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Wire size for wall charger

kishkaru

Member
Feb 3, 2020
247
271
The Bay, CA
So found this handy calculator, and loaded in some sample values. Is it wrong? Read the fine print, and it references the 60C rating of NM-B cable.


View attachment 666679

*The calculated ampacity value is the minimum of: the corrected and adjusted ampacity; the equipment temperature limitation default values as described in NEC® 110.14(C)(1); or the cable type temperature limitation. The initial input values are also shown for verification, and they will be emailed along with the calculated ampacity if the email option is chosen. The insulation temperature rating is the rated temperature of the insulation surrounding the conductor metal, and the termination temperature rating is based on the default values shown in NEC 110.14(C)(1). If a cable wiring method was chosen, the temperature limit is 60℃ for NM-B and UF-B, and it is 75℃ for SER, SEU and MC cable.
That calculator is horribly broken. Looks like it's hard-coded to use 100% rating ampacity rating until it gets to 132-140F correction factor, where it starts lowering ampacity in the calculated output. Check the JS console for debugging output if you're technical.

NM-B wire is not even allowed to be used at that high of a temp (132-140F), so the calculations doesn't even make any sense. A halfway decent calculator should drop the ampacity to 0A if its out of range for the conductor type (like in the calculator below).
Also check the fine print on the page:
... Ampacities are based on the 2017 NEC and do not reflect any temperature correction or ampacity adjustments that may be required. Please consult a qualified electrician or professional engineer to determine the appropriate values for your specific application. ...

👉Use an actual working calculator👈


---
reference tables for the calculator above.
Namely: default ampacities + temp correction factors = calculator output.
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,297
8,990
Boise, ID
The other thing about those ampacity/wire size calculators is that it seems many of them are set with a default to preserve a very tight % for allowed voltage drop. I don't see in your screen shot what it's using, but I've seen some that were set for about 3% or 4% before it would decide it needed to bump up the wire size. For car charging, lower voltage isn't really important, so it's not necessary to go so aggressive about preventing voltage drop.
 

kishkaru

Member
Feb 3, 2020
247
271
The Bay, CA
For car charging, lower voltage isn't really important, so it's not necessary to go so aggressive about preventing voltage drop.
I'd argue that it is important:

At 60 ft, a hypothetical 5% voltage drop would mean 228V at the load, under load:

240V * 48A = 11.52 kW
228V * 48A = 10.94 kW
= a difference of ~600W!

To put it into perspective, that's 2-3x the power required to keep Sentry Mode on.

That ~600W is dissipated as heat in the wire. That extra heat will be partially trapped in the cable insulation or conduit, further raising the ambient temp.
If your circuit was already running in an area with high ambient temp (e.g. attic, garage), then you're adding even more to the problem. Definitely keep voltage drop in mind as another variable when designing your EV charging circuit. As you can see, doing it properly is not simple as "just use #4 NM-B (romex)".

I'd personally try to aim for no more than a 2% voltage drop at the load, under load. For example for the above case, it means 235V at the load:
235V * 48A = 11.28 kW
=
a difference of 240W, less than half the heat generated compared to 5% voltage drop.
Fortunately, only a #8 cable is required to get the 2% at 60 ft (1.88%). There's a calculator for that too.

---
Ignoring the heat issue, a lower voltage drop also means your car will slightly charge faster. Not a huge deal for short charging sessions of 1-2h, but if you're charging 5-6+ hours, then you can save 20-30 mins.

On top of that, I know that Teslas monitor the input voltage and automatically reduce the current pulled as needed to avoid overheating the wire. But I don't know when this behavior kicks in (230V? 220V? 210V?).
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,297
8,990
Boise, ID
= a difference of ~600W!

To put it into perspective, that's 2-3x the power required to keep Sentry Mode on.

That ~600W is dissipated as heat in the wire.
You seem to be forgetting something, though. This is a problem that already regulates itself somewhat by length. Dissipating 600W in 6 feet of wire is a far greater temperature rise than dissipating that 600W all along the much greater mass of 60 feet of wire. The much longer wire runs do two things that help with this. First off--thermal inertia. That's a lot more mass to heat up, so it takes much more heat energy to raise its temperature and takes much longer. And second--that is much more room of air space and volume around the wires in which to spread that heat dissipation out. It's not concentrated all in one place where the heat would accumulate and build up more/faster.

So for really long wiring runs, you can have significant resistance and voltage drop and still have the wires be stone cold because there's just SO MUCH wire there!
a difference of 240W, less than half the heat generated compared to 5% voltage drop.
Fortunately, only a #8 cable is required to get the 2% at 60 ft (1.88%). There's a calculator for that too.
I was trying to find one--thanks for the link. And when I looked at the chart you linked to 48A continuous at 4 gauge copper wire (like Romex) is such a small amount of temperature rise it is not even showing up on the chart. And spread over 60 feet would be even less of an issue.
 

rjpjnk

Active Member
Mar 12, 2021
1,015
626
NJ
Is this MC 6/2 wire suitable for a 60 amp circuit for a Wall Charger install?

