Tesla Wall Connector: Hard wiring Vs NEMA 14-50

[ajexpert]

I’ve my Tesla Model Y delivery date finalized and now I’m exploring the options for charging. I ordered Tesla Wall Connector and thinking to connect to NEMA 14-50 for flexibility. Appreciate if I get response to following questions.

• I understand that 60 Amp hard-wired wall connector works best, however, what are the cons of installing NEMA 14-50 with 50 Amp wire? Appreciate if someone let me know how slow the charging will be if Tesla Wall Connector is connected to NEMA 14-50 (50 Amp)
• Assuming I opt for NEMA 14-50, would it affect the installation rebate or any State/Federal(?) rebate? One electrician said if I do not opt for hard wiring, the rebate can be denied. Is it true?
• Any EV rebates available to apply for state of MD (Montgomery County)?

TIA

 

[MY-Y]

I still prefer the 4/3 NMB I put in. (4/2 is even better if you don't want the option of putting a 14-50 in later.)

The thicker copper makes less heat (less loss). I prefer a cooler wire over a hotter wire with higher temp insulation.

 

[Rocky_H]

True, but if folks think doing 60 amps with #6 NM is okay, I guess I would be fine also with a 70 amp breaker in my setup?

Two different unrelated things. If people are going to be clueless about why the round-up rule cannot be used, and violate code, that's one thing. Your announcing a simply incorrect amp rating number is another thing.

 

[Eric33432]

True, but if folks think doing 60 amps with #6 NM is okay, I guess I would be fine also with a 70 amp breaker in my setup?

90˚C column is only for de-rating, since breakers are only good to 74˚C. And as Rocky_H so sweetly pointed out to me the other day, can be used even for de-rating NM-B cable!

But you do not want to install a 70 amp breaker on a 65 amp circuit any more than you would install a 60 amp breaker on a 55 amp circuit. And there is no need to either.

I am curious why you need charging for 5 EVs?

 

[h2ofun]

90˚C column is only for de-rating, since breakers are only good to 74˚C. And as Rocky_H so sweetly pointed out to me the other day, can be used even for de-rating NM-B cable!

But you do not want to install a 70 amp breaker on a 65 amp circuit any more than you would install a 60 amp breaker on a 55 amp circuit. And there is no need to either.

I am curious why you need charging for 5 EVs?

I never needed for 1, but now have an ev

I had my garage torn apart. It physically can fit 5 cars. So to future proof, I just went to max possibility. Plus I can do multiple free charging for friends. Go big or go home

 

[240vPlug]

#6 NM-B is derated. If it were able to use the full rating it would be 75 amps as it's 90C. I wish the code was more clear about these types of things. I think the concern is environmental (think hot attics).

Also doesn't help that 4/2 or 4/3 NM-B ROMEX is expensive...a lot more expensive...and equally hard to come by.

 

[ucmndd]

I still prefer the 4/3 NMB I put in. (4/2 is even better if you don't want the option of putting a 14-50 in later.)

The thicker copper makes less heat (less loss). I prefer a cooler wire over a hotter wire with higher temp insulation.

20 feet of 6awg copper has a resistance of about 0.008 ohms. At 48 amps that’s about 18.4 watts of loss (heat).

20 feet of 4awg copper has a resistance of about 0.005 ohms. At 48 amps that’s about 11.5 watts of loss (heat).

Needlessly using 4awg wire instead of 6 because of “preference” is an extremely expensive way to save those 7 watts.

 

[h2ofun]

20 feet of 6awg copper has a resistance of about 0.008 ohms. At 48 amps that’s about 18.4 watts of loss (heat).

20 feet of 4awg copper has a resistance of about 0.005 ohms. At 48 amps that’s about 11.5 watts of loss (heat).

Needlessly using 4awg wire instead of 6 because of “preference” is an extremely expensive way to save those 7 watts.

code is code

 

[MY-Y]

20 feet of 6awg copper has a resistance of about 0.008 ohms. At 48 amps that’s about 18.4 watts of loss (heat).

20 feet of 4awg copper has a resistance of about 0.005 ohms. At 48 amps that’s about 11.5 watts of loss (heat).

Needlessly using 4awg wire instead of 6 because of “preference” is an extremely expensive way to save those 7 watts.

I agree with your point, but get different numbers than you do.
See: Copper Wire - Electrical Resistance vs. Gauge
Resistance per 1000m
4 gauge copper: 0.253 ohms
6 gauge copper: 0.403 ohms

My run is 40', but let's use 20' from your example. A 20' run has 40' of conductor (both hot legs).

