I am connecting to a 200 amp service panel and running 300 ft to a 100 amp sub panel. At the 100 amp sub panel it will be running mostly lights and a occasional small appliance. Do I need to use #2 for the hot leads or can I get by with #4 ?

Before you start trenching, can your main service panel accept a 100 breaker?

Next issue, if go from the main service panel to a downstream panelboard, you must use a 4 wire feed.

Next, to deliver 240 volts - 100 amps a distance of 300' will require 2/0 AWG copper conductors. For Aluminum, you would need 4/0 conductors.

That's allowing for a 3% Voltage drop. The recommended allowance is 5% end to end (feeder and branch circuit).

Before you start trenching, can your main service panel accept a 100 breaker?

Next issue, if go from the main service panel to a downstream panelboard, you must use a 4 wire feed.

Next, to deliver 240 volts - 100 amps a distance of 300' will require 2/0 AWG copper conductors. For Aluminum, you would need 4/0 conductors.

That's allowing for a 3% Voltage drop. The recommended allowance is 5% end to end (feeder and branch circuit).

Thanks