Table 310.16 Allowable Ampacities of Insulated Conductors

[PLCStudent]

Greetings,

I am a student in the Electrical Technology field at a local Community College. In our Branch Circuit and Service Entrance Calculation class the Instructor has asked that we research and write a five paragraph report on, "Why a 30 amp breaker can not be used on a #12 copper wire". Listed under the 90 degree C (194 degree F) column in Table 310.16, we should notice that it calls for a 30A breaker for #12 wire. There is an asterick by the "12", which tells us at the bottom of the page to "see 240.4 (D). In 240.4 (D) (5) it says "12 AWG Copper 20 Amperes". After looking around in the NEC, I was able to find some useful information regarding this in 110.13 (C)(1)(A) regarding conductor termination provisions and conductors with higher temperature ratings. Then even if I used this information and used the 60 degree C (140 degree F) column; I would still be using a 25A breaker on a #12 copper wire.. Later I ended up in Article 310.15(B). Any assistance will be greatly appreciated.

 

[Smart $]

Ultimately, the premise implied by your instructor's title is wrong... but only under specific installation conditions. As for addressing the project as dubbed, what Code article and section(s) sets the overcurrent rating in general and/or says that you have to look elsewhere for specific cases...????

My response is based on not doing your homework for you, but figured I can give you a little nudge

 

[infinity]

You have a few issues to consider. The condcutor ampacity based on it's insulation, the terminal temperature rating, the equipment temperature rating and whether or not 240.4(D) even applies. I'm sure I left a few out.