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I am taking power from my 200amp entrance panel to a separate building with woodworking tools. I'll start with 100amp breaker at the panel then travel 75' inside my basement before going out through 1 1/2" conduit another 75' to the building where i'll have another 100 amp breaker at the subpanel. My question is cable type. I know i'll need URD for outside and am looking at either 2-2-2-4 or 2-2-4-6. Which will suffice? Also what should i use for wire inside the house between service panel and the exit to conduit? Thanks for your help

  • Why 4 conductors? Normally this would just be two hots and a neutral? – Jeff Pritchard May 26 '15 at 10:50
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    @JeffPritchard That's not true anymore. Modern codes call for 4 wire feeders. – Tester101 May 26 '15 at 14:39
  • @tester101, so the 4th wire is for a ground going back to main panel instead of having a separate ground rod at sub-panel? – Jeff Pritchard May 27 '15 at 15:00
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    @JeffPritchard Yes, the 4th conductor is a grounding conductor. If the second panel is in a separate structure, there needs to be a grounding electrode (rod, etc) at the separate building too. – Tester101 May 27 '15 at 15:42
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First off, you likely want type UF cable for this application -- URD isn't even a recognized NEC type to begin with, and even URD/USE cable can't be run indoors as per 338.12(B)(1) as it lacks the fire-resistive insulation of NM, UF, and SE cables.

Second -- while 2AWG copper USE has an ampacity of 115A as per the NEC, since you cannot use USE in this application, you must use 1AWG wire here, as UF has a lower temperature rating than USE (60deg vs USE's 75deg) and thus a lower ampacity (2AWG copper UF can only carry 95A, while 1AWG copper UF is limited to 110A). This may also require you to upsize to 2" conduit in order to keep the conduit fill acceptable. Of course, since you are doing the run in conduit, 2AWG copper THWN is also an acceptable choice of wire for this task, provided the indoor run is done in conduit as well, and would allow you to keep the existing 1.5" conduit.

As to the neutral and ground wires, a 2-2-2-4 configuration would be used -- this is as per 215.2(A)(1) Exception 2, which would prevent an undersized neutral in your situation when the feeder is fully loaded. Of course, 2-2-2-2 is also acceptable, and may be easier to manage when working with individual THWN wires in conduit.

If you do decide to use URD/USE for the outdoor run -- you will need to use a different wire type for the run indoors. You'll either want copper THHN/THWN in conduit (2AWG works), or a 2-2-2-4 copper SE (note, NOT USE) cable, either in conduit or run as if it were NM.

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