Originally Posted By: API This post was automatically imported from our archived forum.

Folks, I got myself a little confused here. I was reading through my new 2003 IRC, Table E3605.1 I thought that 14 AWG copper was rated at 15 amps, 12 AWG copper rated at 20 amps, etc… Now when I look at this table and NEC table 310.16 I see that 14 AWG copper is rated at 20 amps, and 12 AWG is rated at 25 amps. Can someone please explain?

Originally Posted By: bking This post was automatically imported from our archived forum.

The electrical code has a statement in there that changes that to 15 amps (14ga.) and 20 amps (12ga.) The wire manufacturer allows more than the code. Probably has to do with average cable length in houses and the enclosed spaces it is installed in (heat build up).

Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.

The answer is in 240.4(D) where 14 gets 15a, 12 20a and 10 30a.

That is unless the 310.16 is allowed to be used such as 430.

The thinking is 15, 20 and 30a circuits have receptacles and the installer has no control over what the user may plug in. They will keep loading the circuit unrtil the breaker trips, then unplug the clock.

By limiting the overcurrent device they are effectively giving you the 80% maximum load factor we see in the design of dedicated circuits.

If you get into hard wired motor circuits you will see conductors used at 80% of FLA using the appropriate 310.16 column. That could even be the 75c column for THHN in pipe. The overcurrent device can be 250% of FLA so you can see some strange looking stuff. A/C condensers are probably the best example for an HI. Go by the label. It will tell you what the smallest O/C device should be and what the circuit ampacity (wire size) should be. It won’t conform to 240.4(D)

Originally Posted By: apightling This post was automatically imported from our archived forum.

There’s lots of variables and tables in the NEC. Explaining them can be convoluted.

Excerpted example:
"If you are required to calculate an ampacity de-ration calculation for either ambient temperature or more than 3 current carrying conductors in a raceway, you may start your calculation by using the ampacity rating found in NEC Table 310.16 column for 90 degree C if the conductor?s insulation is originally rated for 90 degrees. You may also start your de-rating calculations by using the ampacity rating found in NEC Table 310.16 column for 75 degree C if the conductor? insulation is originally rated for 75 degrees. You must then finish your de-rating calculation for the ampacity rating according to NEC Articles 110.14.C after you have completed your ampacity de-ration calculation, you must then compare that calculation de-rated ampacity answer that you calculated using the original temperature degree rated ampacity in your calculation, with the either the 60 degree column ampacity rating, if the conductor is smaller than 1 awg or with the 75 degree column ampacity rating. If the conductor is 1 awg or larger. NEC Article 110.14.C.1 & 2 You must use the worst scenario [lowest amps in the comparison, comparing your de-rated ampacity answer to the ampacity rating dictated by NEC Article 110.14.C.1 & 2 ] The lowest ampacity rating must be chosen concerning the ampacity rating of that conductor after comparing the de-rated ampacity you found after learning your de-rating calculation answer to the ampacity rating found in the column found in NEC Table 310.16 column for 60 degree C that NEC Article 110.14 requires, if smaller than 1 awg = 60 degree column or if 1 awg or larger = 75 degree column. NEC Article 110.14.C.1 & 2

This NEC Article 210.14.C.1 & 2 requirement of a lower ampacity rating than found in NEC Table 310.16 is because the terminal ends and wire nuts that are listed and approved on the market in the electrical industry are only rated at 60 degrees centigrade if smaller than 1 awg or 75 degree column if larger than 1 awg conductors as per NEC Article 110.14.C.1 & 2."

Clear? Right . . .

I look for #14 wire from a 15A breaker, #12 from a 20A breaker, and #10 from a 25 or 30A. Unless. . . the run is a long one through a hot place with lots of conductors. . then I refer the client to an electrician or engineer.