NEMA 14-50 cable / conduit size question

[Dmagyar]

So if I were to run 600V 2/3 NM-B through an insulated wall, would that be able to support 80A charging on a 100A breaker?

Check with your AHJ (Authority having jurisdiction) (city/county) electrical inspector and your electrical professional just to make sure. Those are copper conductors in the nm cable?

 

[gavine]

Check with your AHJ (Authority having jurisdiction) (city/county) electrical inspector and your electrical professional just to make sure. Those are copper conductors in the nm cable?

Yes, copper.

 

[eprosenx]

So if I were to run 600V 2/3 NM-B through an insulated wall, would that be able to support 80A charging on a 100A breaker?

No.

#2 AWG copper NM cable is only allowed to 95 amps because NM cable is limited to the 60c rating.

#3 AWG copper NM cable is only good for 85 amps.

If in conduit you can use the 75c rating so #3 is good to 100a and #2 is good to 115a.

You only need two current carrying conductors for a wall connector and one ground.

But do note that even if you use a 90a circuit that will max out any single Tesla vehicle sold today (72a max charge rate). Where the full 80a is nice is for charging more that one vehicle (shared) or charging old dial charger model S cars that can take the full 80a.

 

[gavine]

No.

#2 AWG copper NM cable is only allowed to 95 amps because NM cable is limited to the 60c rating.

#3 AWG copper NM cable is only good for 85 amps.

If in conduit you can use the 75c rating so #3 is good to 100a and #2 is good to 115a.

You only need two current carrying conductors for a wall connector and one ground.

But do note that even if you use a 90a circuit that will max out any single Tesla vehicle sold today (72a max charge rate). Where the full 80a is nice is for charging more that one vehicle (shared) or charging old dial charger model S cars that can take the full 80a.

The ultimate plan is to run #2 AWG NM from my main panel in the basement to a 100A-capable sub-panel in the garage and have two 50A breakers with NEMA 14-50 outlets off of the sub-panel (#6 wire for those). What breaker should I use to feed the sub-panel via the #2 copper wire? Sounds like I can't use 100A but would need to use 90 or 80A. Correct?

If I were to use 100A breaker, what wire would I need instead of #2?

I appreciate the help.

 

[tga]

Professional" very very often means "gets paid to do" which has relatively little to do with how knowledgeable, thoughtful and quality minded someone is.

"Just because you're professional, doesn't mean you're competent." I once repaired a ceiling-mounted garage door opener outlet that had been wired with the ground hot, making the entire garage door opener frame and mounting straps live. I discovered it when trying to hang a grounded trouble light from it. The spark show was impressive.

So that is why you see much smaller gauge power company wire spliced to larger gauge wire at the weatherhead.

The utility feed for my old 100A service was 4ga Al, connected to 2ga Al SEU at the weatherhead. After the 200A upgrade, the utility feed is 2ga tied to 4/0 AL SEU.

The ultimate plan is to run #2 AWG NM from my main panel in the basement to a 100A-capable sub-panel in the garage and have two 50A breakers with NEMA 14-50 outlets off of the sub-panel (#6 wire for those). What breaker should I use to feed the sub-panel via the #2 copper wire? Sounds like I can't use 100A but would need to use 90 or 80A. Correct?

If I were to use 100A breaker, what wire would I need instead of #2?

I appreciate the help.

NEC allows you to round up to the next standard breaker (100A) in this case, but that doesn't allow you to round up the wire's rating (that's still 95A). Or you can use a 90A breaker and toss the extra 5A of capacity.

Realistically, though, any (competent) pro would install 1AWG Al SER cable for a 100A subpanel feed (possibly 1/0 if derating to 60C for insulation). Most subpanels I've seen are wired with Al; it's perfectly safe in this application.

 

[gavine]

NEC allows you to round up to the next standard breaker (100A) in this case, but that doesn't allow you to round up the wire's rating (that's still 95A). Or you can use a 90A breaker and toss the extra 5A of capacity.

