I wish people would stop spreading the lie about the 80% rule on this forum, or at least use it appropriately. If you have a 15 amp breaker on a branch, you can use 15 AMPS CONTINUOUS, 24/7/365.25. 14 ga wire is rated for 20 amps minimum in residential settings per the NEC. But, the NEC also requires overload protection of 15 amps for 14 ga wiring. If you do the math, 15/20 = 75%. YOU WILL NOT EXCEED 80% CAPACITY IN YOUR HOME! I hate to yell like that, but internet electricians have been pushing this false fact for as long as I have been coming to this forum.
Source:
http://ecatalog.squared.com/pubs/Ci...eakers/QO-QOB Circuit Breakers/0600DB0103.pdf
Now, I am a mechanical engineer, not an electrical one, but I know enough to know how to read and I have taken my share of EE courses. I do encourage someone to disprove me though if I am off base here.
As for wire ratings, I run 25 amps continuous through 14 ga conductors in my control panel. That 15/20 amp rating is only for bundled conductors. The rating is higher for single conductors in open air like in chassis applications. It all comes down to temperature rise and the NEC rules for residential applications are VERY conservative just because you never know.
Source - pg 184
http://std-wire.com/downloads/SWC_Catalog.pdf