Wiring for NEMA-14-50

MikeC said:

Hoping someone can give me a quick answer on this. I'm buying a newly-constructed home and received the following information about wiring in the garage:

"Home is pre-wired with #8 wire that will accept 50 amps per table, 310.16 of the national electrical code."

Is that sufficient for a NEMA 14-50 or does it need to be #6? Is it sufficient for two NEMA 14-50s when we get a Model X? Thanks for any help.

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I am not an electrician, but if you search around here for posts on the subject IMHO most would say you need #6 because of the continuous draw during charging (even though you will be charging at 40A--derated). FWIW, in my home, I have 2 NEMA 14-50 on 2 separate 50A breakers with #6 NM fished inside my walls. You certainly need 2 separate circuits & breakers if you are charging 2 Model S simultaneously (...and is your home electrical service able to handle the continuous load???).

It sure is nice to have a dedicated line for each car so that you can plug in as soon as you get home and forget about it until the next morning.

I believe the table they reference as far as #8 being good for 50 amps is just the beginning or initial rating. There are other tables that specify derates to that, including the type of wire and the temperature rating of that wire, how many wires are in the conduit, etc.

I'm not an electrician, but as far as I can tell, It's going to depend on what kind of #8 you have. If it's just Romex (NM-B), then no, #8 won't work because it's only rated for 40 amp. If you have something like THHN in conduit, then yes, #8 is good enough even after de-rating for the relatively high ambient temperature you'd see in a garage. References:

NM-B ampacity chart:

http://www.encorewire.com/wp-content/uploads/EncoreWire-NMB.pdf

Wire ampacity chart:

Wire Chart

pgiralt said:

I'm not an electrician, but as far as I can tell, It's going to depend on what kind of #8 you have. If it's just Romex (NM-B), then no, #8 won't work because it's only rated for 40 amp. If you have something like THHN in conduit, then yes, #8 is good enough even after de-rating for the relatively high ambient temperature you'd see in a garage. References:

NM-B ampacity chart:

http://www.encorewire.com/wp-content/uploads/EncoreWire-NMB.pdf

Wire ampacity chart:

Wire Chart

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That is correct. If it's "Romex", then #8 is only good for 40A. If it's THHN copper, in conduit, then it's good for 60A.

Yes, assuming that info is correct, you should be fine for a single nema 14-50 plug, but not two. If you do get a second car, you'll want to install its outlet closer to the new car anyways with its own wire run.

I had final inspection on a house in Santa Clara County jurisdiction in October 2012. I specified two NEMA 14-50 outlets in the garage. As far as I can tell, the electrician put #8 NM-B in the walls and the inspector passed it. At the time there was no EVSE, but I got a permit for the EVSE install and the same inspector didn't ask about the wire. He just asked me if I was using the circuits that were put in during construction. I've been using one of them for nearly a year now with a Leviton EVB40 for my RAV4 EV. The plug does not even get warm after several hours of charging. I know #6 is better, but it seems that this happens a lot.

Depending on how long your your run is and especially if it is x2 for a second car, you will be able to save money and not waste copper by installing either 6-50's (since the neutral on a 14-50 is extraneous and not even recognized by the "S") or some 14-30's (unless you need a quicker "turn around time"). I have been using a lowly 14-30 (a 10-30 is no-no in new construction after around '95) device for a 160 mile commute for 15 months, plus I am "old school" and prefer a slower charge rate (it's hard to teach an ol' dog new tricks). My HPWC is for guests only.

The FAQ in my signature covers this question (in case you want more detail), but the other posters are correct.

#8 in conduit, #6 Romex (NM-B).

That 60 degree restriction is something I'd lobby to change if I were on the code-making panel. Today's NM cable isn't your father's NM cable--had to work with some of that old 1970's white-with-orange-lettering NM cable this weekend... today's SIMpull stuff is worlds ahead!

You could use a 40A breaker (which would be code compliant with your romex), install a NEMA 14-50, but set your car to charge at 32 amps. No risk of excess heat there. Charging every night at 32A instead of 40A should be fine as far as range recovery goes.

MikeC said:

So it's #8 Romex. After reading FlasherZ's excellent FAQ, I guess my next question should be if it is 8/3+ground NM-B or 6/3+ground NM-B.

What is the risk if I were to keep wire that wasn't rated for 50A? Fire? From what miimura said, it sounds like this is relatively common issue.

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#8 Romex is 8/3+g (anymore we just leave the +g off if we talk NM-B because that's the default). It won't be 6/3 because you said #8.

As to your last question, I have my opinions but they're outside the scope of this forum. The conductors inside the NM-B sheathing are usually 90 degree rated conductors (THHN-2) and the NEC's requirement that it be treated as a 60 degree conductor is a bit old IMO. I can understand the concern - NM-B can be packed together more tightly than when you follow conduit fill rules with individual conductors and under certain circumstances could cause a failure of insulation - but my personal opinion is that they could raise it to the 75 degree level on the ampacity table and it would still be very safe. Code is code, however, and in most jurisdictions represents enacted law, which can influence liability or insurance.

