A friend of mine wants to put strings of Christmas lights outside around trees. He has a subpanel with a 1" PVC conduit run out to the 6 trees he wants to light up. There are 6 J-boxes evenly spaced in the run. The run is 165 feet to the last J-box. He wants a 15A receptacle off each J-Box to plug in the light strings. What size THHN wire would I use for a total connected load of 20A? 30A? I will put the circuit on a GFCI breaker.

Using 20 amp receptacles maximum, and a 20 amp breaker, I would recommend #10 THHN/THWN wire.

You cannot use a 30 amp breaker to protect 15 or 20 amp receptacles.

Be sure to add up the wattage of all the strings of Xmas lights and do not exceed 1920 watts for a 20 amp circuit. If it does exceed, another circuit is needed.