I would go deeper then 18". I went 4' when I put the power to the shop. Some boxes are also set up for the main on the bottom, at least the 200 amp I put up in the garage is. I believe you will also need ground rods for the sub, not one but two of them. Noticed Memike mentioned power at the main buss. It is live at all times, even with the meter out I believe. Just had the meter replaced on our house and everything was still on when they pulled the meter.
