I was reading my IAEI magazine about breaker marking and the part about "75c only" was interesting. When you look at larger breaker sizes for residential panels they do seem to be 75c only (80a and up). Does that really mean you couldn't replace an old breaker if the wiring method was 60c like TW?

Greg- That's the way I'd read it. So I guess we'd have to put a short piece of 75° wire ( 10' or 10% or maybe 18" but not more than 6') for a heat sink. Code ref. 310.15(A)(2) or 410.67(C) could set precedent. This should be interesting.

I believe the markings are derived from the requirement that above a certain rating the terminals are rated for 75 degrees.

The markings are WRONG.

Quote ["I was reading my IAEI magazine about breaker marking and the part about "75c only" was interesting. When you look at larger breaker sizes for residential panels they do seem to be 75c only (80a and up)".]

The breakers I have in my catalog show the temp rating for 10 thru 100 amps are rated
at 65/75C. Above 100 amp they are rated at 75C.

Quote ["Does that really mean you couldn't replace an old breaker if the wiring method was 60c like TW?"]

No. You can replace the 60C breaker with a 75C breaker thats has the same ampacity assuming that the conductor will fit the breaker terminals. The temperature rating determines the conductor size to be installed in the breaker. If you look at table 310.16 you will see that conductor sizes 14 thru 10 cu have an * that limits
the breaker size to 15 thru 30 amps even thought the 75C rating for the conductor is higher. If you have #12 tw and needed to change a 60C breaker the #12 at 60C and 75C
would meet that ampacity requirements.
The temp rating on the breaker is the MAX
temp allowed for that breaker. So operating it at a MAX of 60C would not be a
problem.

George Quoted ["I believe the markings are derived from the requirement that above a certain rating the terminals are rated for 75 degrees. The markings are WRONG."]
George
What do mean by "The markings are WRONG"?


[This message has been edited by Bob (edited 11-21-2006).]

I think most of you are reading it "backwards."

What the marking on the breaker terminations means is that the wire needs to be sized using the 75 degree column ... and not the 90 degree column ... in the tables.

If you had a 60 degree wire, the result of using the 60 degree column would be a larger wire, not a smaller one.

More important, the 75 degree is an improvement; in former times, we were limited to a 60 degree limit to the terminations.

Think of it as a "chain," we can only load up the wires based upon the 'weakest link,' or lowest rating, in the circuit.

Or, looking at it from another direction, that 75 degree termination means that THHN -a 90 degree wire- is limited to 75 degrees in your calculations.

So I just happen to have a 20a breaker made by American Circuit Breaker Corporation and it plainly say "Use only in Listed enclosures" and "Use 75 degree conductors only" 120/240v.
Gentlemen- that's the call. I'm kinda stubborn that way sometimes.

That is what the IAEI article says. A dual 60/75 can use either, 75c alone means you can only use 75c conductors. I was surprised at IAEIs take myself.

renosteinke's Quote
"If you had a 60 degree wire, the result of using the 60 degree column would be a larger wire, not a smaller one." That would not be true for sizes 14,12 and 10. True for larger sizes.

George Little's Quote
"So I just happen to have a 20a breaker made by American Circuit Breaker Corporation and it plainly say "Use only in Listed enclosures" and "Use 75 degree conductors only" 120/240v."
George I believe you but I do not understand
the reason. Under the 60C and 75C col, #12 conductor is rated for 20 amps. If you use
the 60C conductor at 20 amps continuous load the temp will rise to 60C. Should be no problem for the breaker.
Actually the conductor is rated at 25 amps. The NEC limits the load to 20 amps.
[This message has been edited by Bob (edited 11-22-2006).]

[This message has been edited by Bob (edited 11-22-2006).]

Bob- You raise a very interesting point that I have never been able to get an answer to.



How warm does a #12 AWG wire get when it conducts 20a and how warm do terminals get that are rated 60/75 degrees? I know the wire won't be damaged and that wire is rated for continous load and I know that terminals <600v typically are not rated for 90 degree use? What with that? I Think fretwell should be complemented for bring up this sorry subject

Bob- You raise a very interesting point that I have never been able to get an answer to.



How warm does a #12 AWG wire get when it conducts 20a and how warm do terminals get that are rated 60/75 degrees? I know the wire won't be damaged and that wire is rated for continous load and I know that terminals <600v typically are not rated for 90 degree use? What with that? I Think fretwell should be complemented for bring up this sorry subject