Tesla certified installers?

[M3P Heavy]

I requested a quote from multiple Tesla certified electricians over the past week. My panel is located on the back right corner of my ranch house inside a finished basement with the garage in the front right corner.

The simplest routing is to exit at the rear, go up to the attic, run across the entire house and drop it down into the garage. I'm looking at roughly 150'.

All three electricians that have come out have provided quotes that say 60 amp breaker and either 6/2 or 6/3 Romex. I asked the two installers I thought about using, "Is the conductor correctly sized? I thought Romex is only rated to carry a max of 55 amps. And since this is a 48 amp continuous load it has to be able to carry 125% of the expected load for a total of 60 amps."

One told me "I'm thinking backwards, that since there is no 55 amp breaker you can round up", which doesn't actually make any sense in this situation to me.

The other said, "You have to use 310.16 in NEC Code book, # 6 copper at 75 degrees. 65 Amps"

Am I wrong in thinking any 6 gauge Romex is not going to work here per code since you can only use the 60C 55 amp rating. 6 thhn/thwn would work but none of them are taking about running conduit other that the 15 or so feet up the outside back corner of the house. Also, I thought Romex couldn't be used outside? And lastly, it's a long-ish run in an attic, should some temperature derating be planned in? In my mind 4 thhn/thwn or something like 4/3 mc are my logical options.

After going back and forth with multiple electricians on this, maybe simplest answer is I'm WRONG on this? But if I'm not wrong what kind of training/requirements make someone Tesla certified? I've spent way too much time reading/researching this over the past few days and I'm wondering if I should even bother discussing it any further with them. Should I just do it myself???

And thoughts, comments, insights on this are greatly appreciated!

 

[Locash]

I don't know the exact answer because I only play an electrician for myself, but I am under the assumption that you are correct here. Just tell them to quote the job with #4 THHN, worst case you have a larger safety factor and a lighter wallet. If they don't want to run #4, tell them to pound sand.

 

[Sophias_dad]

You are not wrong. I'm also not an electrician by trade.

Type NM-B (nonmetallic-sheathed cable) may be used for both exposed and concealed work in normally dry locations at temperatures not to exceed 90 degree C (with-Ampacity limited to that for 60 degree C conductors) as specified in the NEC 1.

That's taken directly from a southwire spec sheet at homedepot.com.

Regarding derating of NMB, 334.80 Ampacity apparently says that for the purposes of derating NM cable should be treated as 90C but never allowed to exceed the 60C rating.

If you switch to conductors in conduit, you definitely DO have to consider derating the conductor for the attic temperatures. At ~130F, 310.15(B)(2)b suggests reducing by a factor of 0.76 for THWN(75C rated) or 0.84 for (THWN-2, THHN, XHHW-2, all 90C rated). That means that #6 THWN(65*.76=49.4) would have a lower usable rating than #6 NMB(75*.84=64, but because its not allowed to be greater than the 60C rating, its 55 anyway).

So you are okay WRT NMB in the attic.

Even if the round up rule could be used, the #6 NMB isn't allowed/rated to carry 60 amps, which is what you'll be asking it to do if you install the 48 amp wall charger.

Per a handy voltage drop calculator even if you could use #6 for this run at 48 amps you'd be near the limit of range, with a 2.65% voltage drop. That right there would get me to next-size-up this thing, even if #6 NMB were allowed to run at 60 amps.

There's no 4-2 NMB made for some reason, so jumping to 4-3 would be stupidly expensive.

And yes, the interweb tells me that NM is not allowed to be outside, even in conduit.

If it were me, I'd go 2x#4 in conduit, with a #8 bare conductor for the ground, even though #6 and a #10 ground would technically work.

Tesla certified probably just means they pay $1k a year(or whatever) for the label. Think of it as the blue checkmark for Twitter.

One final question that might change some of these answers... do you really NEED 48 amp charging? Most people don't.

 

[ucmndd]

Agree with the above. Tesla “certification” means nothing in terms of capability or quality. Your interpretation of the rules is correct. You can’t round up breaker size when the calculated load of the circuit exceeds the ampacity rating of the conductors. NM-B is a no-go outside.

I’m less worried about voltage drop and upsizing conductors, #6 will be just fine and keep the cost of the project more reasonable.

This sounds like a perfect job for 6/2 MC (metal-clad) cable.

 

[ATPMSD]

Amazing how many so called electricians do not understand the round-up rule! Since there is no 55-amp breaker you can use a 60-amp breaker with wire rated at 55-amps IF the device on the other end will never exceed 55-amps. If you set the wall connector to a 60-amp circuit you violate this rule (even though it should not draw more than 48-amps). If you set the connector to a 50-amp circuit you can use a 60-amp breaker, but why bother as you would be better off with a 50-amp breaker.

And of course a 55-amp wire is not suitable for the wall connector when set to 60-amps anyway.

 

[Gauss Guzzler]

First, you should decide how many conductors you want. 2 for a charger, vs 3 for a subpanel or outlet. Most household circuits need 3 conductors so electricians tend to default to 3, but EV chargers only need 2 so it's up to you to specify that to the electrician.

