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

George
Look at the top of table 310.16. It gives you the information related to the table. If you have #12 TW with 25 amps continuous(3 hrs)with an environment temp of 30C then the conductor temp will rise to 60C. If the
conductor is #12 THWN(90C rating) with a
continuous load of 25 amps with the environment of 30C, the conductor temp will rise to 75C. If the load is 30 amps the #12 will rise to 90C. The type of insulation makes the difference. The code only allows #12 to have a breaker rated at 20 amps so the temp would be lower than 60C.

I don't believe any such thing. You certainly should not see anything more than slightly warm to the touch with a 12 ga copper with 20a on it. I found my dryer was on a #12 shortly after moving in my current home and it never really got warm. That is more than 20a. I was surprised because I expected it would be cooking. 60c is far too hot to touch.

Bob- I think you are mistaken in your analogy of T.310.16. The reason you can load the same gauge of wire more with higher temperature rated wire is because it will dissipate the heat better. Greg is correct the wire doesn't get to 60 or 75 degree C when it carries the current identified in the table.

Bob is correct in his understanding of the intent of the table. Based upon the assumptions made to calculate the table, when a conductor is loaded to the tabulated ampacity it should just reach the temperature rating of its insulation.

The big reason that conductors run cooler than this is that there is better thermal conductivity away from the conductors than the table assumes. With better cooling than table 310.16 assumes, the equilibrium conductor temperature will be lower than the table suggests.

Back to the original post, I wonder if 75C only terminations have limitations to protect the wire from device heat in addition to the temperature ratings to protect the device from wire heat? Similar to the instructions on some lighting fixtures that require 90C rated wire.

-Jon

I'm confused- I still don't think the temperature rating in the column headings tells me the temperature that the wire will get to when the specified ampere rating is on the wire. This really is my question that no one seems to answer, even UL have not answered me. I almost accepted the fact that it's a stupid question. I'm sure there is some kind of tolerance factors involved that allow the continuous load rating.

We are getting into some testing esoterica. The wire ratings date from some IEEE work, originally done back in the '30's. There is a rather convoluted method used to determine the temperature an insulatio may be exposed to, over extended periods, without 'significant' deterioration in the insulating ability of the insulation.
When a new wire comes out, it is generally compared to existing wires, and classed accordingly.
Take, for example, THHN. A pvc insulated wire with a nylon jacket. Sounds simple? Sure- ask DuPont. There are thousands of varieties of "nylon", and an infinite variety of "pvc." Yet, wires from different manufacturers, using varying compounds, all get clased as "THHN."

There are sundry tests used to class, or verify the classification, of a wire type.

We drift, here, however. What is important is to note that (for example) a 60 degree wire may be exposed to 60 degrees for a long time, before the insulation deteriorates. We use that figure ... adjusting for ambient temperature, bundling, etc., ... to adjust the table "ampacity" to our overcurrent protection.

Now... we've done all that. The wire ought to never exceed 60 degrees, as we have calculated. We next attach it to a breaker rated to withstand 75 degrees ... how can there be a problem?

Or, looking at the reverse situation: You have THHN - a common 90 degree wire - that your calculations show can reach 80 degrees. Are you OK? Not if the breaker termination is only good to 75 degrees! The wire might be fine, but the termination will fail.

The breaker rating does NOT influenc the wire type; it DOES influence HOW that wire is used. ANY insulation type can be used on ANY breaker ... IF the calculated temp of the wire will not exceed the rating of the termination.

George
What I said is true. If the load on #12 is 20 amps and is run for 3 hrs or more the conductor temp will approach 60C or 75C degrees. Read this article below and see.

http://www.iaei.org/subscriber/magazine/03_d/magazine_03d_hartwell.htm

George,

I am working backward from what I see in table 310.16, not from any knowledge of how 310.16 was generated.

Take a look at the formula in 310.15(C) Engineering Supervision.

This formula is pretty darn simplistic. You presume an allowable conductor temperature rise (TC - (TA + {delta}TD) ) and divide it by the thermal resistance to the surrounding ambient RCA. This tells you how many 'thermal watts' you can dissipate, and is essentially 'ohms law' for heat. You have a certain amount of heat that flows, and this requires a certain temperature difference across a thermal resistance.

