Free air cables sizing
- Thread starter anbm
- Start date
- Location
- Illinois
- Occupation
- retired electrician
How are you supporting the conductors? What is the spacing between the individual conductors?
Cables run very short distance like 3 feet, from overhead bus duct to utility xfmr secondary terminal.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
If they are terminating in an enclosure, then I would not consider them available for a free air rating
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Contact a busway company. My guess is that their bus bars are sized based on an allowable heat rise, which probably means they are expecting 75°C sized conductors.
Schneider Electric's 4000A busway transformer tap (open air style) is supplied with lugs for (10) 1/0-600kcmil/phase.
I guess I needed to explain more. The 480V/3PH, 4,000A bus duct was already connected to (6) sets 4#600KCM cables, the cables then connect to transformer (this cable length is around 3 feet plus). Going with NEC table 310.17 for free air cable, 600KCM at 75 degree C is rated for 690A. So (6) sets will be equal to 6 x 690 = 4,140A, this works fine to me as this cable rating is greater than bus duct rating. My question is whether we need to derate 4,140A value as cable runs without (6) conduits?