the load (assuming that the circuit will not incur future modification) on this circuit is 8A, less than the derated capacity of the branch circuit.

Is this allowable? Is there a citation in the code book that addresses this?

Thanks!
Jim

 darren 08-21-2007 04:32 PM

You have to remember that your breaker is there to protect your wire. Now that 14 is rated for only 10A, if you could find a 10A breaker you could do it but not on a 15A breaker. You know that it has only 8A but what happens in a few years and someone else adds somehting to it and it starts to draw 12A.

 Stubbie 08-21-2007 04:37 PM

Jim you are making a common error in your calculations. Deration for thhn is done from the 90C column of table 310.16. 12 awg is 30 amps before deration and 14 awg is 25 amps before deration. So .70 x 30 = 21 amps for 12 awg thhn and .70 x 25 is 17.5 amps for 14 awg thhn. The reason this is allowed is because the insulation of thhn is rated at 90 c and therefore deration begins at that rating in column 3 of 310.16. If the insulation is rated 75 c then you would derate from the 75c column. In no case however can you use the ampacity of the 90 C column. There are special exceptions in this regard for 14,12 and 10 awg copper wiring. They can only have 15, 20 and 30 amp protection respectively. See 110.4(C) for any exceptions.

The minimum allowed branch circuit ampacity must be at least 15 amps.... any deration that takes you below that is not allowed for 120 volt systems... you would have to upsize the the wire size until you have at least 15 amps.

Stubbie

 BigJimmy 08-21-2007 05:10 PM

Quote:
 Originally Posted by darren (Post 58811) You have to remember that your breaker is there to protect your wire. Now that 14 is rated for only 10A, if you could find a 10A breaker you could do it but not on a 15A breaker. You know that it has only 8A but what happens in a few years and someone else adds somehting to it and it starts to draw 12A.
#14 is good for 10A? Since when??

Also, this was a somewhat hypothetical question and I indicated that I was assuming that there would be no growth.

 BigJimmy 08-21-2007 05:12 PM

Quote:
 Originally Posted by Stubbie (Post 58813) Jim you are making a common error in your calculations. Deration for thhn is done from the 90C column of table 310.16. 12 awg is 30 amps before deration and 14 awg is 25 amps before deration. So .70 x 30 = 21 amps for 12 awg thhn and .70 x 25 is 17.5 amps for 14 awg thhn.Stubbie
Sorry my friend; I do understand how the insulation rating affects ampacity. I was rushing under the watchful eye of Big Brother while at work!

 darren 08-21-2007 05:20 PM

[quote=BigJimmy;58820]#14 is good for 10A? Since when??
You did the math and you came up that your #14 is only good for 10A so you tell me.
[quote]

 Speedy Petey 08-21-2007 05:46 PM

Quote:
 Originally Posted by BigJimmy (Post 58821) Sorry my friend; I do understand how the insulation rating affects ampacity. I was rushing under the watchful eye of Big Brother while at work!
The point was not about the insulation. You already confirmed that the insulation is THHN.
The point was we use the actual insulation temperature when figuring derated ampacity. For THHN this is 90 deg C.

So as Stubbie said, #14 THHN uses 25 amps as a starting point for derating, NOT 15 amps.

 BigJimmy 08-21-2007 08:30 PM

Quote:
 Originally Posted by Speedy Petey (Post 58831) The point was not about the insulation. You already confirmed that the insulation is THHN. The point was we use the actual insulation temperature when figuring derated ampacity. For THHN this is 90 deg C. So as Stubbie said, #14 THHN uses 25 amps as a starting point for derating, NOT 15 amps.
OK, OK, thousands of appologies for my errors in quoting the code. I posted in haste and in doing so, made a bunch of rookie errors. Underneath, I did have a valid question that I wanted you all to consider but the point has been muddied by my mistakes. That being said, let me think more about it and repost!

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