Opinions about 40A vs 30A house wiring for EVSE installation

[LTLFTcomposite]

If you put a continuous 40a load on a 40a circuit with #8 wire will it overheat the wire and/or trip the breaker or is it just that there is no cushion if the breaker is a little weak?

 

[theds]

If you put a continuous 40a load on a 40a circuit with #8 wire will it overheat the wire and/or trip the breaker or is it just that there is no cushion if the breaker is a little weak?

You can use a 100% rated breaker. And continuous load is defined as a load last for 3 hours or more.

 

[wwhitney]

If you put a continuous 40a load on a 40a circuit with #8 wire will it overheat the wire and/or trip the breaker or is it just that there is no cushion if the breaker is a little weak?

It may trip the breaker, or it may not. The breaker is not required to hold, but it is not required to trip. Depends on the ambient temperature at the breaker, heating from other breakers, etc. The #8 wire will be fine, it is rated for 40a continuous.

Cheers, Wayne

 

[LTLFTcomposite]

So if the Tesla draws 40a and you have a 40a circuit instead of the preferred 50a circuit the worst you are looking at is nuisance trips?

 

[mitch672]

EV charging is considered a "continuous load" (more than 3 hours).
You are only supposed to draw %80 of the circuit breakers ampacity to avoid overheating the breaker and thermal tripping. 40A breaker, do not draw more than 32Amps. A 50A breaker, do not draw more than 40Amps. That is from N.E.C. Code book rules.. 8AWG will get hot, even though 90 degree THHN is rated for 40A, it depends on the wiring method... NM-B is only rated to 75C, versus strands of THHN in a conduit/pipe.

 

[pchilds]

So if the Tesla draws 40a and you have a 40a circuit instead of the preferred 50a circuit the worst you are looking at is nuisance trips?

You are good as long as you don't charge for more than 3 hours. I do it all the time, haven't popped a breaker yet.

 

[GlennD]

So if the Tesla draws 40a and you have a 40a circuit instead of the preferred 50a circuit the worst you are looking at is nuisance trips?

You are good as long as you don't charge for more than 3 hours. I do it all the time, haven't popped a breaker yet.

Not at my house. My EVSE is set to 32A on a 40A breaker. When you charged here it was less than 40A. I have since set it from 30A to 32A but that's it! Since I have the infamous Federal Pacific Electric panel I am not going to push it.

 

[wwhitney]

So if the Tesla draws 40a and you have a 40a circuit instead of the preferred 50a circuit the worst you are looking at is nuisance trips?

I'd say that the worst you are looking at is starting a fire, because the circuit has a marginal connection that works OK at 32A continuous but overheats at 40A continuous. If the circuit is installed properly, the worst you'd be looking at is nuisance trips. However, I wouldn't be comfortable doing it on an unknown circuit.

Cheers, Wayne

 

[nsayer]

I've never heard of the continuous duty derating being based on the length of time the load is engaged. I would have assumed that any load with a very high duty cycle would require a derating. I believe I've been told or read that the NEC considers EV charging to be continuous duty regardless of how long you do it.

The wiring in my garage for the EVSE (8 gauge, 40A breaker - I set my EVSE to 32A max) gets noticeably warm when the EV is charging. Not warm enough to be a concern, but I certainly would not consider anything that would make it get any warmer.

 

[QueenBee]

I've never heard of the continuous duty derating being based on the length of time the load is engaged. I would have assumed that any load with a very high duty cycle would require a derating. I believe I've been told or read that the NEC considers EV charging to be continuous duty regardless of how long you do it.

Check out NEC a continous load is one that runs for three hours or longer or is specifically called out such as EVSEs. I'd quote it properly but it's a pain on a phone

 

[LTLFTcomposite]

When I installed my charger a couple years ago I went to HD for 8/2 and they were out of it, but they had a hunk of 6/3 off the end of a roll that was just the length I needed they gave me for cheap. Looks like that might have been a fortunate turn of events.

Still wondering if that neutral can be put to any use though.

 

[mitch672]

Still wondering if that neutral can be put to any use though.

Sure can. If you wire up a 14-50 (which is the standard connector Tesla supplies with the Model S UMC), you will be "code compliant" for the 14-50 outlet, to pass inspection... Even though Tesla doesn't use the neutral, an RV could plug into it, and that does need a neutral.

 

[zac1992]

Hello all! New Leaf owner here :mrgreen: and I've been enjoying my ride for 5 whole days! After much research (thanks to this forum), I've decided to upgrade the stock EVSE using EVSEUpgrade instead of purchasing a dedicated unit. I'll need to install the correct NEMA L6-30R receptacle in my garage but I'm wondering what you all think about what size breaker and what gauge wire to use. I realize I only need a 30A circuit for the EVSEupgrade, but installing a larger capacity best Modular Electrical Systemwould provide a more future proof installation should I decide to get a larger capacity EVSE in the future. There is also the question of 4 vs 3 conductor wire in case I need a neutral at some point. Thoughts?

Hello
May I know, where should I use these circuits? And which wire be used in this?