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#181869 - 11/04/08 10:28 PM temperature rating of 40 degrees???
Niko Offline
Member
Registered: 08/17/06
Posts: 356
Loc: Campbell, CA
I have noticed that many circiut breakers have 2 temperature ratings on them, one is 75 and the other is 40.
Do you guys know what that 40 represents?

Thank you
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#181870 - 11/04/08 11:05 PM Re: temperature rating of 40 degrees??? [Re: Niko]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
75c is the terminal rating and 40c is the ambient temperature they are calibrated at ... the way I have heard it.
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#181872 - 11/05/08 09:12 AM Re: temperature rating of 40 degrees??? [Re: gfretwell]
SteveFehr Offline
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Registered: 03/19/05
Posts: 1195
Loc: Chesapeake, VA
Yep, 75C terminals and 40C ambient. Cabling needs to be derated to never exceed 75C, I'm sure you're familiar with this. Exceeding 40C (104F) ambient, though, can lead to nuisance tripping of the breaker.
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#181874 - 11/05/08 10:24 AM Re: temperature rating of 40 degrees??? [Re: SteveFehr]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
I am still waiting to see what happens when you have a panel stuffed with AFCIs and installed in an un-airconditioned garage.
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Greg Fretwell
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#181892 - 11/06/08 04:23 PM Re: temperature rating of 40 degrees??? [Re: gfretwell]
Niko Offline
Member
Registered: 08/17/06
Posts: 356
Loc: Campbell, CA
very nice, hey it does not hurt to learn something everyday.
thanks for your time.
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#181896 - 11/07/08 01:00 AM Re: temperature rating of 40 degrees??? [Re: gfretwell]
Trumpy Offline


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Registered: 07/05/02
Posts: 8211
Loc: SI,New Zealand
Originally Posted By gfretwell
75c is the terminal rating and 40c is the ambient temperature they are calibrated at ... the way I have heard it.

What temperature (in C) is the conductor insulation rated at?
Reason I ask this, is if you can have up to a 75C connection, what is that actual connection doing to the wire connected to it?
Any heat that comes from a connection, means resistance, and therefore a BAD connection, think pyrolysis.
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#181900 - 11/07/08 06:37 AM Re: temperature rating of 40 degrees??? [Re: Trumpy]
renosteinke Offline
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Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
The tempertaure rating of the wire depends on the type of insulation, which is marked on the wire. Probably the most common type used here is THHN/THWN.

THHN/THWN has a 90 degree rating in dry applications, and 75 degree in wet.

The wire can attain those temperatures in a number of ways; generally, the ambient temperature (say, in pipe running across a rooftop) provides a significant rise; simple resistance to the amount of current passing through the wire provides the rest.

Since the temperature rating of the connections is usually lower than that of the wire, we are not able to use the wire to it's fullest potential. That is, we are limited to, say, 65 amps in a wire that otherwise might be rated for 75 amps. Since there isn't a 65 amp breaker out there, if we can't use a 60 amp breaker for the application, we would have to use a larger wire.

Other factors ... wire fill, for instance .... are also considered in 'de-rating' the wires.

While the ambient temperature can cause one section of the wire to be hotter than another, the one factor that heats the entire wire, from one end to the other, is the current carried by the wire. In the example given above, running 75 amps through that particular wire, at room temperature, can easily result in the wire inside conduit reaching 90 degrees - and that's without any connections or splices entering into the equation.

PS: I like the number 17 laugh
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#181904 - 11/07/08 10:29 AM Re: temperature rating of 40 degrees??? [Re: renosteinke]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
I just think of it as a chain with the weakest link determining the ampacity. That can be the real rating or a code mandated rating like you have in most cable wiring methods that put you in the 60c column or 240.4(D) that force the ~80% derating on you in small conductors, no matter what else you have.
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#181934 - 11/10/08 03:54 AM Re: temperature rating of 40 degrees??? [Re: gfretwell]
SteveFehr Offline
Member
Registered: 03/19/05
Posts: 1195
Loc: Chesapeake, VA
Originally Posted By gfretwell
I am still waiting to see what happens when you have a panel stuffed with AFCIs and installed in an un-airconditioned garage.
I think we already know that answer... CLICK

CLICK

CLICK CLICK ... CLICK

...and that's it. The rest would cool down, see? laugh Thermal derating applies to the temperatures in the panel, not just in the garage. If that environment is going to exceed the rating of the panel, the panel shouldn't be there.
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#181938 - 11/10/08 05:58 AM Re: temperature rating of 40 degrees??? [Re: SteveFehr]
Zapped Offline
Member
Registered: 10/01/02
Posts: 482
Loc: Huntington Beach, CA, USA
Steve - an excellent interprutation of an AFCI in action!

Even without the AFCIs in the panel, 104F is a very easy ambient temperature to reach and excede in many environments. Let alone deserts, you could easily have a panel in direct or partial sunlight in a temperate area reach well above 125F. It's a metal box!

Now add some heat from heavy currents, such as AC, etc. 40C seems like an awful low testing temperature to rate electrical equipment, IMHO.
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