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I want to place a sub panel on the opposite side of my house (from my main panel) on the outside so I can run a heater in our new green house as well as some outdoor power boxes.

I'm either going to do a 70 or 100 amp sub. The cable will run will be approximately 100'.

What size cable will I need to carry this amperage for this distance?

I've attempted to contract a few electricians as this is slightly out of my depth but it will be 2 months before anyone can get to me, and I want to be prepared ahead of time.

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    You will first need to do a load calculation for your main panel, to see if you have enough. Aluminum wire will be much cheaper and is recommended. A fuel type heater might be better than an electric type heater for running costs and amount/size of electrical service you need.
    – crip659
    Commented Dec 2, 2022 at 12:38
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    What are you running off the power boxes, and how many watts of heat are you putting in? Commented Dec 2, 2022 at 12:47
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    @crip659 I like your suggestion of a fuel based heater. Electrical resistance heaters are notoriously expensive to run. But unless natural gas was available, that would mean installing a propane tank and lines and a more expensive heater. Electrical resistance heaters are dead simple....basically giant hair dryers! and inexpensive, but you pay for it in high operating costs. I dk if you remember that old Fram filter commercial: "You can pay me now or pay me later!" Commented Dec 2, 2022 at 15:47
  • The potential future load is greater than my current needs. At some point this will power 2 heaters and some shop equipment like am air compressor, table saw. bandsaw, welder etc. Commented Dec 3, 2022 at 6:30

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The size of the sub-panel doesn't determine the gauge of the wire unless you intend use its full capacity at the same time. If so, you'll need to break into Fort Knox to pay your electric bill!

An extreme example is you could supply a 200 amp as a sub with 10ga wire (good for 30 amps) as long as the feed is protected by a 30 amp breaker. It would be dumb to do that, I'm only mentioning that because many times people want a large subpanel for breaker space but not incur the cost of the size of wire to supply it at its rated capacity. Aluminum is your friend here. It's much less costly than copper and perfectly acceptable for feeders. Just remember to put anti-corrosion paste on the connections.

What matters is what the maximum concurrent load will be. Then upgrade the wire size just to be safe and future-proof. OK , that said, what will the wattage of the heater be? How many heaters? Bear in mind that electrical resistance heaters are considered a continuous load so they can only use up to 80% of circuit capacity.

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  • @isherwood Thanks for the edits! Sometimes I get in a hurry! The only issue I have with your edits is to change "Goop (NOALOX)"" to anti-corrosion. And it's not really a paste, more of a semi-liquid goop, which is why I refer to it that way, but always include the brand NOALOX to be clear. If there are other brands of anti-corrosion I'd be glad to know that. Thanks again for the edits! Commented Dec 2, 2022 at 17:06
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    No need to tell me about it. Just revise further as you like. I don't see our objection, though. That's just about the very definition of "paste". :)
    – isherwood
    Commented Dec 2, 2022 at 17:26
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The correct size for your feeder wire is #2 aluminum.

Large feeders like this are safe in aluminum. Even the old, brittle #1350 alloy has proven reliable. And the stuff is no more costly than #10 copper (30A). This makes it a "no-brainer" for any feeder requirement between 30A and 90A.

The amps of the subpanel doesn't matter. Spaces do.

The first priority in subpanel selection is plenty of breaker spaces. Spaces are cheap, and nobody ever said "man, I sure am glad I bought a couple of lattés with the money I saved chintzing out on a small panel"... but the line is around the block for "Help, my panel is full because I did not anticipate my future needs!"

The size (in amps) of the panel is not a consideration (except obviously you want it to be not less than the feeder size.) Amp ratings are a maximum, like the speed rating on your tires. (you don't have to drive 112 mph).

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