Hot breakers

[Masterapprentice]

I looked at a job that had some breakers reaching 120-158 deg on single pole circuits and two pole circuits. Most are tandem 20 amp breakers in and older qo panel, Cooper bus bars. I took thermal imaging of the panel. Some breakers are getting very hot. The circuit wires are not too hot, maybe 110 at max. The amperage at each breaker never exceed 12-13 amps. All connections are tight. It’s a 200amp panel and a and b phase max out at about 98-100 amps. I cannot figure why they are heating up so much. I replaced several breakers to see if the stab connections are loose, but it didn’t seem to change temp much. Should I just purchase a larger circuit capacity panel and make sure all circuits have its own breaker? What is causing such excessive heating without even coming close to overloading the circuits and all breaker connections are good. Would the qo breakers lose a tight connection over time and cause this?

 

[LarryFine]

Welcome to the forum.

The heat must be coming from either the breakers' guts or bus contact.

 

[4x4dually]

Can you post up the thermal image? Is it just a particular section of the panel that get hot? So when you replace a hot breaker with a new one, the new one heats up to the same temp?

 

[jim dungar]

A breaker's hottest spot is tested at its operating handle, I recall this can be as high as 60°C. Their lugs should be one of the cooler areas, maybe not more than 55°, even though the conductors have much higher temperature ratings.

 

[kwired]

In dwellings we typically don't notice this kind of thing as loads in a dwelling tend to have a lot of diversity. You may find an occasional single load that is pretty constant, usually HVAC, maybe a pool pump where you have one breaker that gets noticeably warmer than most others. Now go to a place where you have more continuous loading even if only around 50% of the breaker rating and you often will find many breakers that are operating pretty warm. Consider that each breaker does have a thermal element in it as part of how it detects overload conditions. Then as you mentioned you have a lot of tandem breakers in your application, so you have even higher density of these thermal elements within your panel. I can see it likely would get pretty warm in that panel. They can get to where they are almost too hot to touch and still be within their designed operating range.

 

[Masterapprentice]

Yes, I will post the images tonight, thank you for reading.

 

[Jraef]

A breaker's hottest spot is tested at its operating handle, I recall this can be as high as 60°C. Their lugs should be one of the cooler areas, maybe not more than 55°, even though the conductors have much higher temperature ratings.

UL 489 calls for a max. 50C (122F) RISE over ambient at the terminals and max ambient is 40C (104F). So 158F is really not a problem.

 

[GeorgeB]

UL 489 calls for a max. 50C (122F) RISE over ambient at the terminals and max ambient is 40C (104F). So 158F is really not a problem.

50C RISE is 90F RISE.

 

[don_resqcapt19]

50C RISE is 90F RISE.

You cannot compare the two temperature systems like that.

 

[GoldDigger]

You cannot compare the two temperature systems like that.

Yes you can.
When you are looking at temperature differences that pesky 32 degree offset drops out. You are left with 180 Fahrenheit degrees (212 minus 32) corresponding exactly to 100 Celsius degrees (100-0).
The formal distinction, not always properly observed in print or in speech, is between Fahrenheit degrees (a temperature difference) and degrees Fahrenheit (a location on the temperature scale.)

 

[kwired]

Yes you can.
When you are looking at temperature differences that pesky 32 degree offset drops out. You are left with 180 Fahrenheit degrees (212 minus 32) corresponding exactly to 100 Celsius degrees (100-0).
The formal distinction, not always properly observed in print or in speech, is between Fahrenheit degrees (a temperature difference) and degrees Fahrenheit (a location on the temperature scale.)

You have me a little confused. I agree when looking at differences that 32 the degree offset drops out of the equation but 212 F is the same temperature as 100C .

It should be for every 1 degree C of change you should have a 1.8 degree F change. The 32 degree offset is only used for conversion between F and C.

 

[LarryFine]

He's saying there's a difference between comparing temperatures and temperature differences.

You're both saying the same thing. I use 9:5 and 5:9 to convert in my mind; the math is easier.

 

[Masterapprentice]

 

[Masterapprentice]

 

[Masterapprentice]

 

[Masterapprentice]

I thought I understood that tandem breakers had a 40c rating, is that not correct? Where do I go to see these ratings? Someone mentioned a UL reference, is there a book on equipment and their capabilities from testing that shows safe working limits?

 

[retirede]

I thought I understood that tandem breakers had a 40c rating, is that not correct? Where do I go to see these ratings? Someone mentioned a UL reference, is there a book on equipment and their capabilities from testing that shows safe working limits?

You always need context when discussing temp ratings. 40C is likely the max ambient temp rating, not the overall (ambient plus rise).

 

[Masterapprentice]

Ok, that’s good to know. Ambient in back room was likely 75 deg

 

[kwired]

Ok, that’s good to know. Ambient in back room was likely 75 deg

75F or 75C?

75C = 167F. Probably not impossible to have a room that hot, but not too common either. Usually for specific process and limited people access when it would happen. I would think this is nearing the upper limit of what a human body can take and would be for short time as well.

40C = 104F That is hot for people not so hot you can't handle it at all. Is where electrical items are mostly just beginning to need to consider temperature for design and use.

 

[Masterapprentice]

F