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Old 03-08-2012, 05:26 AM   #1
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Default 40 degree c

What does the 40 degree c mean on the side of most circuit breakers? I have seen it marked on breakers of all sizes from all manufactures.

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Old 03-08-2012, 05:30 AM   #2
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methinks you want to go to 110.14 Cable.....~CS~

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Old 03-08-2012, 05:38 AM   #3
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The breakers are usually rated for 60 or 75 degree c for the lugs, but on the side they have a 40 degree c molded into it.
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Old 03-08-2012, 06:08 AM   #4
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Ambient temperature rating.

UL 489 listed breakers are tested to hold 100% load at 40 degrees C in free air. If you are running the breaker above that temperature it needs to be derated.
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Old 03-09-2012, 03:39 AM   #5
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Thanks jlarson, providing the ul number opened up a vast search on google.

I have always seen this exception in the code that works around going 125% for stuff like continuous loads.

Exception: Where the assembly, including the overcurrent
devices protecting the branch circuit(s), is listed for operation
at 100 percent of its rating, the ampere rating of the
overcurrent device shall be permitted to be not less than
the sum of the continuous load plus the noncontinuous
load.

I wasn't sure if I had ever seen a device rated at 100 percent before. I still would not use the exception, or if that would fit it. Is a breaker in free air when installed in a panel with other breakers?


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Old 03-09-2012, 05:13 AM   #6
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Quote:
Originally Posted by cabletie View Post
Is a breaker in free air when installed in a panel with other breakers?
No, when they say free air it means no enclosure.

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