100-amp circuit breaker boxes are the minimum size allowed by code. They are generally used as sub-panels instead of main service panels, because 100-amp service really only provides enough electricity for a refrigerator, washing machine and lights--with just enough left over to run a microwave or hair dryer. Adding an electric dryer, oven or water heater will require more electricity, which is why the minimum recommended residential service is 200 amps. If you are using your breaker box as a sub-panel, follow the steps below to wire it properly.
Run the cable from the sub-panel to the service panel. The location of your sub-panel and your local building code will determine the specific type of cable to use and whether you will need to run it through a conduit. 100-amps of current requires #4 copper cable.
Remove the knockout on top of the sub-panel and insert an NM connector. Pull enough cable through to reach the main breaker and side buses, and screw the connector securely to the cable.
Slice the sheath off the cable inside the panel to expose the wires. Locate the tie bar at the bottom of the panel--it connects the two side buses--and remove it.
Connect the red and black wires to the terminals on the top of the hot bus, one wire per terminal. Connect the white wire to the top of one side bus--this will be your neutral bus. The bare or green ground wire goes to the top of the other side bus--this is your ground bus.
When wiring 120-volt circuits in the sub-panel, the white wires can only go to the neutral bus and the ground wires to the ground bus. Grounds and neutrals must be separate in any sub-panels to avoid sending current through grounded appliances or pipes. 240-volt circuits require the white neutral connected to the breaker.
Turn off the power to the main panel and remove the cover. Remove a knockout near the location for the 100-amp sub-panel breaker and insert an NM connector. Pull the cable through far enough to reach the breaker and secure the connector around the cable.
Slice the sheath off the cable in the panel. Connect the red and black wires to your 100-amp breaker and seat it into the main panel. Insert the white and ground wires under separate screws on the neutral bus.
If you have any doubts, consult a licensed electrician.