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I will be installing a 100A subpanel in a detached garage. 100A for an EV charger and shop tools, and future proofing for a potential laneway house conversion.

The garage is relatively close at just 16 feet from the house, and the main panel is 80ft away on the other side of the house. Preference is to run the line above ground outside, rather than bury the cable. I have reviewed wire size and amp ratings, conduit fill tables and local codes, and think I have the solution. My natural tendency is to "over spec" things, and so I'm curious what those with hands on experience would say about the setup, and if there is any room to optimize cost with alternatives? Here's the current plan:

  • 100A Double Pole breaker from main panel
  • Aluminum 1/0 SER / USE (80ft, whatever jacketed wire is cheapest)
  • Junction box just inside house
  • Splice to outdoor run, Aluminum 1/0 R90XPLE, THWN-2, USE-2 / RHW-2 / UF-B inside conduit (16ft, whatever type of wire is cheapest)
  • Conduit schedule 80 PVC 2" to garage, bolted to fence
  • 100A subpanel, bonded to ground with secondary ground stake

All subpanel cabling will be in a 2 hot, 1 neutral, 1 ground configuration.

The things I am contemplating to make things more cost effective:

  • Downgrade from 1/0 to #1 AWG
  • For the inside section, switch to XHHW-2 / THHN / THWN-2 wire in conduit, or NB-B CCA cable instead of SER or USE cable (TBH, I'm not sure if this will be cheaper!)
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    The quickest / easiest cost optimization is to drop to 90A because then you can use 2-2-4-6 mobile home feeder. It's much cheaper per foot than almost every similar wire option. This might mean you can't do 48A 'fast as possible' EV charging but unless you have identified a hard need for that like a short ToU window and long commute you probably don't need it, and 32A or 24A charging will be Plenty.
    – KMJ
    Commented Jun 6 at 15:38
  • I think you may need or want to stick to 2-2-2-4 on the MHF here, as the neutral reduction is often disallowed for feeders that are not the full service, or at least requires convincing your inspector that enough of the loads on it are and will always be 240-only and the maximum unbalanced load is in fact less than or equal to 65A. 6 AWG aluminum ground is big enough, but I haven't seen 2-2-2-6, personally.
    – Ecnerwal
    Commented Jun 6 at 16:51
  • Given the EV charging I would expect enough of the load to be balanced that 65A is not a problem, but you're right, it might require convincing the AHJ. And 2-2-2-4 is also common and cheap.
    – KMJ
    Commented Jun 6 at 16:53

1 Answer 1

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First and foremost, don't just guess a size, do a load calculation. With the right car charger and associated current monitoring equipment, the car charger load can be reduced or eliminated by it automatically reducing its use when you are drawing near-maximum current, and going to its maximum current when your other uses are not enough for it to affect the overall load. In the usual case of "car charges overnight while shop is not used all night" that works out very well, in practice. But it does require the right parts to make that happen automatically.

No practical reason to choose 1/0 at this distance. Voltage drop on 1 AWG is under 2% and you need 2/0 for a 125A feeder, so 1/0 is just wasting money.

Unless you hate yourself do not put UF-B in conduit. It requires HUGE conduit for legal fill, (or you get to replace it or the conduit after failing inspection for that, having struggled to get it in place) and is hard to pull, and there's no good reason to use it in conduit rather than individual wires.

As commented by @KMJ, using a 90A feed breaker offers the greatest cost savings, as the 10A peak/8A continuous load reduction (.vs. 100A) buys you the ability to use 2AWG aluminum which for market reasons tends to be lower cost than the mere volume of aluminum would indicate.

The panel can (and often should, to get plenty of spaces) be a 200A panel - it's the size of the feed breaker that sets the size of the wiring. The panel merely needs to be "at least as big as" the feed breaker rating, and exceeding it is just fine. You will need to pick up an accessory ground bar kit or two, but you might also get a few free breakers with a main panel kit (you want a main panel as you need the local shutoff the main breaker provides, since the garage is a separate building; and that's the most economical way to get it. You remove or don't install the bonding screw, and install ground bars to make it a proper sub-panel.)

You will also need a local grounding system since the garage is a separate building. That is tied to the panel ground and ground from the supply. You did mention it, but you'll need a minimum 2 rods spaced minimum 8 feet apart if driving rods.

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  • Great answer. One quick add: if the garage is being constructed now, consider an Ufer ground as it's better and often very easy to add.
    – KMJ
    Commented Jun 6 at 17:00
  • The use of "laneway" house suggests the OP may be in Canada which may change some details, but not the main point.
    – Jon Custer
    Commented Jun 6 at 17:01
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    Makes 3% voltage drop a rule, not just a guideline, but that's well-met anyway. I read the question as a garage already built, but yes, any time concrete is getting poured, the concrete encased electrode is the best one you can get (and does not need an additional rod, normally, because it's THAT good.)
    – Ecnerwal
    Commented Jun 6 at 17:04
  • Wonderful answer, very rich in content. Thanks for the time you spent writing this up! Marked as the answer.
    – Levi
    Commented Jun 7 at 12:34

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