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I built a shed a while back and. now I'm ready to get power to it.

I have a 50 amp GCFI breaker, that I plan to run the service line from to the shed. The shed is roughly 100ft away. I called a supplier and they recommended I go with 6 gauge copper or 4 gauge aluminum.

I'm curious which would be the better option?

To cavitate, I prefer the cheaper route as long as it is suitable for the application. The Aluminum 4 gauge is half the price of the 6 gauge copper, but both are reasonable.

Also, I plan on digging an 18-inch trench and running wire through PVC conduit.

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    There are many duplicates to choose from. There's nothing wrong with aluminum for running a sub panel.
    – JPhi1618
    Jan 24, 2020 at 16:48
  • @JPhi1618 I've read a few posts and all seem to mention the same 6 copper vs 4 aluminum. Will both provide equivalent voltage drop over 100 ft? Or is one going to be better. I haven't found a clear answer on this.
    – Discoveringmypath
    Jan 24, 2020 at 16:51
  • This answer has a voltage drop calculator: diy.stackexchange.com/a/135812/43874 The results shown in the answer are that the drop is very similar between Cu and Al.
    – JPhi1618
    Jan 24, 2020 at 16:58
  • The larger size of the aluminum wire permits the voltage drop to be very similar - you could even afford to oversize the aluminum wire (so, less voltage drop) and still come out cheaper than copper.
    – Ecnerwal
    Jan 24, 2020 at 17:07
  • Okay, it seems very similiar in that it doesn't matter so much for voltage drop. Are there any other considerations with alluminum that could be an issue?
    – Discoveringmypath
    Jan 24, 2020 at 17:09

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Your voltage drop is almost the same for a 240v feeder at 100’ at a full load. 1.84% with copper and 1.91% for aluminum. I would save a few bucks and go with aluminum for a residential feeder, code had no hard standard for voltage drop but the recommendation is 3% at the sub and 5% an the end of the branch circuit. Remember you need 4 wire , 2 hot a ground and a neutral. Since it is detached you will also need a grounding electrode. If the 50 amp GFCI panel is in the shop ok but I would not recommend a GFCI feeder from your main panel, that’s a long way to walk for trips. Keep the GFCI protection local to the shed. The terminals on your panel and breakers are aluminum and copper compatible.

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It's called feeder not service, unless it has a separate meter and electric bill.

Your trench needs to be 20" to give 18" of required cover over top of the conduit.

At only 100' you don't need to worry about a wire size bump for voltage drop.

If you are running cable for your feeder, your numbers are correct. If you are running cable in conduit, that is masochistic and conduit is not designed for that; as such the conduit needs to be bigger than you think. (not least to make the pulling possible, though it'll really be an awful task any way you slice it).

On the other hand, if the conduit runs the entire way, you are better off running THWN-2 or XHHW-2 individual wires through it, and then conduit works very well. You are able to run the feeder wires at 75 degrees C, allowing a higher ampacity:

  • 4 AWG aluminum THWN/XHHW in conduit maxes out at 70A breaker.
  • 6 AWG aluminum THWN/XHHW in conduit maxes out at 50A breaker.

As far as a GFCI protecting the feeder, that protects the feeder proper from leakage (all the more reason to use the correct rated wire)... but that is usually not considered valuable. (well, I like it for hot tubs, pools and docks).

Since GFCI trips on power tools do happen, having the GFCI at the shed makes more sense. The $80 breaker you have already sunk money into could be moved there, by using a 2-space "subpanel" to house the GFCI. It could not be installed in a backfeed position on a normal panel, because GFCI breakers cannot be back-fed.

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