Realistically, though, any (competent) pro would install 1AWG Al SER cable for a 100A subpanel feed (possibly 1/0 if derating to 60C for insulation). Most subpanels I've seen are wired with Al; it's perfectly safe in this application.

Thanks. I’ll use a 90A breaker then and make sure I limit total simultaneous charging to 72A, realizing that, if I accidentally exceed the 72A, the 2 AWG wire will be protected by the 90A breaker.

It would be a VERY rare situation where I would need both cars to charge quickly so 72A shared is plenty.

 

[eevv]

For all you folks with electrician background out there, I have a question regarding the derating code. I recently had a NEMA14-50 outlet installed for charging my standard range model 3. I requested 6 gauge copper wire, outlet was only a couple of feet from the panel and everything seemed to have gone smoothly. I didn't realize until later that the electrician only installed a 40A breaker instead of 50A. When I asked him, he stated the 6/3 NM-B wire he used, which has a rating of 55A at 60C, must be 125% over the breaker size (hence 40A), which then must be 125% of the charging load of the car (32A). If a 50A breaker is to be installed, a #6 HTTN wire would be needed.

According to NEC 210.19:
"The minimum branch circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."

And the overcurrent protection code 625.40 also calls for a breaker that is no less than 125% of the maximum load of the EV car.

Both of these codes provide guidelines of the comparative amperage between the circuit/breaker and the charging load of the car. I don't see it mentioned, and I haven't come across it in my research, that the circuit amperage must be higher than the breaker. Am I missing something?

A 40A breaker satisfies my needs as my model 3 is charging at its max. But I read on various forums that a 50A breaker is usually what's installed with the #6 wiring and I want to make sure the job was done correctly and keep my options open for future upgrade. Thanks!

 

[eprosenx]

For all you folks with electrician background out there, I have a question regarding the derating code. I recently had a NEMA14-50 outlet installed for charging my standard range model 3. I requested 6 gauge copper wire, outlet was only a couple of feet from the panel and everything seemed to have gone smoothly. I didn't realize until later that the electrician only installed a 40A breaker instead of 50A. When I asked him, he stated the 6/3 NM-B wire he used, which has a rating of 55A at 60C, must be 125% over the breaker size (hence 40A), which then must be 125% of the charging load of the car (32A). If a 50A breaker is to be installed, a #6 HTTN wire would be needed.

According to NEC 210.19:
"The minimum branch circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."

And the overcurrent protection code 625.40 also calls for a breaker that is no less than 125% of the maximum load of the EV car.

Both of these codes provide guidelines of the comparative amperage between the circuit/breaker and the charging load of the car. I don't see it mentioned, and I haven't come across it in my research, that the circuit amperage must be higher than the breaker. Am I missing something?

A 40A breaker satisfies my needs as my model 3 is charging at its max. But I read on various forums that a 50A breaker is usually what's installed with the #6 wiring and I want to make sure the job was done correctly and keep my options open for future upgrade. Thanks!

Your electrician has a fundamentally wrong belief. That is pretty bad (though it least this is not a mistake that will burn your house down).

He is correct about using the 60c rating which is 55a. He is also correct that for continuous loads like an EV you have to calculate them as if they were 125%.

But the 125% requirement for the conductors and the breaker are separate, not compounding.

So the best thing to have done here would have been to use a 50a breaker. 40a would have made sense if it was 8 gauge wire.

So what you have is fully safe and code compliant for your 32a UMC, but it is lame since if you later wanted to connect a 40a EVSE you would need to upgrade the breaker to a 50a.

 

[Dmagyar]

NEC 334.80 further interprets that the 60C column (NEC 310.15b) should be used for NM cable except when the 90C column can be used for derating, where the final allowed ampacity does not exceed the 60C column for the respected wire size...

Second the no compounding of derating...