breaker heating

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wiredad

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rtu min breaker size 59 amps max 70 fla 44 amps wire size #6 rtu less than 100 feet from panel breaker is heating to 113 F ??? checked amps at panel with meter 43.6 it is a 4 wire delte high leg 3p 120/240 system
 
How did the breaker to bus bar connections look?

If all the connections looked good I would swap out the breaker.
 
Are you having tripping problems? 113 deg F is not very hot and may be expected, especially on a roof top. 113 deg C is hot. Were conductors derated for ambient temperature. 2008 code has new requirements for determining ambient temperature on rooftops.
 
Breakers do give off heat. Don't expect a that is loaded to be cool to the touch. And remember that heat is relative, that is some may think that when something they would expect to be cool to the touch is luke warm it is often refered to as being hot.
Often time consultants requests the heating watts of breakers in a switchboard so that the A/C unit can be properly sized for that area.
 
Perform a FOP test across the CB under load it the millivolt readings.

Is the CB in the middle of the other CB's on the bottom or top?

What is the ambient of the panel?
 
wiredad said:
rtu min breaker size 59 amps max 70 fla 44 amps wire size #6 rtu less than 100 feet from panel breaker is heating to 113 F ??? checked amps at panel with meter 43.6 it is a 4 wire delte high leg 3p 120/240 system
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That temp sounds right. What are the termination ratings at the breaker and RTU. If not listed than you would need to use the 60 deg. C (140 deg F.) column from 310.16 per 110.14 C 1. I would think that the 59 amps is the minimum circuit ampacity not minimum breaker size. I usually see MCA and MOCP listed on these units. MCA for the minimum wire size and MOCP for maximum breaker size. A #6 in the 60 deg. column of 310.16 would reach 140 deg. F with 55 amps on that conductor. If the termination is not listed you should use a wire that can handle 59 amps without exceeding 75 deg C. It appears that a #4 would be the smallest size wire for this RTU if the 59 amps is the MCA without knowing the termination temp. ratings. If the terminations are all rated for at least 75 deg. C than the #6 is the correct wire size and the temp of 113 deg F would be expected.
 
kwired said:
Are you having tripping problems? 113 deg F is not very hot and may be expected, especially on a roof top. 113 deg C is hot. Were conductors derated for ambient temperature. 2008 code has new requirements for determining ambient temperature on rooftops.
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This new rule only applies if the conductors are installed a distance of 10 feet or 10% whichever is less of the total run on the rooftop. (310.15 (A) (2) Exception)
 
People are often amazed at how hot breakers can be.
The hottest spot on a breaker can be the operating handle. The lugs are often the coolest spot.

Many breakers are rated for use in a 40?C ambient which is 104?F, so an operating temperature of 113?F is probably acceptable.
 
yired29 said:
This new rule only applies if the conductors are installed a distance of 10 feet or 10% whichever is less of the total run on the rooftop. (310.15 (A) (2) Exception)
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We do not know if it applies to the OP that is why I mentioned it.
 
According to our IR tech a molded case 40 amp CB at 80% load in a 77 F ambient can run as high as 138 F and be within operating temperature.

Seems high to me, I double check his math.
 
I agree with your tech, a breaker operating continuously at near 70% load will generate some heat. Important thing is that there isn't one hot spot causing this heat to generate like a bad internal contact on one phase.
 
wiredad said:
rtu min breaker size 59 amps max 70 fla 44 amps wire size #6 rtu less than 100 feet from panel breaker is heating to 113 F ??? checked amps at panel with meter 43.6 it is a 4 wire delte high leg 3p 120/240 system
Click to expand...



harmonics with a positive sequence generally cause overheating of circuit breaker. Negative sequence harmonics can cause the same heating problems as positive.

Eddy current and hysteresis losses also cause heating.
 
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