4 gauge: 0.253 * 40 feet * 3.28 ft/m / 1000 = 0.0332 ohms

6 gauge: 0.403 * 40 feet * 3.28 ft/m / 1000 = 0.0533 ohms

The power loss in the wire is I^2 * r
4 gauge: 48 * 48 * 0.0332 = 76 watts
6 gauge: 48 * 48 * 0.0533 = 122 watts

With a short 20' run, the difference is 46 watts. For my 40' run, it's 92 watts.

You're point is still valid though. For my usage, the delta is ~ $7/year. The lower temperature wire is still nice though.

 

[Sophias_dad]

See: Copper Wire - Electrical Resistance vs. Gauge
Resistance per 1000m
4 gauge copper: 0.253 ohms
6 gauge copper: 0.403 ohms

Just FYI, you are using the table incorrectly, those numbers are per 1000 feet. To actually see the resistance per 1000m you need to press the (ohms/1000m) button which then gives you:

4 gauge copper: 0.83 ohms
6 gauge copper: 1.32 ohms

 

[HMHM]

I have a Mobile Connector which is permanently plugged in my garage to a 110V outlet. It is sufficient to keep the battery topped off at 80%. I use it every 2-3 days because my daily commute is only 20 miles. Initially, my thought was to have the 14-50 installed and keep using the Mobile Connector. However, now I'm buying a Wall Connector for hardwiring. I decided against 14-50 receptacle for my Mobile Connector because recently I have been reading about a consistent 'nuisance tripping' of GFI circuit breaker. As per the NEC, since 14-50 is a Wall receptacle, it can only be installed on a 50A or less GFCI breaker. On the other hand, to utilize the full 48A capacity of the hardwired Mobile Connector, I need a 60A breaker in my electrical panel, but not a GFCI kind. The GFCI protection is built in the Wall Connector, but not in the Mobile Connector.

 

[jcanoe]

The GFCI protection is built in the Wall Connector, but not in the Mobile Connector.

Regarding this last part; the Mobile Connector does have built-in GFCI protection as does the Wall Connector. The NEC requirement for GFCI protection for the receptacle is there to protect the user when plugging or unplugging the equipment. (When there is no receptacle or power plug there is no need for a GFCI circuit breaker.) The built-in GFCI in the Mobile Connector and the Wall Connector is there to protect the user when plugging the charging cable connector into the vehicle's charge port.

 

[ucmndd]

code is code

For sure. But there are cheaper ways to be code compliant than upsizing to #4 conductors.

 

[MY-Y]

Ih

Just FYI, you are using the table incorrectly, those numbers are per 1000 feet. To actually see the resistance per 1000m you need to press the (ohms/1000m) button which then gives you.

4 gauge copper: 0.83 ohms
6 gauge copper: 1.32 ohms

Good catch. The danger of posting before coffee... That makes my point 3.28x stronger.

I'd lose 276 watts more to heat with a 6 gauge wire compared to my 4 gauge. That's $22 / year charging 2 hours / day, $0.11 /kWh.

276 watts less heat in my wire every time I charge? Yes, 4 gauge for me.

 

[Eric33432]

I still prefer the 4/3 NMB I put in. (4/2 is even better if you don't want the option of putting a 14-50 in later.)

The thicker copper makes less heat (less loss). I prefer a cooler wire over a hotter wire with higher temp insulation.

According to Cerrowire, #4 NM-B is only available in 4/3

https://www.cerrowire.com/wp-content/uploads/2019/03/Cerrowire_CerroMax-NMB_sheet_211220.pdf

Same for Southwire:

Romex® Brand SIMpull® Type NM-B Cable | Southwire

Romex wiring for easy installation. Buy Romex brand SIMpull Type NM-B cable for your residential wiring needs.

www.southwire.com

MC (Metal Clad) cable is also only available with 2 conductors up to #6. Larger than that and it is only available with 3 conductors:

https://www.cerrowire.com/wp-content/uploads/2022/09/Cerrowire_Type-MC-Cable_Stranded_sheet_220906.pdf

According to the Southwire voltage drop calculator, my 75 foot run of #6 MC cable has a 1.33% voltage drop, which would be 3.192 volts. Ohm's law says this will be ~153 watts that are dissipated as heat in the #6 MC cable.

If I were to use #4 MC, this would drop to 0.86% voltage drop, or 2.06 volts, resulting in ~99 watts being dissipated as heat in the #4 MC cable.