I have a bunch of 6/2 NM-B on hand and was going to use that for a 50 amp run due to the Romex 60C temperature limit, but am considering buying this if it will allow for upgrade to 60 amps. I believe I read somewhere that the MC allows for that but would like to confirm with the experts here.

 

gfunkdave

Member
Aug 10, 2016
167
219
Chicago
Is this MC 6/2 wire suitable for a 60 amp circuit for a Wall Charger install?

I have a bunch of 6/2 NM-B on hand and was going to use that for a 50 amp run due to the Romex 60C temperature limit, but am considering buying this if it will allow for upgrade to 60 amps. I believe I read somewhere that the MC allows for that but would like to confirm with the experts here.


I think it depends on the temperature rating of the other things in the circuit you're planning to build, namely the outlet and your circuit breakers/panel. I think (but am not sure) that most residential panels, breakers, and outlets are rated with the 75C max. 6/2 MC cable has a 65A limit at 75C, so you should be OK.

 

kishkaru

Member
Feb 3, 2020
247
271
The Bay, CA
Is this MC 6/2 wire suitable for a 60 amp circuit for a Wall Charger install?

I have a bunch of 6/2 NM-B on hand and was going to use that for a 50 amp run due to the Romex 60C temperature limit, but am considering buying this if it will allow for upgrade to 60 amps. I believe I read somewhere that the MC allows for that but would like to confirm with the experts here.

Be careful of any product that doesn't list a datasheet or specifications. In the Amazon listing, it says the Brand is SouthWire, so the seller is just reselling another brand.

The closest thing I was able to find on SouthWire's website is Armorlite® Type AC cable, which has a 6/2 variant. Here is the datasheet for it. But you notice that the ground conductor is un-insulated and also is aluminium. But the Amazon listing says theirs is green insulated and is copper. There are several other things that doesn't seem to match up. I'd check with the seller before buying to see exactly what the cable they are selling is.
 
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iluvmacs

Member
Jan 27, 2014
519
899
Madison, WI
I would suggest you think long and hard about if you really should be buying building wiring from random sellers on Amazon. There's a lot of knock-off products out there these days. Electrical equipment and wiring is one of the last things I would buy on Amazon or eBay. You have no idea what you're really getting. Go to your local electrical supply house, home improvement store, or if you really want to order online, at least use Home Depot or Lowe's or similar.
 

rjpjnk

Active Member
Mar 12, 2021
1,015
626
NJ
Good point. I have seen some Chinese knock-offs (actually counterfeits) of name brand products from time to time. I definitely don't want inferior wire.

I will probably just stick with the 6/2 NM-B since I already have a bunch left over from an AC install years ago. I don't really need the additional 8 amps of charging current. Just thought that since this MC wire was so cheap I'd run it by the experts.
 
Does burying the wire in a wall that is then filled with closed cell spray foam insulation impact the choice of appropriate wire type and gauge for a tesla gen3 charger?

The calculator recommended by @kishkaru does not have an option to calculate this, but I would imagine that a wire surrounded by 2 inches of polyurethane would not be able to dissipate heat very easily so I would expect it to get pretty hot during a long charging session. Does anyone have any advice on the appropriate wire to use in this case? I have one quote from an electrician so far and he is quoting 6/3 Romex which, from what I've read in this thread so far would not allow the maximum 48A charging speeds.
 

Rocky_H

Well-Known Member
Feb 19, 2015
7,297
8,990
Boise, ID
I have one quote from an electrician so far and he is quoting 6/3 Romex which, from what I've read in this thread so far would not allow the maximum 48A charging speeds.
I'm answering your last question first because it's the simplest. No, 6/3 Romex definitely isn't allowed for a 60A rated circuit with 48A charging, under ANY circumstances. That would be allowed for a 50A circuit, which is still a fine choice. You say he's quoting, but quoting on what? What did you specifically ask for the job to be? Did you specifically ASK for it to be a 60A circuit? If so, and he is planning to try to use 6/3 Romex for that, then that is a very straightforward code violation, and I would never use that electrician.

Does burying the wire in a wall that is then filled with closed cell spray foam insulation impact the choice of appropriate wire type and gauge for a tesla gen3 charger?

