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Wire size for wall charger

bbwex

Member
Oct 25, 2020
52
22
Pittsburgh
I am going to connect a wall charger -- because I have one -- and it is going to feed off of a 40A circuit. Since it is only a 40A circuit, it seems to me (certainly not an expert) that 8 gauge Romex should handle the load without a problem. Anyone have any thoughts on this????
 

mspohr

Well-Known Member
Jul 27, 2014
10,420
13,232
California
I am going to connect a wall charger -- because I have one -- and it is going to feed off of a 40A circuit. Since it is only a 40A circuit, it seems to me (certainly not an expert) that 8 gauge Romex should handle the load without a problem. Anyone have any thoughts on this????
8 gauge is minimum. Use 6 gauge for less heating and better efficiency.
 

jcanoe

Active Member
Oct 2, 2020
4,378
4,563
Maryland
The circuit wiring has to be rated for the maximum load of the circuit. This does not change because the Wall Connector will only charge at 32A on the 40A circuit. For a 240V / 40A circuit 8 gauge is the minimum. If you use a thicker wire, i.e. 6 gauge you could later upgrade the circuit to 50A but otherwise 6 gauge wire would not be required.
 

drtimhill

Active Member
Apr 25, 2019
2,389
3,094
Seattle
I am going to connect a wall charger -- because I have one -- and it is going to feed off of a 40A circuit. Since it is only a 40A circuit, it seems to me (certainly not an expert) that 8 gauge Romex should handle the load without a problem. Anyone have any thoughts on this????
If you need to ask you need someone else to do it. Not being insulting but power at this level can kill or burn your house down . Get an expert .. really.
 

Prof

Member
Jan 6, 2020
51
42
Florida
The Tesla gen 3 wall charger had a max charge rate of 48 amps. That requires a 60 amp circuit as 80 percent of 60 is 48. 6 gauge Romex is only good for 55 amps so you would have to use a 50 amp breaker limiting you to 40 amps. 6 gauge thnn wire in conduit is good for 60 amps. This example shows that Romex and write in conduit can have different capacities even with the same gauge wire. If it were me, I would use the 6 gauge in conduit so I could upgrade later if needed.
 

MY-Y

Active Member
Mar 4, 2020
1,241
1,411
MD
For both my HPWC and a spare 14-50, I used 4-3 NM-B (it does exist). This allows the full 60 amp circuit for the HPWC, and for future upgradability if I replace the 14-50 with another HPWC.

In hindsight, I would have used 4-2 NM-B and used a 6-50 for the outlet. I went with a 14-50 because I have an RV, but I have a different 14-50 for that. The 4-3 was very hard to pull with small holes in the drywall, especially through holes in the studs.
 
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The circuit wiring has to be rated for the maximum load of the circuit. This does not change because the Wall Connector will only charge at 32A on the 40A circuit. For a 240V / 40A circuit 8 gauge is the minimum. If you use a thicker wire, i.e. 6 gauge you could later upgrade the circuit to 50A but otherwise 6 gauge wire would not be required.
This is the way. The cost difference between 8AWG and 6AWG isn't very big, and if you do decide to upgrade to 48A charging down the road, you won't have to throw out a bunch of expensive copper wires because they're now too thin, and the upgrade will be as easy as swapping out the 40A breaker for a 60A one and calling it a day.
 

iluvmacs

Member
Jan 27, 2014
519
899
Madison, WI
This is the way. The cost difference between 8AWG and 6AWG isn't very big, and if you do decide to upgrade to 48A charging down the road, you won't have to throw out a bunch of expensive copper wires because they're now too thin, and the upgrade will be as easy as swapping out the 40A breaker for a 60A one and calling it a day.
Mostly true. 6AWG Romex can't do a 60A circuit, but 6AWG THHN wire in conduit can. So there's more to this story than meets the eye.
 

2101Guy

Active Member
Jan 6, 2020
2,516
3,929
USA
jcanoe provided very helpful answers. To add to that, in case you also needed documented info from Tesla:

https://www.tesla.com/sites/default/files/pdfs/installation-guides/nema-14-50-installation-guide.pdf

Circuit Breaker: 50 amps
 Voltage: Single phase, 208-250 volt AC supply, 60 hertz
 Four Wire Configuration: Line 1 - Line 2 - Ground - Neutral
 Conductors: 6 AWG copper wire for circuits up to 150 feet
 Outlet: Use a high quality, industrial grade outlet
 Ground Pin Orientation: Top position of outlet
 Ventilation: Not required
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,309
9,003
Boise, ID
jcanoe provided very helpful answers. To add to that, in case you also needed documented info from Tesla
Huh, I thought that was going to be the old document from back when it was the Gen1 UMC, but I see that it is updated with the Gen2 UMC.

Back when it was the Gen1, it had to be a 50A circuit, because that device could draw 40A. And I really liked the call-out in the install to use 6 gauge for that instead of using 8 gauge wire in conduit, which was just barely passable for a 50A circuit. And I guess you could make the same case where 8 gauge Romex is just barely rated for a 40A circuit, so 6 gauge is still a good idea.
 

iluvmacs

Member
Jan 27, 2014
519
899
Madison, WI
I really liked the call-out in the install to use 6 gauge for that instead of using 8 gauge wire in conduit, which was just barely passable for a 50A circuit. And I guess you could make the same case where 8 gauge Romex is just barely rated for a 40A circuit, so 6 gauge is still a good idea.
I've always assumed this is just to reduce the complexity of what they have to write as a requirement and to reduce the chance of misunderstanding/misapplication. In the end, it's no more or less "passable" than using #14 for 15A circuits and #12 for 20A circuits, which is how 99.9% of 15 & 20A circuits are installed... but yes, I'd probably always use #6 for 50A, even THHN.
 

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