Wire amperage

[berzerk]

Hello Mike,

Q. In a U.L.891 switchboard, are you allowed to de-rate the wire or cable inside the switchboard? I have been seeing other manufacturers doing it, so it appears. I've also read somewhere that you cannot use open air values to de-rate the wire. For an example, if i have a 200A disconnect mounted inside my switchboard, normally, per the NEC chart, i would need to use 250kcmil wire(assuming the wire is aluminum). I've seen other manufacturers using 3/0 which is only good for 155A. I'm confussed on this. Can you clarify this for me?

Thanks Dave R.

 

[Dennis Alwon]

Mike doesn't post here so you'll have to deal with us. :lol:

The difference is if the wire is part of a listed equipment then it has been tested for the purpose. Field wiring we must abide by the NEC. I just wired a 400 amp meter main combo and the wires from the load side of the meter to the 400 amp panelboard was done with one set of 3/0 copper compact conductors. If I did it I would need 2 sets of 3/0.

 

[Jraef]

UL has it's own set of charts for wire ampacity. As Dennis said, the NEC is for field work, UL only applies to factory assembled and tested / listed equipment. If you do field wiring inside of a UL listed piece of equipment (assuming it was designed to allow that), then you must follow the NEC rules, no ifs ands or buts. If you ARE the equipment mfr, someone in your organization will have a copy of UL891 that tells you the wire rating rules to follow for your assemblies.

By the way, one difference is that the factory gear must use minimum 90C rated cable.

 

[berzerk]

So, if i understand you correctly, If as the OEM, i use 3/0 aluminum wire(155A) to feed a 200A breaker, then that would be fine only if i had that combination tested by U.L.? Otherwise if not, i would need to follow the NEC charts?