The calculator recommended by @kishkaru does not have an option to calculate this, but I would imagine that a wire surrounded by 2 inches of polyurethane would not be able to dissipate heat very easily so I would expect it to get pretty hot during a long charging session.
Huh--I haven't heard anything specifically about that kind of spray foam insulation, but being closed inside a wall with no air flow and surrounded by the fluffy fiberglass insulation is already a pretty hot kind of environment. But that is accounted for by the rating tables, where Romex, which is to be used in those areas, is already derated to much lower currents. So I think that in wall heat consideration is already...baked in. (snicker)
 
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Vaildriver

Member
Sep 16, 2021
148
100
Colorado
We did a 6 AWG copper wire. 60 Amp circuit. This is what the Tesla manual recommends. Either way, all wires were individually wrapped and a conduit used. We didn't need to use our indoor breaker box as our main service was on the outside of the garage and had space. Had 3 bids from different companies (these guys wire huge hotels/hospitals, etc) and all recommended this setup.
 

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You say he's quoting, but quoting on what? What did you specifically ask for the job to be? Did you specifically ASK for it to be a 60A circuit? If so, and he is planning to try to use 6/3 Romex for that, then that is a very straightforward code violation, and I would never use that electrician.
I would have thought that it would go without saying that if you are paying to install a tesla wall charger, whose main purpose is to charge more quickly at higher amps that any solution proposed by the electrician should at a minimum be capable of the max 48A of the charger. At a minimum one would hope that a professional would discuss any cost/benefit trade-offs during the face to face on-site meeting to avoid any unpleasant surprises for the customer later. The electrician's quote also specifies a 60A breaker, and from what you are saying a 60A breaker is not safe to use with 60°C 6 awg romex.

What makes this even more perplexing it that I already have another tesla wall charger on the other side of the same garage that uses 4 awg / 3 conductor cable connecting to a 60A breaker. I'm surprised he didn't at least propose a solution that is equal to my existing charger (by the way, I plan to set the two chargers up using power sharing so don't worry about having 2 60 circuits in a 100A panel).

I have sent an email to the electrician asking for clarification so I'll let you know what he says regarding his rationale for proposing 6 awg 60°C romex.

I wonder if Tesla itself has been training these electrician that 6 awg 60°C romex is suitable since this is now the second electrician I have met who installs lots of tesla chargers in my area, all with 6awg 60°C romex. I found the electricians I am getting quotes from on the tesla website listing electricians and supposedly "Each electrician listed below has been trained to install Tesla charging equipment and adhere to strict quality, permitting and inspection requirements. "

Page 5 of the gen3 wall charger installation manual is fairly clear though that 6 AWG 90° C-rated copper wire is the minimum for full charging speed:
If installing for maximum power, use minimum 6 AWG, 90° C-rated copper wire for conductors.
NOTE: Upsize conductors if necessary.
 

roc-noc

Member
Sep 30, 2021
117
405
Rockford, IL
I just received my VIN this morning and am now planning my Gen 3 wall connector install for 48 amp 240v direct wire. 10 days before my delivery window.

60 amp breaker Square D Homelite breaker - 3/4" EMT Run about 35 feet. Goes up into our garage attic from 200 amp panel, across the garage and back down the other side from the attic to the wall near left rear of my parking space. Plan to pull 2- 6awg thnn copper (90 deg C) for power and 1 - 10awg thnn copper for ground.

I have the tools and experience. I have a licensed electrician that I occasionally hire when I get in over my head and can't complete something by myself.

Anybody see code problems?

Thanks, Tom
 

jcanoe

Active Member
Oct 2, 2020
4,374
4,548
Maryland
I just received my VIN this morning and am now planning my Gen 3 wall connector install for 48 amp 240v direct wire. 10 days before my delivery window.

60 amp breaker Square D Homelite breaker - 3/4" EMT Run about 35 feet. Goes up into our garage attic from 200 amp panel, across the garage and back down the other side from the attic to the wall near left rear of my parking space. Plan to pull 2- 6awg thnn copper (90 deg C) for power and 1 - 10awg thnn copper for ground.

I have the tools and experience. I have a licensed electrician that I occasionally hire when I get in over my head and can't complete something by myself.

Anybody see code problems?

Thanks, Tom
Curious why you are not using MC wire as no conduit would be required.
 
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TBrownTX

Member
Dec 25, 2020
968
1,136
Dallas, TX
I just received my VIN this morning and am now planning my Gen 3 wall connector install for 48 amp 240v direct wire. 10 days before my delivery window.

60 amp breaker Square D Homelite breaker - 3/4" EMT Run about 35 feet. Goes up into our garage attic from 200 amp panel, across the garage and back down the other side from the attic to the wall near left rear of my parking space. Plan to pull 2- 6awg thnn copper (90 deg C) for power and 1 - 10awg thnn copper for ground.

I have the tools and experience. I have a licensed electrician that I occasionally hire when I get in over my head and can't complete something by myself.

Anybody see code problems?

Thanks, Tom

Planning to use any conduit?

Tim
 

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