When can we exceed the 60?C rating?

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Jon456

Jon456

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I understand the temp ratings of conductor insulation. And I understand that ampacity is limited by the lowest-rated device or conductor in a circuit. What I don't understand is, in these modern times when much (most?) of the wire we work with is THHN rated at 90?C, why are we still constrained by 60?C ampacity calculations?

In other words, why must we use the 60?C ampacity column in Table 310.16 for up to 100A or #1 conductors? Is it a standard "rule of thumb"? Or are we allowed to use the 75?C ampacity column if we can determine that both the wires and the terminations are rated at least 75?C? And, if so, then how do I determine the rating of terminations? For example, when I look at the product data sheet for this breaker, I see no temperature ratings. I find it hard to believe that devices today are still manufactured to 60?C ratings.
 
short answer to one part.. you can use the higher ratings if terminations allow (see 110.14) but be careful on NM or SE interior wiring is limited to 60?
 
You are not limited to 60C unless one of the termination is 60C. The weakest link is your answer. Most terminals are rated 75C so a 90C wire may only be protected at 75C. You can use the 90C to derate but you cannot protect it at anything higher than the 75C rating. Now NM cable is limited to 60C by art. 334.80 so there you are limited to 60C
 
You are permitted to use the actual temperature rating of the terminals that the wire is connected to.

As for determining the temperature rating of a terminal you would need to check either on the device or in the manufactures literature.

Chris
 
Thanks all. It makes sense to be able to use the higher ratings, I just wasn't sure it was permitted.

But that brings up the problem of determining the ratings...

raider1 said:
As for determining the temperature rating of a terminal you would need to check either on the device or in the manufactures literature.
Click to expand...

In the example link I posted above (for a QO load center main breaker manufactured by Square-D/Schneider Electric) the product data sheet makes no mention of the breaker's termination's temp ratings. This is obviously an important specification, and yet it's conspicuously absent. So am I supposed to call a company's engineering dept every time I need to determine a device's temp rating?
 
augie47 said:
You are correct in finding that info isn't easy :)
15 min into SquareD I did find the following:

http://www.squared.com/us/products/...ABA3AF485256C2200692460/$file/faqFrameset.htm
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Thanks Gus. Using the search page you linked to, I searched for the words "temperature rating" by product "circuit breaker" and came up with a list of FAQs including: "What is the wire temperature rating for a QO or QOB circuit breaker?" Clicking on that topic, I received the following answer:

"10 to 30 amp circuit breakers are suitable for use with 60C or 75C conductors. 35 to 125 amp circuit breakers are suitable for use with 75C conductors."

Well that's clear (not)! "Suitable for use"? I'm sure that means that the terminations of their 10A to 125A breakers are all rated to 75?C, but why didn't they just say that? And I still don't understand why they wouldn't list the temp ratings in their product data sheets.
 
I agree. I was suprised to not see it there.
On the HOM breakers I found on my truck, it is stamped in the molding on the side.
 
Jon456 said:
Thanks all. It makes sense to be able to use the higher ratings, I just wasn't sure it was permitted.

But that brings up the problem of determining the ratings...



In the example link I posted above (for a QO load center main breaker manufactured by Square-D/Schneider Electric) the product data sheet makes no mention of the breaker's termination's temp ratings. This is obviously an important specification, and yet it's conspicuously absent. So am I supposed to call a company's engineering dept every time I need to determine a device's temp rating?
Click to expand...

Instead of the product data page, all you had to do was choose the product catalog option instead, where the termination information is on page 4.
 
augie47 said:
short answer to one part.. you can use the higher ratings if terminations allow (see 110.14) but be careful on NM or SE interior wiring is limited to 60?
Click to expand...

And of course there are others (Like UF and AC installed in thermal insulation). If I were you I would also get familiar with table 310.13(A).

You know funny side note I had a startling revolation when studying 310.13(A). I never new that THHN/THWN did not retain it's 90 degree insulation for ampacity derating in wet locations...

Don't think I ever violated it though in my ignorance.
 
Dennis Alwon said:
You are not limited to 60C unless one of the termination is 60C. The weakest link is your answer. Most terminals are rated 75C so a 90C wire may only be protected at 75C. You can use the 90C to derate but you cannot protect it at anything higher than the 75C rating.
Click to expand...

Just to clarify that a little, the restriction to use 60? or 75?C rating is limited by the termination/device rating at either end of the conductor, and not what is connected thereafter.

For example, a conductor is landed on a CB rated for 75?C termination max. This conductor runs to a terminal junction box a few feet away and lands on one of the terminals. All terminals and terminations within the box are made up to be 90?C rated. An extendsion of the circuit can be made using the full 90?C rating of such an extending conductor provided the termination rating at other end of this conductor is also 90?C (or greater).
 
Twoskinsoneman said:
You know funny side note I had a startling revolation when studying 310.13(A). I never new that THHN/THWN did not retain it's 90 degree insulation for ampacity derating in wet locations...

Don't think I ever violated it though in my ignorance.
Click to expand...

That is an excellent point. However, THWN-2 is good to 90 deg.
 
raider1 said:
You are permitted to use the actual temperature rating of the terminals that the wire is connected to.

As for determining the temperature rating of a terminal you would need to check either on the device or in the manufactures literature.

Chris
Click to expand...
You have to be careful here. You can have application where the actual termination is rated for 90?C, but the equipment is rated for 60? or 75?C. In this case you are limited to the lower temperatures. This often happens on equipment with mechanical termination lugs. The lugs themselves had the 90?C rating but the equipment does not.
 
don_resqcapt19 said:
You have to be careful here. You can have application where the actual termination is rated for 90?C, but the equipment is rated for 60? or 75?C. In this case you are limited to the lower temperatures. This often happens on equipment with mechanical termination lugs. The lugs themselves had the 90?C rating but the equipment does not.
Click to expand...

Yes, you are corrct, I should have said the equipment instead of the terminals.

Chris
 
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