In commercial services such as common office buildings, what is an acceptable temperature for panel mounted (as compared to discrete disconnects) branch circuit breakers in the 20-70A range? We often find a few breakers in the 80-95deg F range when scanning a panel. Curious if there is any guidance on this beyond the 'can your thumb tolerate it for 5 sec." advice Thank you.
Temperature of Breakeers
- Thread starter avtech
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Are you certain about the 85-90?Farenheit?
Many breakers are specified (rated) for operation at 40?C ambient temperature. That's 104?F... but referring to the air surrounding the breaker, not the breaker itself. The actual operating temperature of the breaker is relative to ambient temperture and current passing through it... and it would increase over ambient temperature the greater the current. For the sake of discussion, and assuming an ambient of 29?C (84.2?F), an operating temperature in the 85-90?F range would not be abnormal...
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renosteinke
Senior Member
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- NE Arkansas
Well, there's 'theory,' and there's 'real world.' I guess you really need to know first is: what is the question you want to ask.
Breakers are calibrated at 20C (about 68F). That means it will perform to the advertised trip curve at 68F. Make it hotter, and it will trip sooner. In extreme situations, this can be a cause of nuisance tripping. Selecting OCPD's for high ambients can sometimes be a challenge. Extremly low temps have the opposite effect- the breaker won't trip soon enough.
The connections on breakers are usually rated at a much higher temperature. Commonly 75C / 135F, this means that at about 135F things in the breaker start failing. Plastics char and become brittle, etc. Parts within the breaker generally become hot because electricity is flowing through them.
What does this mean to you? Well, it means that a high ambient can be the cause of problems. It also means that places that are noticeably hotter than the parts around them need extra attention; there might be a problem brewing.
It's not enough to say "My IR camera says the breaker is 121.37F" So what? Maybe it's supposed to be that warm. OTOH, if everything else in the panel is 90F, and the hot spot is where the wire connects .... maybe the screw is loose.
Breakers are calibrated at 20C (about 68F). That means it will perform to the advertised trip curve at 68F. Make it hotter, and it will trip sooner. In extreme situations, this can be a cause of nuisance tripping. Selecting OCPD's for high ambients can sometimes be a challenge. Extremly low temps have the opposite effect- the breaker won't trip soon enough.
The connections on breakers are usually rated at a much higher temperature. Commonly 75C / 135F, this means that at about 135F things in the breaker start failing. Plastics char and become brittle, etc. Parts within the breaker generally become hot because electricity is flowing through them.
What does this mean to you? Well, it means that a high ambient can be the cause of problems. It also means that places that are noticeably hotter than the parts around them need extra attention; there might be a problem brewing.
It's not enough to say "My IR camera says the breaker is 121.37F" So what? Maybe it's supposed to be that warm. OTOH, if everything else in the panel is 90F, and the hot spot is where the wire connects .... maybe the screw is loose.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Breakers can easily exceed 104?F during normal operation as that is their design ambient temperature.
A fully loaded breaker can have parts that reach 60?C (120?F).
zog
Senior Member
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- Charlotte, NC
My advice, take an IR certification course
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