i am currently trying to figure out the best way to feed a 100amp panel fed from a 200amp Main that is 300 feet away from each other. Do you have any suggestions? and yes i know its going to be expensive.

  • A 100 Amp panel could have a huge load - as much as 80 Amp continuous, or a tiny load - as little as 20A (with occasional higher amounts). Some key questions: What is your actual anticipated load? Is the 300 feet inside, outside or some of each? If outside, how hard to dig a trench for conduit? Jun 8 '21 at 1:56
  • This is pretty much the same as these, barring a small detail or two: diy.stackexchange.com/q/74445/18078 diy.stackexchange.com/q/203665/18078
    – Ecnerwal
    Jun 8 '21 at 1:57
  • 1
    What loads are on this new panel, and can we presume that this is an outdoor/buried run? Jun 8 '21 at 2:02
  • Also similar, varying a different detail diy.stackexchange.com/q/195308/18078
    – Ecnerwal
    Jun 8 '21 at 2:03
  • Won't be so bad, $750 for the good stuff (#2/0). If that's too much, for half the money you could 90A (#2) but the voltage drop would be pretty bad. Depending on what your loads actually are. Jun 8 '21 at 6:19

The minimum legal size for 100A is #1 aluminum, however, voltage drop will be almost 5% with that wire.

2/0 aluminum is at a pricing and availability sweet-spot and is probably just right for that distance.

Voltage drop is proportional to actual load so it matters what the loads actually are.

You're supposed to breaker the circuit for 125% of the expected load, so a 100A breaker implies you expect <=80A of load. At "full" 80A load your voltage drop will be 3.25% @ 300' w/ 2/0 aluminum wire, which is perfectly fine. $2.50/foot so $750 whole job.

The minimum aluminum size you must use is #1, and #2 is legal if you downsize to 90A breaker... but there's not much cost savings to downsize wire. Even #2 is $1.68/foot.

Aluminum wire is the preferred wire for heavy feeders of this sort. Disregard anything you may have heard about aluminum wire for small 15/20A branch circuits; that is not applicable here.

If you really are hell-bent on superstition, you can pay $14/foot ($4200 whole job) for #1 copper, which will give you 2.99% voltage drop at 80A.

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