ATS - 480v Utility side feeding disconnect switch and transformer.

[Raptor1963]

Looking for comment on this scenerio....1500kW 480VAC generator w/ 2500a MCB......3000a 480VAC Utility breaker in the ATS feeding the building load.

Got an ATS being fed from utility service and the emergency side fed from a generator....this is all housed in a weatherproof enclosure....the customer want to supply voltage to the enclosure accessories via tapping off the load buss connection.....there is a fused disconnect switch (65A fuses) feeding a single phase 480:240/120v transformer and load center....so using 240.21(1) tap rule & mounting the disconnect switch right beside the ATS, I would have to size the conductors not less than 1/10 the ampacity of the OC protecting the feeder conductors (300A). Questions is if I just use a 6" nipple out of the side of the ATS cabinet can i get away with rating the conductors in free air? If I could justify free air then I could get away with a 200A fused disconnect switch that would take the 3/0awg cable rated in free air = 310A....Otherwise I would have to jump up to a 400a DS to have the properly rated lugs and bend radius for 350kcmil cable = 310A in conduit....comments????

 

[coop3339]

Hi

If the Gen is outside you can use 240.21 (B)(5) and just size the wire to the fuse in the disconnect. No need to oversize

 

[Raptor1963]

Not sure

Not sure

Not sure if we could get by with this......the ATS and generator are inside a building, but outside the building/structure they are feeding.

 

[coop3339]

Under 2" of concrete is considered outside also I think. Maybe you could make that work?