upping neutral and ground conductors for multiple circuits
I am going to extend at least 2-15 amp circuits to an outbuilding for exterior lighting. 1 circuit will be wired to existing carrage lights on the house, then I will be adding 2 carrage lights on the outbuilding so as to illuminate the house and outbuilding at the same time. The other circuit will be wired to existing spot lights under the soffits of the house which I plan on adding 2 spot lights on the outbuilding in the same manner to illuminate all flood lights with another switch.
I am using 2" PVC conduit buried at least 18" deep. Conduit run is about 50'. Am I allowed to use 1 larger conductor for each the neutral and ground to keep my conductor count down? I know my fill count good, I have plenty of room for expansion later. What I would like to know is if I can use a #10 conductor for a shared neutral, then another #10 for a shared ground? In other words: Can I size my shared conductors for the maximum load of all combined circuits and size accordingly? Or even If I were to run 2- #6awg THWN and use 1 for neutral and another for ground could I use them as neutrals and ground conductors for up to 4- 15 amp circuits since copper #6awg THWN is rated for up to 65amps?
1 - 14awg THWN hot conductor for each circuit. (2#14)
1 - 10awg THWN neutral shared (1#10)
1 - 10awg THWN ground shared (1#10)
You only need 1 ground conductor. The ground conductor must be sized for the largest circuit. If you upsize the circuit conductors for any reason (voltage drop is the most common reason) you must upsize the ground conductor to match the possible max circuit size possible with that circuit conductor size.
You can share a neutral on a multiwire branch circuit. The neutral needs to be no larger than the hot conductor due to how a MWBC functions, it will only carry the difference between the loads on the 2 hot conductors or if only 1 circuit has a load on it, it will carry that total just as the hot conductor does.
You cannot run 1 neutral for 2 circuits on the same "leg" of the panel and you cannot run a single neutral for multiple circuits outside of a MWBC.
So, if you run 1 MWBC of 15 amps (thats 2 hot circuits on opposing legs), you would use 2 #14 black, 1- #14 white, and 1-#14 green.
One neutral can serve exactly two 110 volt hot conductors when the latter are one on each side of a 120/240 volt service. Or (not common in homes) exactly three hot wires one on each leg of a "wye" three phase system. No other neutral sharing is permitted.
In a conduit, one ground wire, sized for the larger (or largest) circuit hot wire, is enough. (For hot conductors larger than #8, there is a table in the National Electric Code specifying how small the ground wire can be.)
Also, only one "circuit" may be run to feed a detached building. For best results it should be a multiwire branch circuit i.e. one neutral, one ground, and two hots one on each side of the 120/240 volt service.
Usually at least a 60 amp MWBC (#6 wires and #8 ground) feed is recommended for a good sized outbuilding (2 car garage or larger).
A subpanel is needed at the outbuilding if/when breakers of more than 20 amps are given to it at the main panel.
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