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This is a breaker from a panel box I recently inspected that supplies power to the HVAC unit heat strips. The breaker got up to 180 degrees under load. The electrician stated the problem ended up being a defective breaker. Once replaced the breaker did not exceed 99 degrees. The first thing that set me off to the problem was an arcing/buzzing sound coming from the breaker. Check it out. The first picture is the breaker once the electrician removed it from the panel. The second is the panel once the new breaker was re installed and ran under load.

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I tried to find an answer to this question a while back, and did not quite believe the answer I was getting - perhaps someone else can do better. What I learned was:

The thermal trip element of a conventional (thermal-magnetic trip) circuit breaker is rated at a reference (ambient panel) temperature of 40° C (104° F).

Above 104° F it must be derated per the manufacturers specifications. (TM breakers must also be derated when used above 6600 ft ASL.).

There is an explanation of how the ambient temperature and the trip element current heating interact starting on page 9 here: stevenengineering.com/tech_support/PDFs/45CBTHER.pdf

An example (Siemens) of the derating charts for both factors is here: http://paragoninspects.com/articles/pdf ... rating.pdf

But... that's not "how hot is the breaker allowed to get?"

There seem to be two answers to that:

1) The UL 489 Standard, which in turn allows for two types of ratings:

1a) Standard circuit breakers cannot exceed a maximum of 50°C (122° F) temperature rise at the wire terminal connection at 100% current in 40°C open air.

1b) 100% rated circuit breakers may have a temperature rise of 60°C at the wire terminal connection in the smallest allowable enclosure if the circuit breakers are connected with wire rated at 90°C wiring insulation sized to the 75°C chart (Table 310-16, National Electric Code®Ã¢â‚¬â€NEC®).

2) The ANSI C37 Standard, which requires a maximum of 55°C temperature rise at 100% in the smallest allowable enclosure and a maximum of 85°C temperature rise on the contacts.

So it appears that per (1a) there is an allowable 122° F temperature rise from 104° F ambient, which would be 226° F at the the wire terminal connection(!)

I also found a (unsourced) statement that breakers should be "should be operated at 20C (68° F) below the maximum temperature specified" here: http://www.interfacebus.com/circuit-bre ... lines.html ,

which would be (158° F).

Of course, we typically never see (apparently) properly functioning TM breakers running anywhere near 226°.

One possibility is that as actually designed and built, a properly functioning TM breaker does not reach these 158° F / 226° F temperatures even under 100% load, and when we see a 180° F breaker (as in the OP's example), it's *because* it's defective.

In which case the question is not "how hot is a 20A TM breaker allowed to be per spec" but rather "outside of what operating temperature range is it suspect"?

In which case the 158° F number might be the limit.

And that's as far as I was able to get.

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I've wondered how hot is to hot for a breaker? Lets say a 20 amp breaker, some heat is to be expected when it is under a load, so at what point do we say that is to hot.

There are so many factors to consider when determining if a breaker is too hot. Such as ambient temperature, amps, types of loads, wire sizes, breaker size, etc. I don’t know the exact answer to that question and I don’t think one exists. Each breaker is different. I was told the only way to know for sure is the same breaker under the same load. Its obvious AFCI, GFCI and double breakers are typically going to be hotter than normal 15/20 amp single breakers. I’ve seen AFCI and GFCI breakers ranging from 80-120 degrees, never any higher. I’ve seen many double breakers 120-130 degrees and didn’t mention them in my report. Never have I seen one 150 degrees or higher. So when I scanned this panel saw the breaker was about 150 degrees and heard this arcing buzzing sound I felt it needed an evaluation by a licensed and qualified electrician.

One thing to note also, when I scanned the panel during the initial inspection the hottest the breaker showed was 150 degrees. I then met the electrician on site a few days later to show him the issue. I didn’t get the 180 degree shot until he removed the breaker and I was able to get a shot from the side and the top.

Kiel Allen

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