I believe you only need three wires since the garage will have its own ground wire and rod.Question 1) I have a boat load of 1/2 inch grey electrical PVC conduit stock. (I'd like to use it ) If I cant fit the 4 wires or if its way to tight will it be ok to run the ground outside the conduit in the trench?
I do not know, but that is why you do not need to run the fourth wire (as far as I know).Question 2) Whats the deal with isolating the neutral from the ground on the sub-panel? ( NEC code )
Because it is assumed each switch or receptacle in the circuit will not have more than a 15A load...and then the wire and breaker for the overall circuit are 20A since it is likely there will be more than one switch or receptacle being served on the circuit.Question 3) Many plain receptacles and switches are rated for 15 amps, but its safe to use them on 20 amp circuits - why?