6AWG in conduit is generally fine for a long run in a hot attic at the full 60A. @Sophias_dad mentioned THWN wire, which is usually used when conduit is run thru mud, but even still, THWN has largely been replaced by the 90C rated "THWN-2" so you still get the full 60A.

Voltage drop can be a nuisance for poorly designed hyper-sensitive junky electronics from the 70's. It's generally not an issue today and absolutely not an issue for EV chargers which don't care what the voltage is, so long as it's not decreasing in an overheating-type pattern. And it's negligible anyway. For example, if you have exactly 240V at the panel and pull 48A over 100ft, your car will get 233.6V with #6 vs. 235.9V with #4.

Lastly and most importantly, just because the charger *can* run at 48A does not mean that you should go to unnecessary lengths to do so. 40A is plenty fast enough for nearly any use case and it allows you to run the much cheaper 6/2 Romex on a 50A breaker. That'll get you 9kW which is about 11% per hour, so a typical daily charge would only take an hour or two. Or maybe ~4 hours twice a week. It's no big deal.

 

[Sophias_dad]

Another idea that might be welcome(or not) would be to use aluminum wire to get a ~90 amp subpanel in the garage. with #2 aluminum(0.88/ft) times three conductors and an appropriate ground, if you have the appropriate main panel capacity.

I tend to shy away from aluminum in general, but when you are dealing with a 150' run the economics may become meaningful.

 

[davewill]

So many electricians and inspectors get this wrong, I figure that the majority of the 60a wall connector installs in the U.S. must be using #6 Romex!

 

[ucmndd]

So many electricians and inspectors get this wrong, I figure that the majority of the 60a wall connector installs in the U.S. must be using #6 Romex!

A good testament to how over-engineered the NEC is, as it’s not like these things are burning houses down with any regularity.

 

[Rocky_H]

@M3P Heavy Yeah, you are right in all counts. Romex can only be used on that 60 degrees C column, and for 6 gauge, that is a 55A rating limit. And it's been covered on this forum a lot about the round-up rule. It exists, but people misuse it where it's not applicable all the freaking time! You could round up the breaker, but the cable still limits it to being a 55A circuit. It DOESN'T become a 60A circuit. So you still can't configure it to run at 48A continuous.

So if you really want a 60A rated circuit, that can't use 6 gauge Romex. Or you could go with the 6 gauge Romex, but you need it to be a 50A circuit.

 

[M3P Heavy]

Agree with the above. Tesla “certification” means nothing in terms of capability or quality. Your interpretation of the rules is correct. You can’t round up breaker size when the calculated load of the circuit exceeds the ampacity rating of the conductors. NM-B is a no-go outside.

I’m less worried about voltage drop and upsizing conductors, #6 will be just fine and keep the cost of the project more reasonable.

This sounds like a perfect job for 6/2 MC (metal-clad) cable.

So this is what i've decided to go with and am going to do it myself. 6/2 MC from the panel to the garage will meet the code requirements, is significantly cheaper than 4/3, and a lot less work than running 4/2 in conduit. And if it's really hot outside and temp derating becomes a worry can just derate it during those months of the year.

Cheapest quotes for 6/2 and 6/3 romex was ~2500, 4/3 romex was 3500-4000. Buying the 6/2 mc myself plus tools, fittings, and all the misc project items should have me ~800 all in.

 

[Sophias_dad]

So this is what i've decided to go with and am going to do it myself. 6/2 MC from the panel to the garage will meet the code requirements, is significantly cheaper than 4/3, and a lot less work than running 4/2 in conduit. And if it's really hot outside and temp derating becomes a worry can just derate it during those months of the year.

Cheapest quotes for 6/2 and 6/3 romex was ~2500, 4/3 romex was 3500-4000. Buying the 6/2 mc myself plus tools, fittings, and all the misc project items should have me ~800 all in.

Are you planning to run the MC entirely indoors? or getting jacketed MC cable?

 

[M3P Heavy]

Are you planning to run the MC entirely indoors? or getting jacketed MC cable?

When I decided to do it myself I was able to find a way to fish it inside and up to the attic, so all of it will be indoors now

 

[Sophias_dad]

When I decided to do it myself I was able to find a way to fish it inside and up to the attic, so all of it will be indoors now

At least when I visit wireandcableyourway.com , the conductors in their MC are THHN which means you shouldn't need to reduce current even in the height of summer(well, 130F at least). I wonder if you could work a deal with them for ~160' at the reduced not-by-the-foot price.

 

[M3P Heavy]

Car is here and the charger is installed. Ended up with the 60 amp breaker, 6/2 MC, about 150' in total length, commissioned for 48amp charging. My panel seems to float between 237-239 at no load but mostly at 239. At no load i'm seeing 239 at the charger, but at full load it drops as low as 229 but mostly sits at 232. The mains voltage sags 1-2 volts under full load. Everything is good but a little more voltage drop than I was hoping for. I'll keep track of it but figured I'd report my findings so anyone doing the same thing down the road an use this to inform their decision. 4 probably would have really been the best choice but for a DIY, working with the 6/2 mc was enough of a headache that I'm glad I wasn't wrestling 4awg up into the attic and through the walls.

 

[davewill]

Sounds perfectly normal to me. 150' is a decent length run.