2

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.

Main panel

enter image description here

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?

plan map

2
  • Where are you in the world? Individual wires always need conduit but some areas do not allowed things like Romex , well it has to be in conduit so why waste the $ for a sheath that makes it harder to pull. What is the requirement for the charger actual load x 1.25 is code , you can have a 50 amp breaker in the 50 amp panel if the actual load is 80%.
    – Ed Beal
    Mar 4, 2022 at 18:43
  • Let us continue this discussion in chat.
    – Ecnerwal
    Mar 4, 2022 at 19:22

2 Answers 2

7

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?

enter image description here

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.

enter image description here

6
  • the EV parks where you would be using the saw and dust collector I was hoping to get an outdoor charger. The plug would be on the inside but the cord could maybe pass through the wall to the outside when in use ( maybe a little access panel door thing?). I've not parked a car in this garage before (woodshop, office, gym takes up all the space). But regardless of location these things are not in use when charging because we're transitioning to time-of-day metering and I'd like to charge at night.
    – Brad
    Mar 4, 2022 at 21:06
  • Re: you're first part, so I'm clear. You're saying there's no need to upgrade the wiring from the main panel to the sub panel at all? Leave it as is and just install a 2 wire (plus ground, maybe grounded via metal conduit) 6-50 outlet. Skip the 14-50 (there is no RV in the picture)
    – Brad
    Mar 4, 2022 at 21:10
  • Or is the through the wall idea not kosher?
    – Brad
    Mar 4, 2022 at 21:23
  • 2
    @Brad Yes, your existing wiring is 50A, and was under-breakered. Normally that's an amateur flub but everything else in the panel is flawless. Yeah, can't run a flexible cord through a wall, just put a J-box outside and hardwire the EVSE to it (that will reduce theft risk, and side-step GFCI requirements for receptacles whose permit is pulled after NEC 2020 lands in your state). Mar 4, 2022 at 22:17
  • 1
    @J... okay............. Mar 6, 2022 at 1:12
2

Conduit from the garage panel to the charger outlet location can be run along the wall, there's no need to go to the attic or even the top of the wall, unless you have unstated reasons you want the wall clear at lower levels.

Use EMT (and you won't need a grounding wire) or Schedule 80 PVC to manage "protection from damage" and you don't have to waste wire going up and down (unless you do want it "out of the way of the wall.")

Above 6'-6" (or 8' in some jurisdictions) you can use less robust conduit - schedule 40 PVC or ENT ("smurf tube") which can be a reason to choose that higher route.

You have plans to hang things on the wall that the conduit would interfere with is also a reason to do that.

As mentioned in comments (now moved to chat as it became extended), 2-2-2-4 MHF (aluminum) is going to cost less and provide more potential ampacity for your feed wire, even if you may not use all that ampacity now (seems unlikely with a 100A main panel.)

4
  • unless you have unstated reasons There's a door, a windows, a workbench, stuff hanging on the wall...it would have to take a circuitous route to arrive where I want the outlet.
    – Brad
    Mar 4, 2022 at 19:50
  • Thinking on this a bit further, you could probably re-use your 40A breaker (if the panels are the same brand/breaker type) and 8AWG THHN copper to feed the 14-50 (yes, you can do that, there are no 40A receptacles) since you are planning for a 32A charger (which only needs a 40A breaker at 125% derate) and save a few bucks that way.
    – Ecnerwal
    Mar 4, 2022 at 19:55
  • No, GE main and Square-D garage, you can't re-use the breaker. The wire, however, you can.
    – Ecnerwal
    Mar 4, 2022 at 20:02
  • I didn't think about re-using the copper from the house to the garage. That's a good idea.
    – Brad
    Mar 4, 2022 at 21:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.