 

[h2ofun]

For sure. But there are cheaper ways to be code compliant than upsizing to #4 conductors.

Yep, which is why I ran #6 thhn

 

[ucmndd]

Ih

Good catch. The danger of posting before coffee... That makes my point 3.28x stronger.

I'd lose 276 watts more to heat with a 6 gauge wire compared to my 4 gauge. That's $22 / year charging 2 hours / day, $0.11 /kWh.

276 watts less heat in my wire every time I charge? Yes, 4 gauge for me.

I don’t believe your math checks out.

6awg = .403 ohms per 1000 feet = 0.000403 ohms per foot x 40 feet = 0.01612 ohms = 37 watts per conductor at 48 amps = 74 watts.

The same math yields 46 watts with 4awg, so the difference is 28 watts.

 

[stopcrazypp]

I don’t believe your math checks out.

6awg = .403 ohms per 1000 feet = 0.000403 ohms per foot x 40 feet = 0.01612 ohms = 37 watts per conductor at 48 amps = 74 watts.

The same math yields 46 watts with 4awg, so the difference is 28 watts.

Yeah, 276W additional is a lot. That is the heat of a mini space heater. I highly doubt the wiring is putting out that much additional heat. 28W difference seems more reasonable.

 

[MY-Y]

I don’t believe your math checks out.

6awg = .403 ohms per 1000 feet = 0.000403 ohms per foot x 40 feet = 0.01612 ohms = 37 watts per conductor at 48 amps = 74 watts.

The same math yields 46 watts with 4awg, so the difference is 28 watts.

Two errors in a row. I cannot believe I did that. I think my original post is correct except for reading the table as meters instead of feet. The error in my second post was multiplying instead of dividing by 3.28. Sigh. Sorry about that.

Now I'm wondering why the calculations from the resistance table don't align with the ones from the Southwire that Eric33432 posted . Either way, the bottom line seems to be that if a cooler wire makes you feel better, or is about the same price (NM-B vs UV of conduit), go thicker. The savings on the electric bill will be small and both are safe.

 

[Eric33432]

Two errors in a row. I cannot believe I did that. I think my original post is correct except for reading the table as meters instead of feet. The error in my second post was multiplying instead of dividing by 3.28. Sigh. Sorry about that.

Now I'm wondering why the calculations from the resistance table don't align with the ones from the Southwire that Eric33432 posted . Either way, the bottom line seems to be that if a cooler wire makes you feel better, or is about the same price (NM-B vs UV of conduit), go thicker. The savings on the electric bill will be small and both are safe.

I think the issue is that you are doing the math as if this is DC power, but of course it is AC power, and the Southwire calculator was designed by power engineers who take the reactive (imaginary) part of the power into consideration....note that they consider both the DC resistance and the AC reactance of the circuit when calculating the loss, and they make the assumption that the circuit will have a power factor of 0.9, which the Tesla Wall Connector may or may not have.

The point I was trying to demonstrate from my post is that upsizing the conductors to minimize voltage drop for any "normal" circuit length that is encountered in most residential situations is not necessary. Sure you can do it, but from an engineering perspective, which is the basis for the requirements of the US National Electric Code, it is not necessary.

For a Tesla Wall Connector operating at 240 volts nominal and 48 amps, a #6 copper conductor can be as far as 169 feet before the voltage drop goes to more than 3% or 113 feet before the voltage drop goes to more than 2%.

From my experience, when a circuit is properly designed to "Good Engineering Practice" it would be designed for an end-to-end voltage drop of no more than 5%, and such Practice advises voltage drop limits of 3% for the "feeder" and 3% for the "branch circuit", which is the only time I have ever seen 3 + 3 = 5!

There is nothing in the NEC that requires this. Here is a link to a discussion of this on Mike Holt's Forum:

Voltage Drop 3% or 5%

How do you know when to use the 3% or the 5% recommendation? [90.5 (C) 210.19 (A) Note 4, and 215.2 (A) (1) Note 2] I'm guessing 3% for 120 branch circuits and 5% for 240 circuits and feeders? Not sure about other voltages. Thanks Test Question- What is the allowable voltage drop for a 240V...

forums.mikeholt.com

 

[dguitarnut]

I just bought a new home and this outlet is in my garage……when I bought the home they said it was wired for EV
I ordered a MY Tesla a week ago. What do I need for charging? The wall connector or the Mobile connector?
Thanks …..a total noobie here