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I am looking for some help in determining the proper size of wiring to run to my detached shop. I wanted to run 100 amps copper at a one way distance of 85 ft. I was told to use THHN 2-#2 hot, 1-#4 neutral, and 1-#6 ground (isolated in the subpanel) in PVC sch 40 1.5 inch pipe. When I look online at calculators they show it needing larger wire. Is there anyone that can help me determine the proper size of wiring to use?

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  • What size are the voltage drop calculators saying? – ThreePhaseEel Oct 31 '17 at 3:22
  • Southwire calculator says #1, but it won't show more than 1.06% voltage drop no matter what I enter. If I change to getting the allowable distance it says that #2 isn't even good for 100 amps. – Jeff Oct 31 '17 at 3:50
  • Why are you running in copper and not aluminum BTW? It's far more cost effective to run aluminum at these large sizes even with the size penalty. (I suspect your calculator is set for aluminum and not copper, BTW, either that or set to the wrong insulation temperature limit) – ThreePhaseEel Oct 31 '17 at 3:53
  • I added pictures of the report. it doesn't let me change the temperature rating. – Jeff Oct 31 '17 at 4:38
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    You (and many others) are caught up in the idea of sizing conductors for a connected load and not the demand load. If you read the last paragraph of your second image, it explains that it's simply processing your input without any design considerations. So to start who gave you the information to use a #2 load conductor and why did you decide to not trust it and use 100A as your demand load? In short there is a difference between installing a 100A panel and needing 100A to operate your shop while work is being performed. – Retired Master Electrician Oct 31 '17 at 15:33
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One thing to remember regarding voltage drop, it is heavily dependent on the actual current draw on the conductor. When you say that you are running 100-Amp capacity to the shop that means you are running wire that can handle that much current. That is not the figure to use to run a voltage drop calculation.
First make a list of all the items in the shop that you would like to run; include all items that are slated to be added in the foreseeable future, be realistic, include lighting and peripherals. Now narrow that list to those items that likely will run at the same time. Now determine the amp draw for each of these items. Add those loads together. You can add a buffer factor if you'd like. This is the number to use in a calculation for voltage drop. Happy calculating.

  • How do you figure what can be used at the same time? If it is just me? If there are two people working? I intend on having the following circuits. 4 circuits for outlets, 220 saw, 220 compressor, 220 instant hot water, 220 Welder, lights, Gas heater – Jeff Nov 2 '17 at 5:40
  • It is not what can be used but actually will be used. And you have to make those judgments. I will give you my input, after 40-years in the business; if this is a residential hobby shop with one or maybe two people puttering around, you are at 80' not going to have a voltage drop problem. Happy puttering. P. – Paul Logan Nov 3 '17 at 15:13

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