It?s common practice to locate a circuit breaker or fuse in an exterior enclosure with winter time ambient air temps under 0F and summer temps over 100F. We know that circuit breakers and fuses are temperature sensitive. Square D circuit breakers, (both thermal magnetic and electronic), are listed for use from 14F-140F. If the temperature is outside the range of 77F-104F then rerating is required. The charts on the Square D Data Bulletin on this subject have rerating info. However, the ambient air temperature that we need for the calculation is that surrounding the OCPD inside the enclosure ? not simply the outside air temp. The temp calculation seems difficult to perform if you want to take all of the heating affects of the sun, the conductors and the OCPD?s into account. I can tell you that I?m aware of a project that with CB?s in an exterior enclosure on a 100F day it was almost 200F inside the enclosure. So it seem fairly likely to me that a temp rise from 104F to 140F in many enclosures is likely ? especially in a sunny Midwest location.
Problem #1: The maximum temp range for the CB is 14F-140F. It is quite possible that this range will be exceeded at certain times of the year for exterior enclosures. I?m not sure how to get around this short of adding climate control to the enclosure or relocating our equipment inside to a climate controlled area. (Keep in mind that warehouses and such with electrical equipment in unconditioned areas will have similar temp issues).
Problem #2: If we ignore these issues as is typically done industry wide then we have the following worries. First, on a cold day a thermal magnetic CB or fuse may not open until it is much higher than designed, posing a fire hazard. Conversely on a hot day we may get nuisance tripping. We may also have coordination issues to worry about when some are inside and some are outside, etc.
Example: Lets say we have an exterior installation in the shade and we do a calculation and determine that the air temp inside the enclosure is 30F higher than ambient air. We need to upsize what normally would be a 350A CB to 400A. We might think just putting 400A worth of wire is OK, but if we consider the rerating needs for a cold day we find that 500A worth of wire is needed since the CB won?t open as quickly as normal.
We frequently have AHJ?s that require a main disconnect outside. We also have clients that like to locate this stuff outside. It seems to me that this issue is regularly ignored because there are not many other good/inexpensive options. My take is that we should try to avoid putting OCPD?s/XFMRs in unconditioned areas whenever we can. If we do put them outside then we should try to put them in the shade and make sure we have plenty of spare capacity to avoid nuisance tripping. Also, if we have an exterior OCPD feeding a panelboard inside we should probably make the interior panelboard an MCB instead of a MLO. Any other thoughts or recommendations?
Problem #1: The maximum temp range for the CB is 14F-140F. It is quite possible that this range will be exceeded at certain times of the year for exterior enclosures. I?m not sure how to get around this short of adding climate control to the enclosure or relocating our equipment inside to a climate controlled area. (Keep in mind that warehouses and such with electrical equipment in unconditioned areas will have similar temp issues).
Problem #2: If we ignore these issues as is typically done industry wide then we have the following worries. First, on a cold day a thermal magnetic CB or fuse may not open until it is much higher than designed, posing a fire hazard. Conversely on a hot day we may get nuisance tripping. We may also have coordination issues to worry about when some are inside and some are outside, etc.
Example: Lets say we have an exterior installation in the shade and we do a calculation and determine that the air temp inside the enclosure is 30F higher than ambient air. We need to upsize what normally would be a 350A CB to 400A. We might think just putting 400A worth of wire is OK, but if we consider the rerating needs for a cold day we find that 500A worth of wire is needed since the CB won?t open as quickly as normal.
We frequently have AHJ?s that require a main disconnect outside. We also have clients that like to locate this stuff outside. It seems to me that this issue is regularly ignored because there are not many other good/inexpensive options. My take is that we should try to avoid putting OCPD?s/XFMRs in unconditioned areas whenever we can. If we do put them outside then we should try to put them in the shade and make sure we have plenty of spare capacity to avoid nuisance tripping. Also, if we have an exterior OCPD feeding a panelboard inside we should probably make the interior panelboard an MCB instead of a MLO. Any other thoughts or recommendations?
