I am installing a new service with two meter panels. One 320 amp meter socket and one 200 amp meter socket. The 320 amp is a 400 amp socket panel with a disconect (two 200 amp breakers). What size of line in from the transformer wire do I need? It's a typical 3 wire, single phase service. Does each leg need to be able to handle 400 amps because it's a 400 amp box? If that's the case it would take a #600 copper conductor. Or, becasuse it's a 320 amp meter does the wire in only need to be able to handle 320 amps, such as #400 copper?
GH

Where are you located and is this for residential or other than residential?

How long is the line?

I'm located in Minnesota. Minnesota Power is our provider up here. The run from the transformer to the meter sockets is about ten feet. They are on poles. All the wiring is underground. The power company wants sockets with disconects because the runs to the house and detached shop are about 175 feet. We have a dual fuel service here so the 320 socket is for general use and my shop (I do a lot of fab work, welding, etc). The 200 amp panel is just for the dual fuel load (interruptable but has a lower rate). My supplier has given me part numbers of:
MIM R208CR2A034 (for the 320 amp meter)
MIM RS45508C (for the 200 amp meter)

I'll have a 200 amp service panel for the house and a 100 amp for the shop.
GH

The underground lateral will be sized to handle the calculated connected load of the sum of both meters. To size for the max the load could be of 480 amps, even #600MCM is not large enough.

My guess, without knowing the calculated load, is that #500 MCM THHN or XHHW copper would be sufficient.

tkrussell,
There will be separate conductors in conduit from the transformer to each meter panel. They will be in parallel from the transformer. It's the 320 meter panel feed that I'm not sure about. There will be two service panels connected to the 320 amp meter, one 100 amp and one 200 amp. The load on the 320 amp meter will be no greater than 300 amps.
GH

500 MCM copper will work fine to feed the 320 amp meter alone. Each meter will have it's own feeder in a separate conduit from the transformer to each meter?

That's correct, each meter has it's own separate conduit from the transformer. The issue I have is for the 320 amp meter. It's a 320 amp meter but the socket panel must also have a disconnect, which it does. But, the disconnects are a pair of 200 amp breakers. (according to the specs it's a 400 amp box with a 320 amp meter socket) So do I have to run wire to the meter for the total maximum load of 300 amps (because the service panels attached to it are 100 amp and 200 amp respectivly) or 400 amps because the box and disconnects are rated for that?
I think it all boils down to either #350 copper or #600 copper?
GH

OK I got it now. Since the max OCPD is 300, the wire needs to be no larger than 300 amp capacity. Or at least large enough to handle the calculated connected load.

If you want to cover for the max, then 350 MCM is fine.

Since you have a 320 amp meter, and two panels fed by that meter, why not make the 100 amp panel a 200 amp panel, will save having to upgrade later. Just a thought.

Actually, as I was posting my last response I was thinking just that. So if I understand your answer, I need to size the wire according to my calculated load, not what the disconnect breakers and box are rated for?

Correct.

Thanks for your help. It's amazing how simple it is when you get asked the right question. Now I have to go see if I can mortgage one of the kids to buy that copper wire.