So the risk is probably less a safety risk and more a question of assumed risk for insurance or liability reasons.

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Cosmacelf said:

You could use a 40A breaker (which would be code compliant with your romex), install a NEMA 14-50, but set your car to charge at 32 amps. No risk of excess heat there. Charging every night at 32A instead of 40A should be fine as far as range recovery goes.

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This is a safer way to protect that wiring, but unfortunatley it still violates code because you're connecting an appliance with a 50A "nameplate" (i.e. default 40A charging + continuous load rules) to a 40A circuit, even if you can set your car lower.

FlasherZ said:

This is a safer way to protect that wiring, but unfortunatley it still violates code because you're connecting an appliance with a 50A "nameplate" (i.e. default 40A charging + continuous load rules) to a 40A circuit, even if you can set your car lower.

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Surprisingly, here in Canada it is very common to see #8 wire connected to a NEMA 14-50 via a 40 amp breaker. I have come across several homes with stove outlets wired this way. The first time, I was running a 50 amp RV extension cord to a stove outlet to charge my car at a friend's place and it came to my attention when the breaker tripped and I discovered it was 40 amps. I had to dial my charge rate down to 32 amps to continue. I started looking around. My own house and my rental property are both wired this way. Several friend's houses I've looked at are wired this way. I agree that it seems "wrong" but am simply pointing out it is not uncommon.

mknox said:

Surprisingly, here in Canada it is very common to see #8 wire connected to a NEMA 14-50 via a 40 amp breaker. I have come across several homes with stove outlets wired this way. The first time, I was running a 50 amp RV extension cord to a stove outlet to charge my car at a friend's place and it came to my attention when the breaker tripped and I discovered it was 40 amps. I had to dial my charge rate down to 32 amps to continue. I started looking around. My own house and my rental property are both wired this way. Several friend's houses I've looked at are wired this way. I agree that it seems "wrong" but am simply pointing out it is not uncommon.

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Yes, there are a lot of stoves out there wired and placed on 40A breakers and it's perfectly legal for those stoves. The NEC says that a 50 amp receptacle is permitted to be installed on a 40A or 50A breaker, but the catch-alls in sections 100 & 110 state that you have to follow manufacturer instructions and cannot attach appliances with a larger rating to a circuit with a smaller rating. Because the Tesla says 50A circuit, you have to follow those instructions.

Most large appliances have a rated ampacity, a minimum breaker size, and a maximum breaker size. Many of them also have selectable load sizes (for example, my hot tub has jumpers that select 40, 50, or 60A operation -- when set to 40A, for example, the heater can't run when pumps 2 & 3 are running, or pump 1 is on high). Generally, the difference between the "installer" and "user" roles are important in interpreting the allowable circuit sizes, like the DIP switches in the HPWC. When the "user" role can change the current (car's display), the circuit must be sized for the largest possible load offered by the user. The "installer" role can, in a non-easily-accessible way, restrict the load so that a smaller circuit size is offered. That's how an HPWC can be installed on a 50A, 60A, etc. breaker.

FlasherZ said:

Yes, there are a lot of stoves out there wired and placed on 40A breakers and it's perfectly legal for those stoves. The NEC says that a 50 amp receptacle is permitted to be installed on a 40A or 50A breaker, but the catch-alls in sections 100 & 110 state that you have to follow manufacturer instructions and cannot attach appliances with a larger rating to a circuit with a smaller rating. Because the Tesla says 50A circuit, you have to follow those instructions.

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With Canadian and US codes largely harmonized, I'm guessing the same rule applies here too. I figured it must be something like that since all of the installations I've seen have (or likely have) passed inspections at the time of construction.

It does seem like a weird loophole, though. Kind of like wiring a 5-15 to a 10 amp breaker 'cause you only want to use it with a small lamp. I think if I was doing any wiring myself, I'd always put a 14-50 on a 50 amp breaker with #6 just to be safe(er).

mknox said:

With Canadian and US codes largely harmonized, I'm guessing the same rule applies here too. I figured it must be something like that since all of the installations I've seen have (or likely have) passed inspections at the time of construction.

It does seem like a weird loophole, though. Kind of like wiring a 5-15 to a 10 amp breaker 'cause you only want to use it with a small lamp. I think if I was doing any wiring myself, I'd always put a 14-50 on a 50 amp breaker with #6 just to be safe(er).

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Retail at Home Depot is something like $160 for a 125 foot roll of #8, and $210 for a 125 foot roll of #6, 3-conductor-plus-ground romex. Not a big deal for an individual (especially when compared to labor), but for a builder doing a lot of spec homes, it all adds up.

Ultimately the quality of EVSE wiring is discovered by comparing:

1. Electric Meter voltage.

2. Main Breaker voltage.

3. EVSE input voltage.

4. EVSE output voltage (@charge cord connections).

5. Tesla Charge Screen voltage.

Test 4 is mainly to determine how your voltmeter tracks with the Tesla voltmeter. Need a person in the car to shout the voltage changes.

Turn off all other major appliances, set Tesla to a steady charge and make your voltage measurements. Then see if you are happy with the inevitable losses.
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