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My septic system requires 3 circuits. A 10Amp & 2 20Amps. Using schedule 40 conduit in a 80ft run underground to the 3 pole emergency service disconnect. Can these three circuits share a ground? And if so what size does it need to be?

  • I don't know if you can combine the grounds or not. But everything I've read says "don't pull 3 12/2, pull the equivalent separate conductors and you can get them in different colors so that you don't get confused (well, at least for the hots - 3 neutrals may be a problem since they'd be white or grey and that's only 2 colors, unless grey and gray look different :-) ). But a small subpanel seems to make sense - they are very inexpensive. – manassehkatz Jan 31 at 15:31
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    The way you distinguish neutrals is with electrical tape. Obviously you can put gray or white tape on white or gray wire. But you can also put colored tape on white wires, because in conduit, it is not allowed to remark white wires to be hot. Therefore red tape on a white wire signifies it not as a hot, but red's neutral. – Harper Jan 31 at 16:39
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    @manassehkatz -- striped THHN is a thing for these applications ;) – ThreePhaseEel Feb 1 at 0:28
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On sharing ground, that's not a problem, it just needs to be sized for the largest circuit. You are vanishingly unlikely to have a ground fault on 2 circuits at once. However:

You're up against a NEC 225.30 limit. You can only run one feeder or circuit to an outside location, unless they are

Different voltages, frequencies, or uses. For example, control of outside lighting...

For instance, you can't run two 120V circuits out there. You can, however, run two 120V circuits if one of them is switched inside the house.

You can also run, separately, a 120V, 120/240V, and 240V-only circuit.

So that gives you a fair variety. Two 120V circuits can be accomplished as a 120/240 multi-wire branch circuit.

And then, of course, each circuit controlled differently is entitled to a circuit - e.g. each section of outside lighting. So if there's any question of putting the well pressure switch at the head vs at the house, put it at the house - it gives you a free circuit.

In fact, hold on -- won't that well pump be 240V*? Of course it will. Your shutoff needs to operate on every hot, so you need 2 blades in your shutoff switch just for the pump. Fortunately, you can have multiple shutoff switches, up to 6.


* By the way, the reason pumps are 240V is voltage drop -- it's not the run from house to well that gets ya, it's the run down the well, which can be hundreds of feet. Well pumps are at the bottom.

  • So you'd have to size the ground for 50A in this case? – isherwood Jan 31 at 17:29
  • @isherwood no, just 20A. – Harper Jan 31 at 17:32
  • Assuming just a single fault at a time? – isherwood Jan 31 at 17:40
  • @isherwood right. – Harper Jan 31 at 17:48
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Eighty feet is a long run. You will likely encounter some line loss at this distance. I would increase the wire gauge by one greater for each of the circuit's conductors in order to reduce the amount of resistance through the wires and therefore the amount of voltage drop. Watch your conduit fill! Always better to go with a sligtly larger sized conduit in case you have need for more conductors in the future. And larger gauge conductors will also affect your box fill calculations. Spend a bit more money and get the larger sized boxes and larger diameter conduit from the beginning.

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