40 degree C. Circuit Breakers

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Greg1707

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Location
Alexandria, VA
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Business owner Electrical contractor
I noticed that some circuit breakers have a rating marked on them of 40 degree C. I was surprised because I assumed all breakers would be a minimum of 60 degrees C. 310.16 does not have a column for 40 degrees. What are the implications of using these breakers with conductors with higher temperature rating?
 

Snorks

Member
Most circuit breakers list the terminal temperature rating, not the ambient temperature as iwire incorrectly assumed.

And 40?C is very low as the average is 75?C with a low of 60?C as Greg1707 stated.
I have not checked this out but I think that since heat is relative that you could apply a differential factor to the 60?C column for the 40?C ampacities.

I would base this factor on the differences between the 75?C column ampacities and the 60?C column ampacities. Please remember that I am only assuming and have not tried this.

btw - 40?C is 104?F which is almost too low for a circuit breaker as the ambient temperature can easily be higher. I think that I would consider replacing the circuit breakers.
 

iwire

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Location
Massachusetts
Most circuit breakers list the terminal temperature rating, not the ambient temperature as iwire incorrectly assumed.

Actually all (modern) circuit breakers list both the ambient rating 40C and the termination temperature. (60, 75 C Etc.)


btw - 40?C is 104?F which is almost too low for a circuit breaker as the ambient temperature can easily be higher. I think that I would consider replacing the circuit breakers.

btw, 40 C is the ambient rating. :smile: If you need to run it in a hotter ambient temperature you will have to derate the breaker itself. The manufacturer would aid you in figuring out the lowered rating above 40 C ambient.

Keep in mind breakers are thermomagnetic. The overload portion works on heat, when the ambient temp goes up the breaker trips sooner, when the ambient goes down the breaker trips later.
 

Snorks

Member
iwire,

I've never seen an ambient temperature rating on a circuit breaker.

And you are confussing heat with temperature as they are not the same. Heat is an energy unit, not a temperature unit.
 

Snorks

Member
I need to mention that my use of the term "heat" was in reference to the Neher McGrath formulation that was used to establish the NEC ampacity tables.
 

jim dungar

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Wisconsin
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PE (Retired) - Power Systems
I've never seen an ambient temperature rating on a circuit breaker.
You are leading a sheltered life.:)

Most molded case breaker list their ambient temperature rating. On a Square D QO it is physically the largest number molded onto the breaker housing.

The only breaker termination ratings are 60?C and 75?C for molded case breakers (actually UL489 listed) and then 90?C for power circuit breakers (ANSI switchgear construction)
 

iwire

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Location
Massachusetts
And you are confusing heat with temperature as they are not the same. Heat is an energy unit, not a temperature unit.

If the ambient temperature is higher the breakers thermo trip unit can not shed heat as rapidly as when the ambient temperature is low.

From the UL General directory for CIRCUIT BREAKERS, MOLDED-CASE AND CIRCUIT BREAKER ENCLOSURES (DIVQ)

Circuit breakers, the performance of which may be affected by a 40?C
ambient temperature within the enclosure, and that have been investigated
for this application, are marked ‘‘40 C.’’
 

Greg1707

Senior Member
Location
Alexandria, VA
Occupation
Business owner Electrical contractor
40 breakers

40 breakers

The markings I am referring to are on the side of the breakers. I checked at HD and several of the breakers on the shelf i.e. GE etc. had 40 degree C. molded on the side. The majority of the breakers, however, had 60/75.
 

iwire

Moderator
Staff member
Location
Massachusetts
The markings I am referring to are on the side of the breakers. I checked at HD and several of the breakers on the shelf i.e. GE etc. had 40 degree C. molded on the side. The majority of the breakers, however, had 60/75.

Look harder, you will find 60 and 75 on there along with the 40

I have a Square D single pole 20 in my hand.

A big 40 C on the side and a small tag just above the terminal that is so small I can barely read it. It has the 60 75 rating along with the torque rating and the UL listing info.
 

templdl

Senior Member
Location
Wisconsin
All UL 489 listed molded case circuit breakers a designed and calibrated at 40deg C ambient. The trip curve is based upon 40 deg C ambient.
 
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