Looking to hook up a new 20KW generator to an existing manual 200A transfer switch.

What size copper wire do I need (2 hot legs, 1 neutral and one ground) and what size PVC conduit do I need for all 4 wires? The distance from the generator to the transfer switch is 15 feet

  • Any reason not to use EMT for such a short distance? Handles your ground for you, one less wire... Sep 26 '17 at 21:39
  • And why copper, for that matter? You're talking fat enough wires that aluminum makes sense here... Sep 26 '17 at 23:39
  • I use cu. wire and also use penatrox (sp) on the connections and also PVC because of outside connections and what other conduit is already in use
    – Billy F
    Sep 26 '17 at 23:56

Oracle says #1 aluminum. Or #3 copper if you own a copper mine.

I don't like copper because the lugs on both ends are usually aluminum, and I don't like mixing copper and aluminum due to dissimilar metal corrosion. You may recall that caused a national scandal when shoddy receptacles intended only for copper were used on aluminum wire.


3 AWG copper should do it. Use http://wiresizecalculator.net/ to calculate wire sizes.

1 AWG Aluminum would be more common in this case.

  • 20kva /240= 83.3 × 125% =104A where do you get 3/0?
    – Ed Beal
    Sep 26 '17 at 22:10
  • My bad. I used 200A instead of 20kva. I'll update to reflect 104A, but where did you get 240?
    – Billy C.
    Sep 26 '17 at 22:13
  • 240v is the standard for calculations. I used 125% on wire size since I don't know if the generator has a breaker/ overload protection. My local code is disconnecting means 115% and wire size 125% unless the manufacture instructions allows 100% wire size. 445.13 = 115% conductors
    – Ed Beal
    Sep 26 '17 at 22:45
  • Took two long to find it and edit comment NEC 445.13 ampacity of conductors from generator terminals to distribution device 115%.
    – Ed Beal
    Sep 26 '17 at 22:52

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