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I am building a barn with a 100 amp subpanel. It is 450 feet away from main panel. What size aluminum wire do I need to reduce voltage drop?

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  • The size of the sub panel only limits the max you can feed it. The size of your main panel and how much extra power you have is more important. 100 amps off a 200 amp panel is a lot, if your house uses a lot of electricity. Have you done a load calculation yet?
    – crip659
    Commented May 20 at 19:57
  • Depending on where the power poles are, a separate service drop for the barn may be more economical than running power from the house. The barn is used for a business (e.g. farm) it also simplifies treating that power cost as a cost of the business. 450 feet is not usually enough to make your own transformers economical
    – Ecnerwal
    Commented May 20 at 23:09
  • Have you run a load calculation for this barn? How many square feet is it, and what are you planning to do with it? Commented May 21 at 3:21

2 Answers 2

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Don't install a 100 amp subpanel. I swear, people buy those things like they're a brand.

If you look at your car tires, you see where it says "speed rating 130 MPH". See, that's not a mandatory requirement, it's a "redline never-exceed limit". Is a 130 MPH tire better than a 112 MPH tire? You bet. And so it is with 100 amp panels. A 125A or 200A panel is better in every way.

And the way that matters the most is number of breaker spaces. Everybody says "I'll never need more than X breaker spaces, and with the savings I can buy a pizza" ... but who saw EV's coming? There's always another gadget. Spaces are cheap, regrets are expensive. And more spaces tends to mean higher amp RATING, and that's fine. If you get a 200A panel "for the spaces", wise plan.

That has nothing to do with the feeder you send to it. On that, I'll discuss the commodity feeder sizes and their practical amp limits at that distance.

2 AWG feeder e.g. 2-2-2-4 will be good for 26 amps at 3% voltage drop, and if you exceed 26 amps, voltage drop will rise in proportion

2/0 AWG feeder will be good for 49 amps at 3% voltage drop, and proportionally more voltage drop at more amps.

4/0 AWG deeder will be good for 74 amps at 3% voltage drop, and likewise.

Some say "I want 100 amps!" I seriously doubt it. When it comes to shed feeders, 100 is just a number people pick out of thin air. You only need enough feeder amps for the loads you plan to run simultaneously. Very few hobbyists weld and run the band saw at the same time lol. However, you need 2 breaker spaces for every 240V load you install. A shop can use up 24 spaces real quick. And you can't use thin/quad breakers anymore.

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A LOT depends on the balance of 120V vs. 240V loads. In addition, 100A should mean 80A maximum continuous usage (e.g., lighting, heaters) but could mean up to 100A if non-continuous usage (e.g., tools).

That gives, broadly speaking, 9 possibilities, based on absolute minimum and, for example, 3% and 5% voltage loss. The NEC does not require a particular value for maximum voltage loss, but some places (e.g., IIRC, Canada) do. Note that sizes listed are for the two hots and the neutral. The ground can generally be much smaller, generally 6 AWG. However, if you get a cable rather than individual wires then the ground wire may be a size larger (typically 4 AWG) for a number of reasons. Larger is always OK. Voltage drop calculations are from this page:

  • Minimum size from ampacity chart: 1 AWG
  • 100A, 120V, 3%: Needs parallel conductors.
  • 100A, 120V, 5%: 500 kcmil
  • 80A, 120V, 3%: 750 kcmil
  • 80A, 120V, 5%: 350 kcmil
  • 100A, 240V, 3%: 350 kcmil
  • 100A, 240V, 5%: 4/0 AWG
  • 80A, 240V, 3%: 250 kcmil
  • 80A, 240V, 5%: 2/0 AWG

So as you can see, the details really matter. So before buying any wire, figure out what your actual loads are. My hunch is that you picked 100A because it seems like a decent size for a subpanel. But you don't have to supply a subpanel to its full capacity. In fact, I would recommend a 200A main panel rather than a 100A subpanel. Why? Because the cost is not much more, particularly since many main panels include "bonus breakers" as part of a package. In addition, you need a disconnect, and the easiest way to do that is with the included main breaker of a main panel. Just separate ground and neutral and it works fine as a subpanel. That also allows for a lot more circuits if you need them in the future.

Depending on what you plan to install, you may be able to just add up the loads or you may want to do a formal Load Calculation. If the result is, for example, 60A continuous with maybe a little more peak demand, mostly 240V loads, then 2/0 AWG will work just fine, and possibly even 1/0. Consider that:

  • Pumps and other large motor-driven loads can often be configured for 240V use, even if they don't require all that much power.
  • Modern LED lighting can often run over a 100V - 277V range.
  • HVAC systems, except the smallest units, generally use 240V.
  • A tank water heater typically uses 20A - 24A on a 30A circuit - much easier on the overall load than even the smallest of tankless water heaters

and you should be able to run quite a bit at 240V, keeping 120V to small stuff (e.g., chargers, computer, etc.) and making it work with reasonable sized wire.

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    Recall that two 120V loads on opposite phases are a 240V load to the extent that they balance. ;^) So if you are running 18A here and 16A there, the neutral is only carrying 2A for those two loads at the same time. Choosing which phase to put which loads on can make a practical difference to your benefit.
    – Ecnerwal
    Commented May 20 at 23:14

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