To identify the wire gauge, the best way might be to take the cover off of the breaker box (advise to do this with power off for safety, but power can be left on - proceed at your own risk) and see if there are any indications on the wire sheathing. If there isn't, use a guide like to identify the gauge of wire.
As for copper vs. aluminum wire, aluminum might be cheaper in larger sizes. However, aluminum requires some special anti-oxident joint compound when you make connections with aluminum wire (see Ideal no-alox for an example of this stuff). If you don't use the compound, its possible that the connections will fail over time.
EDIT: Using the info you provided in the comments, and the wire voltage drop calculator at http://www.southwire.com/support/voltage-drop-calculator.htm, assuming:
- Single Phase 240V
- Direct Burial
- no more than 3% voltage drop
- 2 AWG aluminum wire
Your wire gauge will allow you to maximum run somewhere in the 75-80 amp range. I wouldn't go all the way to 100 amps, but 60 amp service should work. You'll need 1 AWG wire to run 100 amps safely that distance.
No, you can't do that
A 100A manual transfer panel has a 100A breaker for the utility-side input; as a result, it would be a "bottleneck" if you put it inline with your service, restricting the whole service to 100A. However, this does not mean that you have the wrong transfer switch, so do not go running back to the store before you read the rest of this answer!
But, you probably don't want to transfer the entire house
Whole-house transfer sounds good at first, but especially for folks with smaller generators, its not nearly as good a plan in practice as it is in theory. Many of your larger loads, even on a smaller service, are rather large for a generator, and are not nearly as important to have on a generator unless you are dealing with a situation where the power regularly goes out for days on end. (Do you really need your dryer on a generator, or your range for that matter?) Furthermore, you will have to flip a zillion breakers in order to not overload the generator when you transfer to generator power -- probably not the easiest thing to do in the dark!
As a result, what I would do is put a 30A, 2-pole branch breaker in the main panel to provide the utility-mains-side feed to the transfer switch, and then put a subpanel in off the standby side that only has breakers in it for the critical loads -- the ability to have heat so the house doesn't freeze (with a 125A service, I can tell you do not have electric resistance heat), the ability to run a small cooking appliance (such as a plug-in electric griddle or a microwave), your refrigerator (if it's not on the aforementioned small appliance branch circuit, that is), 1-2 circuits for standby lighting + the smoke alarms and selected receptacles, and perhaps the ability to run the hot water heater as well (an electric tank-type heater can be half-volted for use on a generator at the cost of very slow recovery, as well as an extra transfer switch and some clever wiring), as well as any sump or well pumps the house may have.
Best Answer
Yes, but only if the meter base isn't already 125A. I find it curious that they'd run 1/0Al when #1 would have sufficed for a 100A seevice. It was like someone intended to go to 125 later. Perhaps this same thinking guided the selection of the meter base.
Now, as far as that service panel. Far and away the most painful thing about changing a service panel is physically fitting all the cable entries and conduits to the edge of the box proper. Yet, the working innards are what makes a panel 100 vs 125A, main vs sub, 2-pole vs 3-phase etc. But think about it. Manufacturers make hundreds of panel sizes but only a few box sizes.
So it's worth the legwork to see if the manufacturer can sell you a box with the guts you need that fit the same bolt pattern on the box you have wired-in.