I'd like to buy an electric car and be able to charge it from outside my garage. I'm located in Colorado, USA. Right now my house has 100 A service. A 1.25" SCH80 conduit brings 8 Ga THHN wire underground (maybe 50 ft total) to a garage subpanel. That circuit is on a 40 A breaker in the main panel.
The garage has 4-20 A circuits currently (tools, lights, my work desk is out there) but generally very light use, but sometimes a dust collector, and table saw are running.
Is this plan reasonable? Pull 6 Ga wire through that conduit, upgrade the breaker in the main panel to a 50 A, 240 V breaker. Install a 50 A, 240 V breaker in the subpanel. Run 6 Ga. THHN through conduit from the subpanel, in the attic, then down in the wall by the garage door for a 14-50 outlet (maybe a total of 30 ft).
Would that run from the subpanel to the outlet have to run through the rafters or can it be located where the rafters meet the joists since it's in conduit? Or maybe can the conduit just be where the ceiling meets the wall and not be in the attic?
Is it fine to have a 50 A circuit on a 50 A panel? When there are other things in the garage? (Though probably not in use while a car is charging) Ot is the whole system too small to do what I want?
Best Answer
Get an accurate copy of Table 310.15(B)(16)
You have #8 THHN wire going to the garage. The wire is rated 90 degrees C thermal. It terminates on a breaker (75C) and panel lug (75C) so the limiting factor is 75C. The wire is copper.
Now, go look on a correct Table 310.15(B)(16) for Copper / 75C column. What number do you see?
Well, whaddaya know.
The idea that #8 is 40A and #6 is 50A ... is "folk knowledge", probably because "NM type cable" does use the 60C column. Not your problem.
EV breakers
I'm comfortable with a 50A breaker for EV charging, since there won't be much load on that panel when the EV is charging. (the EV parks where you would be using the saw and dust collector, yes?)
EV current settings are "soft" anyway, so you can dial the EVSE to any current which works. So if the local inspector kicks a fuss, you can crack the case of the EVSE and set the DIP switches to 40A instead of 50A (note actual charging is 80% of this)... or replace the 2 hot wires on the feeder with #4 aluminum or #6 copper THHN or XHHW wires (65A). Sky's the limit really; you could run #2Al and bring 90A.
Aluminum wire at these large sizes has proven reliable; however that's because a) it is placed on terminals properly rated for aluminum wire, and b) it is torqued properly. Note that most NEMA 6-50 and 14-50 receptacles are rated 75C and AL, opening the door for using #6 Al wire as your branch circuit wire.
Electric vehicle charging has no use for the neutral wire, so there is no earthly reason to install a NEMA 14-50 instead of a NEMA 6-50. Unless you also plan to plug in a larger RV at some point. Some EVSE manufacturers advise a 14-50, but they are just accustomed to rich customers who already have the 14-50 for their RV. Remember, in conduit, you can always add the neutral wire later. When prices have gone back to normal!
When organizing breakers, note how the panel is phased, and try to distribute your 120V breakers so the load will be reasonably balanced when load is heaviest. For instance I see 3 breakers on phase L1 but only one on phase L2 (middle, right column).
Also, if you ever get a second EV, don't try to brute-force it with a huge subpanel. Use "Share2" power sharing (built into all Tesla Wall Connector v2 and v3; available on certain non-Tesla EVSEs) which shares an allocation between two vehicles. For instance two 60A EVSE's can share an 80A allocation, running 60A when alone (or when the other car is taking only 20A).
By the way, here's the practical copy of 310.15(B)(16). It's a bit of a mess.