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looking for what size wire do i need to put in a 100 amp sub panel with a 150'run off a 400 amp service. new panel will run 60 amp hot tub and sonme lights and ceiling dfans

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I would say 1/0 aluminum from the wire size calculator. Any smaller than that and you'll have significant voltage drop.

You'll need 4 wires - two hots, 1 neutral and 1 ground.

There are two ways to go.

  • Lay a conduit, and have individual wires going through the conduit. Expensive initial construction, but you can obtain 1/0 wires as individual THWN wires at 50 cents a foot (x4 = $2.00/foot). This also allows adding other stuff such as a switched lighting circuit. You can't put data in a power conduit, though. Depending on the type of conduit, conduit can be the ground, which eliminates one wire.

  • use multi-conductor cable. This is a pre-made cable that has all 4 wires wrapped in a single outer sheath. (this bundle will be rather stiff.) Trouble is, multi-conductor cables are not made in every size (or not kept in stock by distributors). What's more, you must use a type of cable (SER, UF, USE etc.) that is correct for your installation route. This limits your selection further, and you could wind up having to go as big as 4/0 ($4/foot).

I often have to go with a larger cable than the calculator said, because my distributor simply doesn't stock my correct size. This is only a problem with multi-conductor cable; single conductor THWN is generally available in every size if you're working in conduit.

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    The trench is expensive in any case (unless you are hanging it on poles overhead) - an extra conduit for data is peanuts and well worth it even if you think there's no use for it when you put it in. With conduit you dig the trench once - with direct burial cable you might be back there digging again... – Ecnerwal Dec 10 '16 at 2:05
  • Form my understanding, 1/0 AL is minimum size for a branch circuit with a 100 amp OCPD. #2 is only 90 amps for a subpanel because it is derated now-a-days. – Damon Dec 10 '16 at 17:47
  • The calculator was indicating nearly 3% so I suspect 150' is near the limit. – Harper - Reinstate Monica Dec 10 '16 at 19:15
  • You can put a smaller ocpd in; the subpanel rating does not dictate the circuit design, wire sizes, etc. I am sure the 1/0 AL will suffice; but if one is truly worried about voltage drop then they can put a 70 amp or so breaker on the feed to the subpanel. – Damon Dec 11 '16 at 9:10

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