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Before I go through the trouble of finding an electrician to come to my apartment to give me an estimate, and to tell me whether he can do it or not, I wanted to ask this question here to find out if this was even possible.

I have a 125 amp panel in my apartment. There is 6 gauge wire running from my panel to my building's main panel. I was interested in having a new 40 amp circuit installed so that I can charge my electric car (my window is on ground level and 10 feet away from where I park my car). I currently charge my car using a 120v 20amp circuit and it is incredibly slow as I expected it to be. I don't know why, but the car's charger shows it drawing 12 amps when I use this circuit.

My car is able to accept 32 amps maximum. I originally wanted to have a new 40 amp circuit installed with a NEMA 14-50 outlet. Can it be configured this way? I couldn't find any NEMA 14-50 outlets rated at 40 amps, only at 50 amps. So if I have to, would I be able to install a 50 amp breaker in my panel, run 6 gauge wire from the breaker to the NEMA 14-50 outlet? Would anything overheat? Would my panel be able to accommodate this new circuit? I would only charge my car at night (once a week) while I sleep and everything in my apartment would be turned off except for the fridge, during charging nights.

Another question is this: What gauge wire should I use? I know 6 gauge wire is rated to 55 amps and the breaker will be 50 amps but since the car accepts only 32 amps max, can I get away with using 8 gauge wire from the breaker to the outlet or do I have to use 6 gauge wire?

Lastly, as you can see in the photos of my panel, there is only one bus bar. So where would the neutral wire go and where would the ground wire go? Can they both be connected to the same bus bar? The sticker on the bottom of the panel says "For this system, neutral is not used".

I don't want to pay an electrician to come out here and start poking around just to tell me that it cannot be done, so that's why I'm asking here first. Thank in advance.

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    I realized that and just added the pics. Thanks!
    – Vlad
    Aug 1, 2021 at 1:00
  • How many square feet is your apartment, and how many of those 20A breakers serve kitchen receptacles? Aug 1, 2021 at 1:11
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    It's possible that your car would draw at 16 amps if it was informed that the circuit it's plugged into was a 20A circuit. But the safe default for a continuous (3 hours or more) load on a 120V receptacle is 12A, since that's a safe continuous load on a 15A circuit, and if your car does not know, it's apparently going to make the safe assumption.
    – Ecnerwal
    Aug 1, 2021 at 1:23
  • ThreePhaseEel - My apt is about 900 sq ft and I'm not sure how many of the 20 amp breakers serve kitchen appliances. Does it help at all that all of the charging will only be done at night when only the fridge is on?
    – Vlad
    Aug 1, 2021 at 1:27
  • Ecnerwal - Gotcha. Thanks for that info! I did now know that.
    – Vlad
    Aug 1, 2021 at 1:28

2 Answers 2

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DO NOT buy the wire.

THHN/conduit allows higher ampacity

Another question is this: What gauge wire should I use? I know 6 gauge wire is rated to 55 amps and the breaker will be 50 amps but since the car accepts only 32 amps max, can I get away with using 8 gauge wire from the breaker to the outlet or do I have to use 6 gauge wire?

I know why you think that, but it's incorrect since this isn't Romex. You are dealing with THHN wires inside conduit. THHN wires are allowed 75C thermal rating (as long as the terminals on the appliances are, and breaker panel terminals always are). As such, ampacity is higher than you're used to:

  • #6 Cu = 65A
  • #8 Cu = 50A
  • #6 Al = 50A
  • #8 Al = 40A

This means a couple of things. First, your subpanel there actually has 65A (i.e. two poles, each with 65A@120V, and a 240V load will draw equally off both poles). Since 65A breakers are not made, it could be breakered at 70A. You can only plan to use 65A.

Second, #6 for the EVSE is wild overkill. Carefully select your NEMA 14-50 socket. If you select one whose terminals are good for 75C thermal, and preferably one that accepts aluminum wire, that gives you more options.

Lastly, as you can see in the photos of my panel, there is only one bus bar. So where would the neutral wire go and where would the ground wire go?

Nowhere. You don't need either one. Really.

Neutral is not needed for an EVSE. The J1772 connector doesn't have a neutral pin, and there's not much else inside the EVSE box.

The EMT metal conduit carries the safety ground for you.

