kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
I never really put much thought to it that the NEC states that #14 is limited to 15A, #12 is limited to 20A, and #10 is limited to 30A. Amazingly then, through divine intervention, suddenly #8 is good for, not 40, not 50, but 60-70A or more, depending on the temperature rating. The limit on #14, #12, and #10 has been in the NEC as long as I can remember (early 80's, anyway). But another post regarding the 80% limit on breakers got me thinking about it.
In a nutshell, why? Where does this come from, and what documentation substantiates this limit? The conductor is by calculation capable of carrying more then the NEC limit of Table 310.16. I looked on manufacturers website, and I looked through IEEE 835. The tables in IEEE 835 appear to have some similarity between them and the NEC, but not exactly, such as ambient temperatures are 40 deg C not 30 deg C. In reviewing IEEE 835, the tables are much more detailed, and specific, whereas the NEC tables seem to be more of a summary, one size fits all.
So, the question is, can anyone provide some insight as to the engineering validity of the NEC cable ampacity tables? Otherwise it seems #14 should be able to be used on a 20A breaker for house wiring. After all, you can only load the circuit to 16A, and the conductor is actually good for 20A.
I hope this brings some good discussion.
In a nutshell, why? Where does this come from, and what documentation substantiates this limit? The conductor is by calculation capable of carrying more then the NEC limit of Table 310.16. I looked on manufacturers website, and I looked through IEEE 835. The tables in IEEE 835 appear to have some similarity between them and the NEC, but not exactly, such as ambient temperatures are 40 deg C not 30 deg C. In reviewing IEEE 835, the tables are much more detailed, and specific, whereas the NEC tables seem to be more of a summary, one size fits all.
So, the question is, can anyone provide some insight as to the engineering validity of the NEC cable ampacity tables? Otherwise it seems #14 should be able to be used on a 20A breaker for house wiring. After all, you can only load the circuit to 16A, and the conductor is actually good for 20A.
I hope this brings some good discussion.