How close can I bury circuits to each other?

Question

I need to run 3 lines (2 - 20amp circuits and 1 - 50 amp circuit) in a trench 140 feet from my 200 amp panel. How close can I bury these circuits to each other?

To follow up with your questions: These lines are 1st going 140 feet to the 1st building (which will be a tiny house) That is the 50 amp. The tiny house will have it's own 50 amp panel.
Then the 2 - 20 amp circuits will go another 70 feet to a green house. I have been looking at running the 2 - 20 amp circuits in one 3/4 PVC conduit, coming up at the tiny house location just because I will need a box to pull the fishing tape through. The 50 amp would be a separate 1" PVC line. I have some depth limitations because of rock, but, for the most part can go down to 24 inches. I have concerns with the proximity of the 50 amp to the 2 - 20 amps. I would choose to run these in the same trench. But, I don't want any current jumping from one circuit to the other.

I plan to have all these circuits coming from an updated 200 amp panel from my barn, and wired with GCFI breakers, grounding them at every box and final locations.

Answer 1

If this run is going to an outbuilding and will be inspected, you can't have more than 2 circuits without a sub-panel. If an outbuilding, you need to run a 4 wire (2 hots, neutral and ground) to a sub-panel, add ground rods at the structure, and insure the grounds are isolated from the neutral. Running fewer wires (the 4 wire feed) feeding a sub-panel might actually be less expensive than running 3 separate circuits (which isn't code legal for that long a run anyway) and give you a much more flexible/expandable setup going forward.

Answer 2

There's no reason to be worried about any sort of interference here

Having multiple mains circuits or conduits in the same trench is a perfectly normal occurrence that won't cause any issues; in fact, it'd be legal under the NEC to pull all these circuits through the same conduit between the main panel and the tiny house, although you'd run into some minor issues with ampacity derating on the 50A feeder forcing you to use 6AWG instead of 8AWG THHN for it if you did that.

However, there are other issues with your plan

There are two primary problems with your existing plan, though. First off, that 1" conduit is unnecessarily constraining for feeding a dwelling unit; I'd run 1.5" for that job if I were in your shoes in order to provide room for a future upgrade to 100 or 125A, although if you already have the 1" obtained or installed, don't fret, as it can be left there for use as a comms duct to the tiny house. (Say, if you want to provide network connectivity there.)

The bigger issue I see, though, is that you're planning to put the whole tiny house on a GFCI. This is a problem because a ground-fault anywhere on the feeder will knock all the lighting out in the tiny house. Given that UL943 Class A GFCIs have no support whatsoever for selective coordination, this will be a big deal if a GFCI trip in the bathroom leaves you in the dark with a hot curling iron in your hands!

As a result, I'd use a plain breaker for the 50A feeder instead; even if the tiny-house you're dealing with falls under RV rules, this isn't an issue, as RV receptacles don't require GFCI protection anyway. As for the 20A greenhouse circuits, while you can feasibly GFCI protect those at the origin end without coordination issues, that may or may not be a good idea, depending on the distance from the tiny house to the greenhouse and the leakage currents introduced along that run simply due to capacitive coupling between wires.

Note that for that 20A run, you'll need to run it as a multi-wire branch circuit (MWBC) in order to comply with the NEC 225.30 rules about not having multiple feeders to a structure. The good news is that this'll make your life a bit easier as it will let you pull 4 wires instead of 5 for the greenhouse and also use a cheap-as-chips non-fusible AC pullout disconnect for the local disconnecting means there.