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I plan to run PVC conduit underground from house to shed. Primary purpose is for a sub-panel in the shed - 4 wires for this. I would also like to run another set of wires "back" from the shed to the house to allow for operating a generator in the shed during power outages and bringing that power to the house. Is it allowed to put both of these wire runs in the same conduit as long as I don't violate the 40% fill rule?

  • How much current are you planning for the feeders? (shed and generator) – ThreePhaseEel Oct 24 '18 at 4:16
  • 60 Amp feed to the sub-panel in the shed with THWN - 6AWG H-H-N and an 8AWG ground. Generator (which I don't have yet) will likely have a 30 Amp 120V RV type connector using only 3 wires. Was going to go for overkill on the wire back from the shed and use 3 8AWG and a 10AWG ground. But realistically I only need 3 wires total coming back for 120. – Dolphran Oct 24 '18 at 14:04
  • Is there a reason you only want to bring 120 back from the generator instead of having the wires for 240 there? What size generator were you planning on getting? – ThreePhaseEel Oct 24 '18 at 23:55
  • The wires back from the generator to house are all about future-proofing since I don't have the generator yet (or transfer switch on the house side for that matter). Until Yesterday I didn't realize that 240V portable generators were even a thing. Now I see they are available and even reasonably affordable (E.g. Briggs Q6500), so I'll definitely be running 4 wires back. – Dolphran Oct 25 '18 at 13:56
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Yes, when dealing with single-phase or split-phase power, you can run up to 3 large circuits in a conduit without needing much math. But aside from all the other amp restrictions, your THHN or THWN-2 wires also cannot exceed this amp restriction:

Copper: 6AWG-60A.... 4AWG-76A... 3AWG-92A... 2-104A... 1-116A... 1/0-136A... 2/0-156A... 3/0-180A... 4/0-208A...

Aluminum: 4AWG-60A... 3AWG-68A... 2-80A... 1-92A... 1/0-108A... 2/0-120A... 3/0-140A... 4/0-164A...

Except you are allowed to round up to the next larger available breaker, so #1 Al is allowed for 100A since they do not make 92A breakers.

As long as a split-phase circuit has 3 live conductors, it counts the same as simple 2-conductor circuits. That's because neutral carries only differential current - any current on the neutral means one of the hots is carrying that much less than its rated current.

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Go big or go home!

Upsizing conduit now is ridiculously cheap compared to putting in a new trench later. I would go with 2" schedule 80 PVC with prefabricated wide sweeps if burial depth is not an issue (you'll need to trench 21-24" deep for this), or 2" RMC if you cannot dig down that far (that gets your burial depth down to 9-12" and saves you a wire but at the price of more expensive conduit).

The same holds true for the shed subpanel, for which I'd use a 125A, 24 or 30 space, main breaker panel -- the main breaker in this panel merely provides a disconnecting means for the shed, so it can be oversized without issue.

As to wires

What I'd do is run 6 6AWG THWN-2 wires, color-coded to distinguish between the two circuits involved (black/black/white for the shed and red/red/grey for the generator, for instance), and then a single 8AWG bare copper ground in the case of PVC conduit (if you are using RMC, the conduit is the ground wire). You will need a grounding electrode system at the shed, by the way -- two ground rods driven 8' deep and spaced 6' apart with an 8AWG bare copper wire connecting them to each other and to the panel is typical for a structure like this, but your local electrical inspector may have other ideas if you have adverse conditions (like shallow, rocky soil) present.

Other notes

You will want to fit an inlet box at the shed to match your generator -- a generator with a 240V/30A outlet uses a L14-30 inlet, while a generator with a 240V/50A outlet uses a CS6365 or CS6375 inlet. That way, you can plug your generator in using a normal generator cord.

Furthermore, when assembling the panel and inlet, you will want to torque all lugs to their manufacturer's specifications with an inch-pound torque wrench or torque screwdriver (the tool to use depends on the torque specification), and you will also need to pull the bonding strap or screw from the panel as it is a subpanel, not a main panel.

  • There are specific considerations in my situation that will cause me to do it slightly differently. In the shed I will be using a 100Amp panel which is currently the house (upgrading the main panel is part of my overall project). If there is ever a generator in the shed it will be a portable, so 30A is the max I would reasonably ever need. If I ever go with a higher amperage fixed installation generator, it will be next to the house, so it could be fed with LNG already in to the house. Part of the conduit run already exists and is 1-1/4", so using 2" for the rest wouldn't buy me anything. – Dolphran Oct 26 '18 at 13:20
  • @Dolphran -- what type of conduit is the existing conduit, and what make and model is the existing panel you plan to re-use? Also, portable-class generators go up to the 12-15kW range, and even 8-10kW class units will have a 50A receptacle most likely... – ThreePhaseEel Oct 26 '18 at 22:23

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