Is #8 THHN with Tesla Wall Charger Up To Code? (On 50A Breaker)

[flai54]

Thanks for all the helpful responses…glad to know I’m up to code at a 50A breaker (which charges at 40A).

Two more questions…

1. Is the #8 THHN good up to 50A or 55A? I’m honestly just curious and want to educate myself better.

2. If using #6 NM-B wire to hardwire to a Tesla Wall Charger, I know the white/neutral is capped off by itself and not used…red and black wires go to the hots on the charger. But is the single copper/ground wire enough (from a #6 NM-B) to be used as the ground? I notice that ground wire is a lot skinnier than the other wires, even within the #6 NM-B.

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Is that a plastic box you are using instead of a metal box? I used a metal box with a grounding screw in the middle for extra safety. Not sure why you just didn't go with the 6 AWG for extra buffer if you are going to install fresh new wire. Most circuit breakers up to 60A can accommodate up to 6 AWG wire.

Also THHN at 55A is only good if your outlet and breaker are rated for 90C which most are not. Safer to assume it is rated on both ends at 75C which puts it at 50A max.

 

[Cosmacelf]

Is that a plastic box you are using instead of a metal box? I used a metal box with a grounding screw in the middle for extra safety. Not sure why you just didn't go with the 6 AWG for extra buffer if you are going to install fresh new wire. Most circuit breakers up to 60A can accommodate up to 6 AWG wire.

Also THHN at 55A is only good if your outlet and breaker are rated for 90C which most are not. Safer to assume it is rated on both ends at 75C which puts it at 50A max.

PLEASE people, read the thread from the beginning before posting, it’s only a page long.

The OP already has 8 gauge THHN wire, so he’s trying to economize and/or reuse. Nothing wrong with that.

He’s only going to use a 50A breaker, so 8 gauge works fine.

And yes, you’d use the 75 degree column for residential installs (60 degree for NM-B cable). And nothing wrong with a plastic junction box.

 

[Rocky_H]

1. Is the #8 THHN good up to 50A or 55A? I’m honestly just curious and want to educate myself better.

Oh, right. We keep throwing out numbers, but may have not been providing where we get them from. I think almost everyone uses that ampacity table from Cerrowire that @ATPMSD linked to:

Cerrowire Resources - Ampacity Charts

Ampacity is the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Cerrowire's ampacity chart helps calculate the load requirement for a circuit.

www.cerrowire.com

The simple answers of which columns to use to look things up are that NM-B has to use that 60 degrees C value. And the THHN wire in conduit uses the 75 degrees C column.

2. If using #6 NM-B wire to hardwire to a Tesla Wall Charger, I know the white/neutral is capped off by itself and not used…red and black wires go to the hots on the charger. But is the single copper/ground wire enough (from a #6 NM-B) to be used as the ground? I notice that ground wire is a lot skinnier than the other wires, even within the #6 NM-B.

Yes, it's correct. The ground wires are almost always thinner than the ones designated "current carrying conductors". There is a table somewhere in NEC that gives the stepdown points where with this thickness of main wires, here's how small the ground wire can be. So yes, it's built into that cable on purpose that it is the right size ground wire for the #6 main wires.

 

[ATPMSD]

And the THHN wire in conduit uses the 75 degrees C column.

THHN is in the 90 degrees C column, isn't it?

 

[Rocky_H]

THHN is in the 90 degrees C column, isn't it?

I didn't want to get off into the weeds on that. People should just use the 75 degrees column, because residential breakers are never rated for use any higher than 75 degrees C conditions. So no one will get to use the 90 degrees C column by default. So it already has a limit of 75 from that and is what people should use.

That listing though is because if people want to do the full calculation of possible temperature derating for use in very hot conditions, you do start from the 90 degree number and work your way down. And then you have to use the lower of: either the calculated derated value or the default 75 degree value. You get to do the same for NM-B too, where you calculate down from the 90 degree value but usually end up with the default 60 degree one too.