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Re: Minimum conductor size
Wendell,
I don't follow what you are saying/asking above, however, let's start with the proper way to do it and see if that answers your question.
(code references are from the 2006 IRC)
Table E3605.5.3 shows that the maximum overcurrent protection allowed for various conductors, with 14 AWG copper having a maximum overcurrent rating of 15 amps. (Note that the two main column headings are split wrong, that the dividing line between "copper" and "aluminum or copper-clad aluminum" is in the wrong place - it is over the right side of the first column and should be over the right side of the second column, reading the chart, you will see that the chart only makes sense if you correct that 'editorial error' ... and it may have been corrected in printings or in newer CDs than I have.)
Thus, 14 AWG copper is allowed to have a maximum of 15 amps overcurrent protection, thus, you can use 14 AWG copper on a 15 amp protected circuit.
However, you now need to go through the derating requirements for ambient temperature (which is why they threw the attic temperature in there) and for bundling (if any). And, you need to do that derating from the conductor insulation temperature rating/ampacity rating (and they gave you a guide to that too).
You cannot exceed 15 amps overcurrent on 14 AWG, but, if the derating does not work out, you might need 12 AWG to make 15 amps with the derating factors applied.
In the question, the 14 AWG insulation rating derating worked out to still allow 15 amps for that 14 AWG NM-B cable.
Did that help?
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