Circuit Breaker Temperature

infinity

infinity

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Journeyman Electrician
This is for the circuit breaker and IR scan gurus. I was having a discussion about temperature rise of a loaded circuit breaker. My take is that a properly sized conductor terminated on the breaker will never exceed the temperature rating of the conductor.

Let say you have a 20 amp circuit breaker in a panel wired with 50' of #12 AWG THHN conductors. For this example the adjacent circuit breakers will have no load. If you ran a 20 amp continuous load on this circuit how much higher than the ambient temperature of the room would you expect to see at the circuit breaker?
 
lots of variables.
What is the ambient temperature immediately around the breaker?

From memory
The hottest part of a breaker should be be around its handle maybe as high as 60°C
The lugs should all be equally hot and not more than 50°C.
 
jim dungar said:
What is the ambient temperature immediately around the breaker?

From memory
The hottest part of a breaker should be be around its handle maybe as high as 60°C
The lugs should all be equally hot and not more than 50°C.
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Ambient would be normal room temperature. My opinion during this discussion was that the 90° C temperature rating of the conductor would not be exceeded when the circuit breaker had a continuous load of the rating of the breaker when everything was properly installed. Loose connections, corrosion of the connection, or other abnormal conditions may present much higher temperatures but with everything being "normal" the conductor could not be damaged.
 
infinity said:
Ambient would be normal room temperature. My opinion during this discussion was that the 90° C temperature rating of the conductor would not be exceeded when the circuit breaker had a continuous load of the rating of the breaker when everything was properly installed. Loose connections, corrosion of the connection, or other abnormal conditions may present much higher temperatures but with everything being "normal" the conductor could not be damaged.
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If I am understanding what you are saying, then yes.

So long as the breaker is suitable in protecting the conductor at the 60°C or 75°C ampacity (depending on rating).

Not to mention, if you had a room hotter than room temperature, you will most likely get a trip prior to the normal point for just that reason.

The temperature rise in the breaker for overload protection should trip when it's rating is being exceeded so as long as the conductor is not less than that, all things should work.

The only, maybe rare, and odd, circumstance that I can think of is in unique scenarios where wire in a box or conduit is bundled and the breaker is in a colder than normal room. The wire might not fail at the breaker but else where in the circuit.
 
jim dungar said:
As I remember breaker lugs should not exceed 50°C rise regardless the insulation of the conductor.
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Looking at my NEMA AB-1 standard for breakers, from 1986, the maximum temperature rise for insulted operating handles was 60°C (based on 25°C ambient) and circuit terminals for external connections 50°C. Insulating material for thermoplastic conductors was limited to 60°C unless the material was investigated to be higher.
 
MyCleveland said:
Jim
Is not RISE the important point here?
25C ambient, 50C rise at full rated ampacity would yield 75C on the conductor...yes?
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Yes, for the most part
The breaker should have an ambient temperature listed on it, many are listed for 40°C and a 50°C rise would be a problem for many conductors.
 
UL 489 7.1.4.2.2 says that when a (non-100% rated) breaker is tested in free air at 100% of its rated current, "The temperature rise on a wiring terminal at a point to which the insulation of a wire is brought up as in actual service shall not exceed 50C." But 7.1.4.2.4 alternatively permits a rise up to 65C if in addition the breaker is tested in the smallest enclosure "in which it is likely to be used" at 80% of its rated current, and the temperature rise is then no more than 50C.

So if room temperature is 25C, and 7.1.4.2.2 is satisfied, the enclosed breaker in the OP will not exceed 90C if the enclosure impedes heat loss by a factor of no more than 50/65.

But if 7.1.4.2.4 was used, and the maximum enclosed temperature rise of 50C at 80% of rated current was observed, and if all the sources of heating follow I2*R, the temperature rise at 100% of rated current would be 1.25*1.25*50C = 78C. That would mean a temperature of 103C at 25C ambient.

No idea if this is a realistic possibility, or just a theoretical one.

Cheers, Wayne
 
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