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Garage is 70ft (Measured as run) from panel to panel. I know it'll need direct bury THHN from panel to panel. 100 amp, 20 slot on garage side.

  1. What size SCh80 at 18in+
  2. What gauge 4 wire (what would you recommend per NEC)do I need and is a AC disconnect switch on the out side OK?
  3. Lastly can I drill and matching hole from inside of panel to inside of switch housing with chase nipples and threaded sleeve for in the interior/exterior transition instead of additional conduit through exterior?
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  • Most thhn is dual rated thwn but not all, you mention direct bury and later mention conduit so I believe you understand that thhn/ thwn require conduit. At 70’ I would be considering aluminum wire and I would probably go direct burial and just sleeve the cable as it comes up from the trench.
    – Ed Beal
    Commented Mar 13, 2022 at 19:37
  • If you don't need the full 100 Amps, it's fine to feed a 100A panel with a 90A breaker, at which point 2-2-2-4 aluminum "Mobile Home Feeder" will get the job done, in a cable that you can direct bury, but need to run in conduit inside the buildings.
    – Ecnerwal
    Commented Mar 14, 2022 at 0:05

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What gauge 4-wire?

For 100A hot-hot-neutral, #1 aluminum or larger.

If 90A had been acceptable, you could have used the more popular and economical #2 aluminum.

The ground wire must be #6 aluminum or #8 copper or larger If aluminum it must be insulated. If #6 or smaller, the natural jacket must be green or yellow/green (or bare for copper).

Aluminum feeder at these large sizes is safe and reliable. The lugs on the subpanel are made of aluminum anyway, because that plays well with both copper and aluminum wire (due to thermal expansion characteristics).

What size SCh80 at 18in+

1-1/2". It's possible in 1-1/4" but a tight fit, which may be a challenge for a DIYer.

It needs to be laid with 18" of cover, so 18" of earth above it. The long run does not need to be Schedule 80, only the part where it curves upward (the "stub-ups").

do I need and is a AC disconnect switch on the out side OK?

No. There is no requirement for a disconnect switch on the outside.

You will need a disconnect inside, but that is super easy -- just choose a subpanel that has a main breaker. They cost only a little bit more (certainly cheaper than a 100A outside disconnect).

Lastly can I drill and matching hole from inside of panel to inside of switch housing with chase nipples and threaded sleeve for in the interior/exterior transition instead of additional conduit through exterior?

Well, the THHN wires need to be in conduit their entire route. Junction boxes and panel enclosures count.

However, the conduit does not need to come up the exterior wall. It can come up the interior wall... or inside the thickness of the wall provided all necessary pulling points will remain accessible.

The conduit must be built, backfilled and tamped complete, before any wires are pulled in. A common novice mistake is to sleeve the wires over the conduit as you go. That is not legal and causes many problems.

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  • Thanks. will price a 2226. Already have a panel and Ill get the conduit and try to complete this week end and schedule a electrician for inspection before e forwarding pics the inspector to close out the permit.
    – Chris
    Commented Mar 14, 2022 at 23:08

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