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I am adding a 100 amp electrical power line to my new shop in a 130-year-old mill. I want to run a service feed from the house panel which has a 200 amp service from the local power provider. I am planning on installing a 100 amp breaker on the house panel and then running a 2 or 3 gauge aluminum wire to the mill building.

The mill has a 45 kw generator that runs off the river, but I want another reliable power source, so I am adding power from my local provider. In the mill, I intend to install a double-throw safety disconnect switch so that the mill service panel can only be powered by either the generator or the power drawn from the local provider thru the panel in the house. Any suggestions or ideas would be greatly appreciated.

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  • Wire isn't big enough for 100 amps to start with. 2Ga Al is only rated for 90 amps barring things that don't apply. You should set up to sell power back to the utility when you are not using it, but that me difficult or easy depending on local rules.
    – Ecnerwal
    Feb 1, 2023 at 2:57
  • So I should use 0 or 1 awg wire? The house panel has 4/0 wire providing service from the local provider.
    – Tom C
    Feb 1, 2023 at 3:03
  • I planned on running the wires thru conduit. Running three wires.
    – Tom C
    Feb 1, 2023 at 3:06
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    Are you running metal conduit @TomC? Feb 1, 2023 at 3:19
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    Because if the conduit isn't metal, you'll need four wires.
    – DoxyLover
    Feb 1, 2023 at 6:26

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I would be going the other way myself, and preparing to run the house off the river! :) River ain't 12 cents a KWH.

Wire ampacity

See Table 310.15(B)(16) 75C column for the ampacity of wires.

  • #3 aluminum doesn't exist but would be 75A if it existed
  • #2 aluminum is 90A
  • #1 aluminum is 100A
  • #3 copper is 100A

Some people are drawn to a smaller, simpler table that calls out #2AL for 100A, but that table is wrong for this. It's for whole services. Since you have 200A service, it's telling you that you can use 4/0 for the service wire and any feeder off that service. "that's not a lot of help" lol.

4-wire feeder (unless running 120V)

You need to run 4 wires with a ground that is separate from neutral, unless you're only running 120V down there. If you learned electricity a long time ago you might've learned something else, but this was made a requirement at least 15 years ago. John Ward nicely describes what happens when you combine neutral and ground, in this video. Basically you don't have ground at all at that point. And that's worse than it sounds.

If you're thinking "local ground rods are just as good", no actually. Local ground rods deal with lightning and ESD, and they're important to protect you from voltage gradients across the ground (e.g. from lightning). But they don't help with returning fault current, which is what you need or this'll happen.

Ground wire needs to be #6 aluminum or #8 copper. Copper can be bare.

Separate neutral and ground in the subpanel

Subpanel neutral and ground must be separated. That means an accessory ground bar may be needed. They're only a few bucks and the panel labeling will recommend ones that dock right up to pre-tapped mount points.

Any neutral-ground bonding screw or strap must be removed. Follow panel instructions there.

Cannot reverse the line to backfeed the house

It might be tempting to rig up something to feed the house off the river generator. And of course then, the temptation is to try to run the 4 wires "in reverse". That is a serious code violation that could get a lineman killed, or worse, get kicked off the utility grid for backfeeding! And yeah, smart meters let them catch you.

An interlock or transfer switch is mandatory to guarantee that can't ever happen. It's not enough to promise to be clever.

The straightforward way, unfortunately is to run a redundant set of wires. If your generator is 48kW this would be 4/0 wires. It's not really practical for a generator to feed 2 panels unless the transfer switch switches neutral.

The high-end way to do that, used on Tesla Powerwalls and the like, is an automatic disconnect switch with electrical handshaking, so the generator will refuse to come up unless the disconnect has been pulled. This can be accomplished with small signal wires instead of three 4/0 wires.

The cheapest way involves what I call "permanent rewiring, temporarily". In which you permanently rewire the house so that connecting to the utility is now impossible, and then wire up the generator. Once the power outage is over, you permanently wire it back.

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    A powerwall or other brand of lithium battery backup will deal with any temporary issue from the river supply, unless it becomes completely dry for long periods. I'd do that and supply the house, and only maintain a grid connection if I could set up to sell the excess, or if there are long periods of no power from the river.
    – Ecnerwal
    Feb 1, 2023 at 13:24

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