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1) The voltage drop calculation for a 100 amp sub-panel, 240 volts at a distance of 170 feet results in 1/0 Stranded Aluminum which provides a 2.4 voltage drop or 2 AWG stranded copper which results in a 2.7 voltage drop. Due to the distance, Aluminum wire will be more cost effective than copper conductors. Code requires the voltage drop to be less than 3 volts.
2) Since a sub-panel, the feeder circuit leaving from your 200 amp main panel needs to be a 4 wire circuit, ie , 2 hots, 1 neutral and 1 equipment grounding conductor.
The 2 hots and 1 neutral can be sized as 1/0 Aluminum and the equipment ground size is a minimum of 6 AWG Stranded Aluminum.
3) If using PVC conduit, you can use Sch 40 gray electrical PVC, 2 inch diameter. For the conduit fill ratio, code requires a minimum size of 1 1/2" inch diameter conduit, however I would suggest using 2 inch diameter as this will make pulling the conductors much easier. The wires will need to be rated as type THWN insulation for wet conditions. Or you can use Underground Feeder (UF) 4 conductor cable as a direct burial. I always recommend using conduit for underground since you only want to trench this one time.
4) You will need space in the main panel to accommodate a 100 amp double pole breaker. The main panel and the sub-panel will both require an equipment grounding bar. Located at both the main panel and the sub-panel, the equipment grounding conductor lands on an equipment ground bar and never on the neutral bus bars. At the barn, you will require a minimum of (1) one 8 foot ground rod.
5) If using PVC, burial depth is a minimum of 18 inches. If using UF cable, minimum depth is 24 inches.
6) The sub-panel in the barn will need to be a main breaker type of panel and not a main lug panel. At the sub, the neutral wire must remain isolated from the panel metal enclosure. This means, do not install the green bonding screw or the copper main bonding jumper at the sub neutral bus bar. The neutral always remains floating in any sub-panel. The grounding electrode conductor to the ground rod can be sized as either 8 AWG or 6 AWG Stranded Aluminum. Depending upon your soil conditions, you may require 2 ground rods installed a minimum of 6 feet apart. Best to ask your local electrical inspector for soil grounding conditions in your area.
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