[engineerscott]

A 220V 125A service would run your 8 x 3100W heater element array with a little overhead. This is the way I would go. Your total heating power would be 24.8 kW which would give you 60.5 W/ft^3. This is of course less than the 79 W/ft^3 that commercial oven that Caswell sells does, so you would expect it to heat up somewhat slower and it will not have as high of a max temperature unless you have better insulation (and since we don't know how well Caswell's oven is insulated we don't know how much better you'd have to be). That said, I suspect that this will be a workable oven for you.

Remember that if you run a oven heating element designed for 220V at 110V it will not pull the same power. At 220V a 3100W element draws about 14.1A. This equates to a resistance of about 15.6 ohm. If we assume that the resistance of this element is constant (which is actually incorrect since resistance is a function of temperature, although this assumption will do for our purposes) then at 110V this element will dissipate only 775 W. In truth since our assumption about the resistance is slightly wrong it will be a little higher, so lets be generous at call it 1000W. So, if you run your 8 elements at 110V you are only going to get at best 8 kW, which is only 19.5 W/ft^3. I doubt you are going to be happy with this amount of heat in your oven.