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Anyone use this Romex 6/3 cable during install?
- Thread starter Dugdale
- Start date
Hey I had an observation that I thought was interesting on this subject, though I'm not out to trigger anybody - just an observation.
My installation is indeed a Romex 6/3, 60 amp breaker, Tesla V3 charger. After I found this thread, I set the limit on the car to 44 amp, which is 80% of the 55 amps that Romex 6/3 is rated for. Frankly, any difference in charge time between 44 amps and 48 amps has been irrelevant to me.
The observation is that whether this charging system is running 44 amps or running 48 amps as it was before, the 18 foot charging cord supplied by Tesla feels warmer to the touch than the Romex 6/3 does. I've sampled both in several places. Is it not appropriate to take from this observation that the Tesla cord is more of a limiting factor than the house wire? Anyway, I'm not advocating for anything, I just thought it was interesting.
My installation is indeed a Romex 6/3, 60 amp breaker, Tesla V3 charger. After I found this thread, I set the limit on the car to 44 amp, which is 80% of the 55 amps that Romex 6/3 is rated for. Frankly, any difference in charge time between 44 amps and 48 amps has been irrelevant to me.
The observation is that whether this charging system is running 44 amps or running 48 amps as it was before, the 18 foot charging cord supplied by Tesla feels warmer to the touch than the Romex 6/3 does. I've sampled both in several places. Is it not appropriate to take from this observation that the Tesla cord is more of a limiting factor than the house wire? Anyway, I'm not advocating for anything, I just thought it was interesting.
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TBrownTX
Active Member
Exactly the same observation here. The only warm areas are the Tesla charging cable where it bends to plug into the car, and the breaker itself, with the cord being the warmer of the two. No detectable heat anywhere else in the chain.
Tim
Johnny Vector
Member
There are two major factors that go into the "ampacity" (a very silly word, but it's what they use) of a wire. One is the gauge of the conductors, which determines how much power they will dissipate for a given current, and thus how warm they will get. The other is the thermal characteristics of the insulation. Nonmetallic B (NM-B) wire has vinyl insulation, and is rated for use at 60°C. The Tesla cable is likely rated to a higher temperature, so it's safe even though it gets warmer.
What a great answer - thanks. In my case both the Tesla cable and the Romex cable seem well below 60°C / 140°F no matter what the amp limit set point is. Maybe I'll find a temperature gun and run a study, just for fun. It has no applicable value for me, however, as the charge time difference between 44 amps and 48 amps has been immaterial.
Johnny Vector
Member
Yeah, the actual wire should be waaaay below the limit in normal operation. As Rocky_H pointed out somewhere else recently, the rating is based on being inside a wall surrounded with insulation. Also, the maximum allowable temperature when they figure the ampacity is almost certainly 10 or 20 C below the rating of the material, for reasons of conservatism. Also also, the temperature rating is probably the point at which it starts quickly breaking down or outgassing the plasticizer (for some value of "quickly"). But that type of damage continues to happen at lower temperature, just more slowly. So if the wire is going to last 100 years, you need to stay well below the rated temperature.
I'm definitely guessing about that last detail, but that kind of conservatism in general is how you handle requirements that don't have an obvious threshold.
Yes. MC cable can use the 90C temperature rating of THHN, so it supports 75A. You may use a 60A breaker or 70A breaker, since 75A breakers are less common.
Why are you choosing MC cable? Do you live in Chicago?
mrau
Authorized Driver
You may want to change the V3 Wall Connector setting to 50 amp instead of 60 amp. Although this would reduce charge rate a bit to 40 amp, I doubt you will notice it. That way you won't have to lower the rate in your car (not sure if lower rate in car carries to other locations). Also, should another Tesla happen to stop by, you won't have to worry about lowering their rate either.
Not that you are doing it wrong now, just another option.
sleepydoc
Well-Known Member
Yup - why do you think thieves steal copper? It’s a commodity so the prices will be high pretty much wherever you look.I will be buying all my supplies from Home Depot.. but that 6/3 Romex is expensive as hell!! I need at least 50 feet.. I was going to buy the 75 feet 6/3 for $120 online order but was sold out quick.. hope that offer comes back
Thanks. The car is remembering set limits by GPS location, so it's a locational setting not a global setting. Perhaps a future update to the V3 wall charger software would be some additional resolution on the setpoint.
Perhaps the part that residential breakers are never rated to 90 degrees C, so that will still limit the entire setup to 75 degrees.
roblab
Active Member
Yes, use a 50 amp breaker and run it at 80%. I can't imagine you are in such a hurry that you need your car to charge a few minutes faster. Mine charges while I sleep, and I honestly never look at how long it will take to finish charging, which is usually in the wee hours of the morning.
Chicago doesn't like MC except for short pigtails. It's all THHN in rigid metal pipe...
Order it from here 6 AWG THHN Building WireI will be buying all my supplies from Home Depot.. but that 6/3 Romex is expensive as hell!! I need at least 50 feet.. I was going to buy the 75 feet 6/3 for $120 online order but was sold out quick.. hope that offer comes back
Johnny Vector
Member
Can confirm. I've ordered from them a couple times, and been satisfied in all cases. It's nice to be able to buy just what I need.
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