I am adding a 70 amp circuit breaker to a 480V 800A 3 phase panel. While searching online for a breaker, the description reads "70 amps at 40C". I am assuming this means that the terminals are only rated to 40C. Is that correct, or is the 40C a reference to ambient temperature? If it is a terminal rating, now I am not sure how to properly size the wire, as the is no 40C column in table 310.15(B)(16)
Breaker temp rating of 40C
- Thread starter csoc64
- Start date
- Status
kwired
Electron manager
- Location
- NE Nebraska
I believe 40 deg C is the ambient temperature at which the trip mechanism is calibrated at. This is common on many breakers. The breaker still has 60 or 75 deg terminals for conductor ampacity selection.
I agree.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You do not need to believe it, as it is an absolute fact.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The variation in the trip curves with ambient temperature is often ignored, although it is known that a hot termination can cause nuisance tripping of a breaker (among other things).
There will be little change in the high current end of the curves, because cooling of the thermal element to ambient does not come into play.
Breakers mounted outdoors in the sun in a hot climate can actually need to use different sizes (although the size of the wires they protect may need to be increased too under the same ambient conditions.)
The corresponding increase in the trip point compared to the rated value at low temperatures is less often a problem.
Sent from my XT1080 using Tapatalk
There will be little change in the high current end of the curves, because cooling of the thermal element to ambient does not come into play.
Breakers mounted outdoors in the sun in a hot climate can actually need to use different sizes (although the size of the wires they protect may need to be increased too under the same ambient conditions.)
The corresponding increase in the trip point compared to the rated value at low temperatures is less often a problem.
Sent from my XT1080 using Tapatalk
You are correct.
It's amazing that we can even protect wire at all.It is common for breakers be installed in temperatures less than 40degC such as 25degC. As such the TC moves to the right which causes the breaker to take longer to trip than at 40degC. The you run NMB through a hot attic and have to wonder how the wire is protect at all. Sometimes I think it's a crap shoot as it is far from an exact science.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I think that is one reason that the NEC's requirements and tables seem to include a substantial margin of error.
kwired
Electron manager
- Location
- NE Nebraska
I think you are right on that one. Just how many 14 or 12 AWG conductors have you seen protected by a 30 amp device that you know were pushed to the limits of the ampacity tables and adjustment factors yet show little or no signs of overheating?
kwired
Electron manager
- Location
- NE Nebraska
You do not need to believe it, as it is an absolute fact.
Well at least I believe, I could have been a non-believer
- Status