I'd want it 6ft deep if it was any of my fields..you never know what you will do with that land in the future ..Land drainage..Sub soiling..fence posts..??
I ended up renting an excavator and dug it 5 foot deep. It may be overkill but its sure nice not to have to worry about it. The cable they ran up to my house is a mainline that normally runs to a pedestal. The stuff looked to be about a 1/2 or more in diameter. Can't be cheap. Not something I'd want to pay for if I had hooked it in the field!