Free Air

Merry Christmas
Does anyone know of an acceptable definition of "Free Air" as allowed in the Table 310.15(B)(17). I know that ladder style cable tray is considered free air as long as the cable / conductor spacing meets requirements. But what is the minimum amount of ventilation required to be considered free air and use the values listed in the free air table?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Does anyone know of an acceptable definition of "Free Air" as allowed in the Table 310.15(B)(17). I know that ladder style cable tray is considered free air as long as the cable / conductor spacing meets requirements. But what is the minimum amount of ventilation required to be considered free air and use the values listed in the free air table?
if you do not know what "free air" means how do you know that a ladder style tray is "free air"?
 
The literature from the cable tray manufacturer says the ladder style is UL listed and considered to be a free air installation method. It's interesting that the NEC does not have a definition in Art 100 or attached to the table for free air ampacity. That's why I'm wondering what the parameters are. I know that wiring within a UL listed cabinet or MCC uses the free air table for internal conductors but where's the line that makes it OK or not OK?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Not so. Ul508a control panels uses the same numbers as table 310.15(B)(16). I think the UL std for mcc uses the same ampacity numbers
Yep, but MCCs might use actual temperature rise tests rather than generic/conservative lookup tables.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Yep, but MCCs might use actual temperature rise tests rather than generic/conservative lookup tables.
I have often wondered about how they get away with the limited ventilation they often have for VFDs. I have been told it is based on actual testing. Supposedly a lot of the heat goes out through the skin of the mcc instead of the relatively small exhaust grilles they provide.
 
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