The wire size for 125 amp service should be a 10-gauge bare ground wire.
How to wire a 125 amp service
Firstly ensure that the power is completely off for safety reasons by disconnecting the power leads from the meter. Put holes in either the top or the bottom of the panel and then secure the knock-out clamps onto the panel and pass the leads through and tighten using a screwdriver. The black and red leads need to connect to the brass bars whereas the white lead has to go in the silver bus.
Connect the silver bus to the ground bus and then feed through the 10-gauge wire through the remaining hole to the ground bus. Then place the wire in a place where you can secure it eight feet underground, and then clamp the wire down.
You will need to put the ground rod near the water lines that feed into your home and secure it with a ground clamp. After that, grab the circuit wires and bring them through the panel, but make sure you fill the clamp with the amount of wires that will fit.
Carefully cut off up to 12 inches of sheathing from the wires as well as half an inch of insulation from each end. Take out the white wire in order to loosen it and put it back in. Continue this process when connecting the bare wire to the ground bus. On the terminal of the circuit breaker loosen the nut and out in the black wire. Then place the breaker into the appropriate place on the brass bar.
Lastly put the red and black wires into the correct terminals and then snap the breaker into the opening that can secure two breakers.