The amount of heat generated in the conductor scales as the electrical resistance of the conductor and the _square_ of the current flowing through it. This means that the current which can be carried at a given limiting temperature will vary as the square root of the amount of heat which gets carried away from the conductor.

If you presume that {delta}TD, RDC, YC, and RCA are _constant_, and presume that TC is the value (60C, 75C, or 90C) at the top of the columns in table 310.16, and then let TA vary, then you will find that the results given by the equation in 310.15(C) will vary in the same way as the correction factors at the bottom of table 310.16. This makes it clear to me that these correction factors were developed using this formula or a very similar one, and that they are based upon the conductor temperatures at the top of the column.

Reading across the columns, the allowed temperature rise doubles (from 30C to 60C). This means that heat carried away from the wire would double, and we thus expect the ampacity to increase by the square root of 2, or 41% more current. Actually looking at the numbers, however, we find that the ampacity increase from the 60C column to the 90C column is about 35%. However we have to note that the resistance of copper wire increases as it gets warmer, so that the resistance of copper at 90C is about 10% greater than that at 60C. 1.35^2 * 1.1 = 2, so again the formula in 310.15(C) fits the values in table 310.16.

This leaves me quite certain that the values in 310.16 are in fact consistent with the conductors reaching their temperature ratings when loaded to their full ampacity. The big assumption made by the table is the RCA value, which we are never asked to calculate or determine. Clearly the thermal resistance to ambient will be vastly different for conductors installed as knob and tube in un-insulated space, versus NM in an insulated wall. My guess is that in any situation where you can actually go and feel the conductors during use, the RCA value is far less than assumed by table 310.16.


If I have 60C rated 12ga wire, connected to a lighting fixture that draws 1A in a 30C ambient, then the conductor should never self heat above its temperature rating. Yet most new lighting fixtures require 90C wire. Why? Because the lighting fixture itself produces heat.

I would be _very_ surprised if a breaker were to produce enough heat to damage a wire. But if the breaker is marked for 75C conductors and _not_ for 60C conductors, then I am forced to assume that the breaker can tolerate no more than 75C, and in addition could damage conductors rated only 60C.

-Jon

Bob, all I got out of that article was that in "worst case" THREE conductors in a cable or raceway would never reach 60c under the load in 310.16 (25a for #12), presumably even if it was buried in thermal insulation. From personal experience I say it is a lot less for 2 current carrying #12 conductors in Romex with a standard dryer load (22-23a).
It never even got warm after my wife did several loads of clothes.

Winnie- Your probably accurate but your way over my head with most of that information. My academic background is not that great. Thanks anyway.

Bob- Thanks for the IAEI article reference- I did get an answer to my question -kinda. T.310.16 is telling us that the insulation on various conductors will not be damaged when "x" number amperes are flowing and that the temperature approaches 60, 75 or 90 degrees. No mention of any exact numbers tho, probably due to tolerances.

So we must be talking about a very special application to specify 75 degree wire only. The terminals are more likely to be the week link in the system anyway.

And Greg, Don't put that #12 back on that dryer just in case

No problem, it went away when I did the service upgrade and fed the dryer from the garage service panel. This place was an electrical nightmare when I bought it but it was such a good deal I had to buy it.
This 12-2 RX was direct buried from the house. I am guessing it worked fine for years since the garage was built in 1978. The dryer wasn't even the scarriest thing I found.

"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"?"

I thought that "I believe the markings are derived from the requirement that above a certain rating the terminals are rated for 75 degrees." explained why the markings are wrong.

Any device that passes the 75 degree test passes the 60 degree test by definition.

The people who determined what markings should be on the device made a mistake.

George
Read this article. Look closely at paragraph
4 and 5. It answers your question.
Bob http://www.iaei.org/subscriber/magazine/03_d/magazine_03d_hartwell.htm

[This message has been edited by Bob (edited 11-27-2006).]