Although just for reference, the bar in the subpanel is the neutral bar. It is not the ground bar and you cannot use it for ground since this is a subpanel. This panel does not have a ground bar, because all circuits are EMT conduit.

You can't use Romex in this building. You think they did all that conduit for fun?

If you have 1-2 other circuits in the conduit

Note that if you run the #8 down conduits with existing wires, a "thermal derate" must be applied. Here is the derate if there are 2 or 3 total circuits in the conduit. The math isn't consistent because the derate comes from a different source (80% of the 90C thermal rating).

  • #6 Cu = 60A (formerly 65A)
  • #8 Cu = 44A (formerly 50A)
  • #6 Al = 44A (formerly 50A)
  • #8 Al = 36A (formerly 40A)

So #8 Cu winds up being the safe choice here. It fits in the conduit and has enough ampacity for sure.

Misc.

My car is able to accept 32 amps maximum. I originally wanted to have a new 40 amp circuit installed with a NEMA 14-50 outlet. Can it be configured this way? I couldn't find any NEMA 14-50 outlets rated at 40 amps, only at 50 amps.

Right, that's because only a limited number of mutually rejecting socket sizes are possible, so NEMA refuses to define a 40A connector. NEC says for a 40A circuit, use the 50A socket with a 40A breaker. That is fine.

Would my panel be able to accommodate this new circuit? I would only charge my car at night (once a week) while I sleep and everything in my apartment would be turned off except for the fridge, during charging nights.

That actually could be a serious problem. I suggest you answer ALL of ThreePhaseEel's questions if you want an answer to that. Also mention any circuits that serve bathroom, laundry room or garage.

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  • Small point: I don't see a garage in the question - I see a "window about 10 feet from where I park."
    – Ecnerwal
    Aug 1, 2021 at 19:50
  • @Ecnerwal... and OP was throwing a cord out the window. Well drat, that changes things. Aug 1, 2021 at 21:46
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For this part:

Lastly, as you can see in the photos of my panel, there is only one bus bar. So where would the neutral wire go and where would the ground wire go? Can they both be connected to the same bus bar?

You have one, properly isolated, neutral bus bar. Your grounds are the metal conduit that the wiring runs in, except the one green ground that comes in the bottom and connects to the box itself.

They CANNOT be connect to the same bus bar in this panel. If the circuit is (as may be required for your location and occupancy) run in metallic conduit, you would not need a ground wire to your panel.

You need to read the WHOLE section that says "neutral is not used" - it's for an OPTIONAL configuration which is not how your panel is configured.

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  • Thank you for your response. Just to make sure I understood you: I should use 6/3 wire right? The red, white, and black go from the outlet to the panel, in the panel the red and black go to the breaker and the white to the bus bar. At the outlet, the green goes from the outlet to the green screw of the metal box? Why do I see some 6 gauge wires have a bare copper wire?
    – Vlad
    Aug 1, 2021 at 1:25
  • Depending on why your wires are run in conduit, you may not be allowed to use cable (what you call 6/3 wire) at all. Because it's an apartment, and/or where it is, you may be required to use wires (individual, THHN typically) in conduit. And you don't use cable in conduit. Side benefit - you might be able to use #8 copper wire in that case, or #6 aluminum; or you may get some break on sizing from following the instructions of the charger (i.e. 40A should be fine.)
    – Ecnerwal
    Aug 1, 2021 at 1:29
  • So it's possible I might be required to use 6/3 wires that are in a metal conduit? I'm sorry if I'm using the wrong terminology, I'm trying to learn. homedepot.com/p/…
    – Vlad
    Aug 1, 2021 at 1:32
  • Or would I need to use this? the 6/3 version of this? Do I need to use 6/3 or 6/4?amazon.com/Southwire-68582621-Solid-Armorlite-Metal-Clad/dp/…
    – Vlad
    Aug 1, 2021 at 1:34
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    @Vlad Well there's already a conduit route from panel to garage, yes? I would use wire that fits in the existing conduit. #6 wire is wild overkill for a 40A circuit, because wires in conduit are allowed more amps than you think. And also #6 won't fit with existing wires. #8 will suffice. DO NOT BUY THE WIRE! Aug 1, 2021 at 18:32

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