400 amp disconnect question

[RAH]

I don't do a lot of comm. electrical work. I have a 400 amp 3-phase disconnect that feeds (2) 200 amp main lug panel. The 400 amp disconnect is fused at 400 amp's. 3/0 copper is used. Is this code/legal? I am adding a 3-phase 100 amp panel off this disconnect to feed (2) roof top units (30 amp each). Load calcuations come out to under 300 amp on the disconnect.

1. Can I use a main lug 100 amp panel? Why?

[Marc]

Ok. You've typed two posts in different categories with a slightly different description in each so I'm struggling to understand what's going on.

If the service goes directly from metering to a gutter and the two sets of 3/0 conductors run from gutter to the 200 amp, main breaker panels, then that seems ok and you can add the main breaker 100 amp to the gutter if you want. All you need is 100 amps of wire.

If the gutter is there, then that 400 amp disconnect is a waste of money. It's not needed. If the load calc yields a value of 400 amps then the service to the gutter needs to be 400 amps worth of conductor.

Does this help?

The roof top units are 30 amp loads?

Got a photo of the nameplates on those rooftop units?

Keep in mind that 3-phase squirrel cage motors have a start current of 6X rated amp load. It's only 2X or maybe 3X for capacitor starts.

Marc

[brric]

IMO the load side conductors must be fused for their ampacity. They are not service conductors coming off the 400 disco, they are feeders and their ampacity is under the disco rating.