240.4(d)

[southernboys]

I am a litttle bit confused by this code. the problem I have with it is if a #12 thhn conductor is rated for 30a in table 310-16 and I only load that conductor to 80% why can I only put a 20a breaker on this conductor and I can only load this to 80% which actually cuts the actual ampacity of that wire in half or to 16a any and all input will be appreciated?

 

[iwire]

Re: 240.4(d)

Originally posted by southernboys:
I am a litttle bit confused by this code. the problem I have with it is if a #12 thhn conductor is rated for 30a in table 310-16 and I only load that conductor to 80% why can I only put a 20a breaker on this conductor and I can only load this to 80% which actually cuts the actual ampacity of that wire in half or to 16a any and all input will be appreciated?

There is no reason that you may not load a 20 amp circuit to 20 amps for non continuous loads so take that out of the picture.

334.80 limits NM to 60 C which is 25 amps for 12 AWG NM.

So why the limit to 20 amps for some types of circuits?

I have no idea but it is what it is.

If this where a motor load you could use the 12 AWG at 25 amps.

 

[charlie]

Re: 240.4(d)

In my opinion, the small conductors are where the abuse comes in, not the larger conductors. The Code says it is not for the untrained persons; however, the rules for overcurrent protection of the smaller conductors seem to be doing just that.

If Joe Homeowner puts a 30 ampere fuse on a #12 Cu wire, what happens? Nothing unless it is really loaded where it is going through the attic. If Joe Homeowner puts a 30 ampere fuse on a #14 Cu wire, what happens? Probably nothing unless it is really loaded where it is going through the attic.

If a conductor is terminated incorrectly, will you have a burnt up connection? Your chances are better is the conductor is lighter loaded than if it were loaded to its capacity.

I think we could come up with more examples but you get the idea. I do not know if this is in the back of the panel member's minds when making the rules or not. I know that we have lived with them for a long time and I don't see a real problem with the rules the way they are now.

Bob mentioned the loading with motors, it is not likely that a DIY will be doing this work. If he does, it is likely that he will not know that he may use a smaller conductor and will still have a fighting chance of being safe.