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I found lots of questions about this, but with many different answers.

I'm looking to run a 100 amp line from my 200A main breaker box to a subpanel in my new detached garage. The run is about 120 ft. A large part of the run will be under my house in a crawl space... the rest will be below ground.

The garage will charge my car and power some equipment, possibly at the same time. I wouldn't expect more than a 60 amp constant draw.

I've read I should probably use at least 1-1-1-3 aluminum cable. I'm not sure if I want to use direct burial or conduit. If I use direct burial USE-2/RHH/RHW-2 cable, will I need conduit in the crawlspace? I think USE-2 wiring is individual - Do I just kind of bury the 4 wires together in a trench and put some danger tape a few inches above?

Honestly I'm not too familiar with electrical work, so I want to make sure I do it right (it will be inspected too, don't worry).

  • If using 1113 al I would have my feeder breaker (in the house) in the house at 80 some of the new car chargers are sensitive to voltage drop, our office admin has a charger here at the plant and I had to boost the output voltage or it kept shutting down for under voltage at 6% , most things work fine with voltage drops up to 10% , this was my first problem with a charging station but I have only installed ~a dozen class 2 chargers. – Ed Beal Nov 12 '19 at 20:47
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    You should expect a voltage drop of 1.45% at 60A on #1Al wire. That is fine. Even at continuous-max 80A Vdrop will be 1.94%. You should not plan to run higher than 80A. This is more than adequate. Even the 3% zealots would not recommend a wire size bump here. – Harper - Reinstate Monica Nov 12 '19 at 20:56
  • What make and model is your main breaker box, and is it indoors in a finished space (flush mounted), indoors in an unfinished space (surface mounted), or outdoors (on the side of the house)? – ThreePhaseEel Nov 13 '19 at 12:40
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Your choice of 1-1-1-3 Aluminum is adequate. Most Level 2 car EVSEs only draw 30-32A continuous. A few years ago I dropped 2 gage Cu from my panel to my car charger spot. It was way overkill for my 32A ChargePoint, but just a couple years later and we have 40A and 60A EVSEs available. I could put two Tesla Wall Connectors on that wire and only need to upgrade the breaker. I future-proofed that circuit.

NEC Rules for Underground wiring

Under Driveways and Outdoor Parking Areas

  • Direct burial wire - minimum of 18 inches deep.
  • Rigid or Intermediate Metal Conduit — 18 inches deep.
  • PVC without cement encasement — 18 inches deep.

Under Buildings

  • Direct burial wire in an approved raceway needs to be buried 0 inches.
  • Rigid or Intermediate Metal Conduit — 0 inches deep.
  • PVC without cement encasement — 0 inches deep.

Otherwise

  • Direct burial wire — 24 inches deep.
  • Rigid or Intermediate Metal Conduit — 6 inches deep.
  • PVC without cement encasement — 18 inches deep.

Conduit?

Under your house you need to be in an approved raceway. That can be sheathing, if you buy wire that way and run it "in" the walls/floor. Otherwise it must be in conduit.

I wouldn't even consider skipping conduit. I like more protection for my underground wires than dirt and wire insulation. If someone is digging in my yard, I want him to hit the conduit before he runs his shovel or pick into the wire insulation. Since it's my yard, guess who that digger is likely to be?

The conduit fill table says you can get your 1-1-1-3 into 1-1/4". However, I would put them in 2". The cost difference for larger conduit is small and it makes pulling much easier. I'd also pull an extra pull cord in the conduit. Consider it a gift to the future owner of your home, which might be you!

Larger conduit also makes it possible to later pull bigger wires without digging. That garage panel might be the best place to plug in that new 120A mega-EV-charger, whole house battery, generator, and/or 20kW solar array. If you live in PG&E territory, these "someday" off-grid upgrades have recently moved up the priority list. For me, those potential future uses of the garage panel are the main reason to consider larger conductors.

Minor reasons to upsize your conductors is reduced voltage drop and less resistance. Larger conductors means less of the energy that flows through your meter to charge your car is lost heating the wires. Since EV chargers draw continuous loads for hours per day, the parasitic losses add up.

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    You can't put data cables in a power conduit, unless they are 100% non-conductive, e.g. fiber-optic. Other than that the answer is excellent. At first, I thought I was reading one of our regulars! – Harper - Reinstate Monica Nov 13 '19 at 17:59
  • Spend a few extra dollars and run fiber-optic. You will thank yourself when this turns into a man-cave in the future. – peinal Nov 13 '19 at 22:21
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    @peinal Spend a few extra dollars and lay a second conduit. – yo' Nov 14 '19 at 13:26
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    I really like @yo's suggestion to drop a second conduit in the trench for data. Digging of the trench is a huge expense and a data conduit insures against having to do it again. In a data conduit, fiber and Cat 6 are both options. I still prefer Cat 6 (POE FTW) and I use a spare Cat 6 for CT clamps back to the main panel and my whole-house energy meter. – Matt Simerson Nov 14 '19 at 19:34
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    @MattSimerson, yes, it's prohibited except in very limited circumstances (which don't apply here). See here for more info: diy.stackexchange.com/questions/122306/… – Nate S. Nov 19 '19 at 17:49
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Wire size:

Nailed it. #1 Aluminum is the minimum allowable size given the 100A breaker supplying it. You are required to provision 125% of headroom on planned loads, so you shouldn't plan to draw more than 80A on a regular basis. Since you know your practical load is 60A, you should compute voltage drop on that basis (not breaker trip).

  • At 60A, expect a voltage drop of 1.45%. <-- what you should design for
  • At 80A, expect a voltage drop of 1.94%. <-- the practical max
  • At 100A, expect a voltage drop of 2.43%.
  • Even the 3% zealots would not recommend a wire size bump here.

Conduit

Conduit definitely helps. You can run USE-2 direct underground, but anywhere else, it will need a conduit, and you'll still need a conduit for the 24" downward into the earth to burial depth. If the entire run is in conduit, you don't need USE-2 - and can just use THWN-2 or XHHW-2, which are smaller and necessitate smaller conduit. Still, follow Matt Simerson's advice about upsizing conduit.

The biggest win in conduit is to go to Rigid type, which allows 6" burial depth. However the stuff is expensive.

Make sure to provide accessible pulling points, such as conduit bodies. You're not allowed to have more than 360 degrees of bend between pulling points. I recommend 90 degrees or 180 at the outer limits, to keep the pull job DIYable.

The other neat thing about conduit is, it constitutes "most of the heavy lifting". At that point you can call in a pro to wrap up, without it sounding like you're calling the pro in to wrap up - you describe the big conduit as "future-proofing that you did earlier". Throwing wires in the pipe is a simple job.

Oh, and one last thing, Harper's Law. Buy your wire last when everything else is done.

  • Some jurisdictions might also give you some grief about using USE-2 in the crawlspace, it could be considered indoor and above ground wiring, which are both prohibited. You could run THWN in conduit then splice in an approved box. – NoSparksPlease Nov 13 '19 at 22:06
  • @NoSparksPlease -- that depends on if you're talking about single-rated USE-2 (which is a no-no for indoor conduit usage) or tri-rated USE-2/RHH-RHW (which is fine in conduit anywhere) – ThreePhaseEel Nov 14 '19 at 0:58

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