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I have an existing 220v x 30a ckt with 10awg copper for a large A/C outlet that is no longer used. I plan to re-route to a j-box and continue this ckt to the garage via conduit with 6awg (< 50'). The "rating" at the Tesla wall connector would be set according to the 30a supply.

In the future the "existing" 10awg and 30a breaker could be upgraded to 6awg and 60amps without re-doing "new" conduit section. A service upgrade may be needed .. currently 100amp.

I hope the plan forward seems reasonable so I'd like to focus on 6awg wire type and conduit sizing so I can proceed with the installation of same. Also, I'd appreciate thoughts on possibly using alum wire (4awg?) and how to best attach 10awg to 6awg or 4awg Al.

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  • Not a complete answer: don't use the aluminum wire. You'll need splices at the copper wire and back to copper for the Wall Connector. It's a headache.
    – KMJ
    Jan 15, 2023 at 3:14

2 Answers 2

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This actually mostly makes sense. However, since:

  • an upgrade to 60A (48A usable because of continuous derate) charging will almost certainly need a service upgrade, so some planning makes sense but not spending much money on it right now
  • 30A is sufficient for most people most of the time
  • copper wire larger than 10 AWG gets very expensive
  • extending copper 10 AWG to copper 10 AWG can be done with ordinary wire nuts, but extending copper 10 AWG to aluminum 4 AWG requires special connectors (both due to the size of the wires and due to the connection between copper and aluminum)

As far as why 30A is enough, take a look at the Technology Connections video, frequently referenced by Harper. Basically 30A (24A actual) will get you on the order of 15 miles of battery charge per hour. Most people, most of the time, have their car sit for 8 - 12 hours most nights. And most people, most of the time, drive well under 100 miles per day except for occasional special trips. The typical number I used to see for general car mileage calculations was 12,000 miles per year = 1,000 miles per month.

I think it makes the most sense to go with this plan, except use only 10 AWG copper wire. You only need 2 hots + ground, no neutral. If you use metal conduit then you don't even need a separate ground wire. Just make sure the conduit is large enough for possible future expansion. It looks like 1" PVC or 3/4" EMT (using the conduit as ground) will work fine.

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  • I agree with your logic. I'm actually leaning toward going minimal now and prefer PVC with smallest possible conduit. I think I found NEC where 10awg uses .0211 sq inches each and the inside of 1/2 sch 40 PVC has .114 sq inches avail at a 40% fill .. I could actually use 5 wires (5.4) so 4 should be fine (with a ground & neutral). Tesla data says 30a will add 22mph. My conduit "path" will have to drill vertically thru a beam (2 2x8) OR horizontally thru a support "post" for that beam (2 2x4). So, keeping things smaller may be better. I realize the trade-off is no provision for growth.
    – Ron C
    Jan 17, 2023 at 2:21
  • Despite my contention that 30A/10 AWG is plenty for one car with typical usage, you may want to consider upsizing a little on the conduit because running 3/4 instead of 1/2 conduit will cost very little (looking at big orange, an extra $2.52 per 10' x 5 = $12.60, plus a little extra on fittings) would let you get to 3 x 6 AWG (hot/hot/neutral) + 1 x 8 AWG (ground) so that you could put in a 60A subpanel someday, which could handle 2 x EVSE and/or a mix of other stuff. All for an extra $ 20 now - the rest paid later if needed. Seems a reasonable way to spend $20. Jan 17, 2023 at 2:39
  • I'm with ya .. just gotta see how big of a hole is reasonable for my structure. Doubt I'll see a second EV or otherwise need a sub panel, but it makes sense if structurally ok. Gotta see if sch40 or 80 has lesser OD.
    – Ron C
    Jan 18, 2023 at 0:47
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Note that 6 AWG copper NM or UF cable is only 55 amps.
6 AWG copper "anything else" is 65A.
6 AWG aluminum is 50A and 4 AWG aluminum is 65A.
2 AWG aluminum (90A) tends to be at a pricing sweet-spot.

If you can do a home-run from panel to garage, aluminum is a real money-saver. The problem comes in splicing from aluminum to copper - really, the cheapest way to do that is using a disconnect switch as a cheap way to get 3 splices. Otherwise you're into Polaris connectors at $20 a pop, which soon negates the savings on aluminum.

The other cheap way to terminate aluminum is a socket. Most larger 30-50A sockets have terminals rated for aluminum. However this may impose GFCI requirements on the circuit, and it will also limit power to 50A.

Remember, EVSEs don't need neutral. But running neutral would let you run a subpanel out there.

As far as 60A exceeding your service capacity, you need to figure out if 30A will exceed your service capacity. The way to do that is a NEC Article 220 Load Calculation, and your municipality may have a worksheet.

Further, there are now "load shed" technologies that can reduce or knock out EV charging when the service is under high load. It's quite possible you can achieve higher EV charge rates without a service upgrade.

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