larrybartowski
Member
From what I've read on other posts, I know that the definition of "ambient temperature" is not clear in the code book, but I'm going to throw this out there anyway.
I have a customer that tests generators. He has a open bottom ladder style cable tray feeder with (6) tri-tied sets of #500 MCM CT rated copper conductors protected by a 2000 amp breaker (100% rated) and feeding a load bank with a max load of 1972 amps (I've debated that the load will get that high. This is figured at 240 volts and I know they don't produce 240V 3-phase generators, but anyway). Another EC has told him that his cables are undersized.
The tray runs outside from the load bank and into a building, through an air conditioned space, and into a "test cell". It does get warm in the test cell, but it's only the last 10' of the cable tray run.
Per the last sentence of 392.80 (a)(2)(d) where it states "supported on a messenger" I'm using table 310.15(b)(20) that has #500 MCM rated for 496 amps at 75 degrees. When I de-rate for continuous load I'm using the 90 degree column that has 500's rated at 580....this gives me 580x6x.8 = 2,784 amps. Even if I use the 496 in the 75 degree column and derate that for continuous load I get 2380 amps.
Am I missing something? Do I need to derate for ambient temperature due to the last 10 or 20' of the run being in a warm test cell? If so can I use the 90 degree column?
I have a customer that tests generators. He has a open bottom ladder style cable tray feeder with (6) tri-tied sets of #500 MCM CT rated copper conductors protected by a 2000 amp breaker (100% rated) and feeding a load bank with a max load of 1972 amps (I've debated that the load will get that high. This is figured at 240 volts and I know they don't produce 240V 3-phase generators, but anyway). Another EC has told him that his cables are undersized.
The tray runs outside from the load bank and into a building, through an air conditioned space, and into a "test cell". It does get warm in the test cell, but it's only the last 10' of the cable tray run.
Per the last sentence of 392.80 (a)(2)(d) where it states "supported on a messenger" I'm using table 310.15(b)(20) that has #500 MCM rated for 496 amps at 75 degrees. When I de-rate for continuous load I'm using the 90 degree column that has 500's rated at 580....this gives me 580x6x.8 = 2,784 amps. Even if I use the 496 in the 75 degree column and derate that for continuous load I get 2380 amps.
Am I missing something? Do I need to derate for ambient temperature due to the last 10 or 20' of the run being in a warm test cell? If so can I use the 90 degree column?