Electrical Service to Detached Garage

Question

I am planning on running an electrical service (240V, 100A) from house to a detached garage.

I am looking for feedback on my plan.

Site:

• Western New York State
• Main House Panel is at the furthest location from the detached garage in an unfinished basement (poured concrete floor and open floor joists)
• Detached garage has an existing 200amp panel
• Pool is an obstacle on the backyard side of the house
• Driveway is an obstacle on the front yard side of the house
• Garage on the opposite side of main house panel (no basement under garage)
• Basement length: 86 feet
• Basement exit to trench entry on side closest to house: 48 feet
• House to detached garage: 100 feet
• Total run: 234 feet (for discussion lets say 250 feet)

Plan:

• Use 2/0 AWG Al THHN/THWN-2 wires for hots and neutral
• Use 6 AWG Al THHN/THWN-2 wire for dedicated ground back to panel (?1)
• Run wires from the Main House Panel in a 2" PVC 40 Sch conduit, in the basement (86 feet), surface mount on side of exposed foundation (48 feet), and in a 24" trench (100 feet) to the detached garage (?2)
• Main house panel breaker will be 100 Amp
• Two grounding rods from detached garage sub panel 6 feet apart with 6 AWG Cu wire (?3)

Question 1: Ground wire size appropriate? Naked ground permissible?

Question 2: Does the basement run need to be in a conduit if using wires instead of a cable? Can the trench run be another wire type, like USE-2? Are junctions between wire types in run lengths like mine common and advisable? Any issues with running PVC conduit surface mounted on exposed foundation?

Question 3: Does this have to be a naked wire?

In general I appreciate any feedback and my main concern is the need to run 2" conduit inside the basement as I would like to avoid it if possible, both for space and cost savings. However, I am worried about splices between different cable types.

Any other advice or recommendations would be welcomed as well.

Answer 1

Load Calculations First!

You have a 200A panel and want to feed it 100A. That's perfectly fine. But do you actually have 100A to spare from your main panel? Maybe. Maybe not. You need to do an NEC Load Calculation first to find out how much you can spare. Maybe it is 100A or more. It might be 50A. It might be almost nothing.

Then do the same for your garage. 100A is likely to be more than enough, even with a huge amount for EV charging. But what if you can only get 60A from the main panel? Then you need to rethink things a bit.

The biggest demand factor, particularly for garages, is EV charging. The reality is that 20A is often enough, and 30A often more than enough, for almost all usage. You can install a 60A-capable charger but configure it for 20A or 30A. If that isn't enough to solve the problem then you can look into a number of different load shedding/load sharing possibilities.

Also note that dropping to 90A lets you use 2 AWG aluminum, which is often priced very competitively.

And now to your actual questions...

Question 1: Ground wire size appropriate? Naked ground permissible?

6 AWG aluminum covers up to 100A. Naked is OK.

But your hot/neutral wires are way oversized! According to an ampacity chart you should be fine with 1 AWG for 100A. 2/0 actually gets you all the way to 135A, but unless you have serious voltage drop issues it is overkill for 100A.

There may be an issue with having to oversize the ground wire based on the capability (even if breakered lower) of the hot/neutral wires. If that's indeed the case (I am not 100% sure) then you would need 4 AWG ground if you stick with the 2/0 hot/neutral.

Question 2: Does the basement run need to be in a conduit if using wires instead of a cable? Can the trench run be another wire type, like USE-2? Are junctions between wire types in run lengths like mine common and advisable? Any issues with running PVC conduit surface mounted on exposed foundation?

Yes, wires need to be in conduit unless they are part of an approved cable type. The problem is that transitions with large wires (e.g., between individual wires and cable or between different cable types) are expensive, and therefore minimized.

PVC all the way through is fine. However, in areas that need protection, which typically is from ground or floor up to 8', need to be PVC Schedule 80 rather than PVC Schedule 40.

Question 3: Does this have to be a naked wire?

It can be naked or insulated. Doesn't matter.