1) Thanks for the replies Steve!
2) Based on a 240 volt circuit, running 80 feet with a 100 amp load, the voltage drop = 4.3 volts or a 1.8 % voltage drop. National Electrical Code recommends a maximum of 3% VD, therefore you are good and will require 2 AWG Stranded Aluminum, Type THWN conductors. The feeder circuit from the 200 amp main panel to the sub-panel requires a 4 wire circuit, ie, 2 hots, 1 neutral and 1 equipment grounding conductor.
The 2 hots and 1 neutral will be sized as 2 AWG Stranded Aluminum, Type THWN. The equipment grounding conductor will be sized as 6 AWG Stranded Aluminum, Type THWN.
3) Since the garage is detached, the sub-panel needs to be a 100 amp Main Breaker panel and not a Main Lug panel. Detached buildings require a means of disconnect and the 100 amp breaker in the panel will serve this function.
4) Suggest using Gray electrical PVC conduit, Sch. 40 for the underground. Minimum conduit diameter for the 4 wires, requires 1 1/4" diameter for PVC. However, I would recommend to increase the conduit size to either 1 1/2" or 2" in order to make the feeder pull easier.
5) Both panels require an equipment grounding bar to land the 6 AWG equipment grounding conductor. You will also need an 8 foot ground rod at the garage. You will need an 8 AWG Stranded Copper Grounding Electrode Conductor from the ground rod to the equipment ground bar at the sub-panel.
6) DO NOT install the green ground screw or the main copper bonding jumper at the sub-panel. All subs, require the neutral bus bar to be isolated or floating from the panel metal enclosure. Your white neutrals for the branch circuits will only land on the neutral bus bar.
Any bare copper for Romex or green grounding conductors will only land on the equipment ground bar.
DO NOT inter-mix neutrals with grounds as that is an electrical code violation and a safety issue.
7) If using PVC Sch 40 conduit, 18" is sufficient under general topsoil and the driveway. Both are OK to use for the entire conduit run.
8) All receptacles in the garage will need to be GFCI protected either using a GFCI circuit breaker or GFCI receptacle. The lights do not require GFCI protection.
Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.
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