Wire size for wall charger

[bbwex]

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????

 

[iluvmacs]

Yes, but since you said you are not an expert, you should hire someone who is an expert to do this job.

 

[mspohr]

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.

 

[haggis444]

Yes, 8 gauge NM-B is rated for MAX 40A.

 

[SSedan]

It is critical that when spending $50k on a car you make sure to spend bare minimum on charging infrastructure, the $50 "saved" by making sure you go as small as possible on wire is going to be the $50 that makes.you financially sound later.

 

[bbwex]

Though I only have 40 amps of service, I want to go as safe as possible, so it looks to me as if I should use 6 gauge wire. Is that the safest route to take??

 

[jeffbco]

Though I only have 40 amps of service, I want to go as safe as possible, so it looks to me as if I should use 6 gauge wire. Is that the safest route to take??

Probably...but why wouldn't you ask your electrician?

 

[MY-Y]

Keep in mind that an EV connection on a 40 A circuit will be limited to 32 amps for charging. (80%)

 

[jcanoe]

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.

 

[bbwex]

jcanoe - thanks for the answer.

 

[drtimhill]

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]

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]

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.

 

[Rocky_H]

Thumbs up to @jcanoe. Yes, the 8 gauge Romex is fine for a 40A circuit. If it's getting into pretty long distance, like having to go from the other side of your house, and it's 80 or 100+ feet, I would probably go up to 6 gauge, though.

 

[CleverUsername]

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]

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.

 

[CleverUsername]

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.

Fair point. Here in Chicago we're not allowed to use Romex, so my brain defaulted to THHN in conduit.

 

[2101Guy]

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

 

[Rocky_H]

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]

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.