Additionally, since its gonna be used continuously(>3 hours at max load), the circuit breaker isn't supposed to be 60 amps... this flies in the face of the next-size-up rule, since there aren't 55 amp breakers out there.
I am not entirely sure what is being referenced here. #6 AWG Copper NM cable must use the rating from the 60c column (even though it likely has insulation rated to 90c). It is good to 55 amps in this configuration. So you could put some load on that wire that was 55 amps non continuous, or 44 amps continuous. Somewhat sadly, the HPWC does not have a rotary dial setting for 44 amps, so your choices are 40 amps or 48 amps. Since 48 exceeds the ampacity of the wire, you have to set it at 40. While you technically could install that wire on a 60a breaker since that is the next size up, there is no reason to do that since you are going to be setting the HPWC to 40 amps and hence a 50a breaker is sufficient (I actually don't know if you are allowed to use the next size up rule if your load does not actually require it - the 50a breaker is the safer choice).
I'm gonna be doing the panel connection of my HPWC using 6-2 romex this morning. I'm sure my 50 amp breaker will be fine for the 40 amp setting on the HPWC, and even though I'd love to get that little extra bit of speed, I really don't need it at all.
BTW, I had originally intended to use a 60 amp breaker, until reading about the continuous load rules. I'm sure it wouldn't burn down my house to run the thing with a 60 amp breaker and 48 amp setting, since the NEC is pretty conservative, and my entire run of romex is exposed in a basement(not buried in an insulated wall, for example). Even the 50 amp breaker would be able to reliably drive the HPWC at 48 amps because its in a basement panel where the temperature stays 65-70f.
So two comments here: Yes, I suspect you are correct that a NM line of 6 AWG would probably be fine with 48 amps continuous on a 60a breaker if it was not installed in any insulation, hot areas, etc... My understanding is that the reason for the NM derating to 60c was because of small gauge circuits in ceilings running to lighting fixtures all surrounded by insulation. There is intended to be a lot of margin in the NEC calculations for good reason. I don't have laboratories with decades of experience in order for me to figure out where it is OK to push the limits and where it is not. I outsource that work to the NEC and just follow their guidelines. ;-)
The second comment about 48amps continuous on a 50a breaker: I am not so sure about that. So breakers are tested under "standard test conditions" which I believe are 104f but with the breaker installed in free air (i.e. in a panel but with no surrounding breakers). They are supposed to break at their rated amperage under those standard test conditions. By putting a breaker in a panel full of surrounding breakers that themselves heat up you could easily find yourself over 104f within the breaker. The breakers are also not that accurate, so you are likely to have nuisance trips.
Thanks for the input. Curious, why aren't you going with a #6 thhn so you can use the HPWC up to 48amps?
Running THHN requires conduit. The labor and cost to run conduit is quite high in many cases. Though I personally like the look of EMT conduit in a lot of settings.
Lets be real however, given the actual amp draw never exceeds 48A, you will never cross the 60c temp rating of the NM-B 6/3. NM-B 6/3 is rated for up to 55amps, and that is continuous load not @ 80%. Everything with electrical code is over engineered, yes for good reason, but in an environment like this I would personally (and in fact do) use 6/3 NM-B on a 60a to pull 48a. The wire doesn't even get slightly warm it is room temperature after 4 hours of charging.
Hrm, I am not sure how you can say you would never cross the temp rating of 60c on NM-B. 48a continuous violates the 60c rating on that cable. If the cable was installed in thermal insulation it might get really hot.
So the whole reason you have to upsize 25% for continuous loads is I believe because the wire gets warm/hot and conducts heat into the circuit breaker which causes the bimetallic strip to heat up more and cause a trip. No, the wire is not likely to melt, but it might cause nuisance trips or damage the connectors on either end.
Using 6/3 NM-B on a 60a to pull 48a is very unequivocally a code violation. If a house burns down that you wire (regardless of the actual cause of the fire), do you want to explain why you thought you knew more than the NEC?
So, my 3 is out there charging on my HPWC... and I must say I'm a bit disappointed. Before I started the charging, I was seeing 236-237 volts at the main lugs of the main panel, a little lower than I'd like, but well within parameters.
I started it at 40 amps, not wanting any surprises. It worked okay enough, but I found the car was reporting it was seeing only 225 volts, and only charging at 32mi/hr, after several minutes of 30/31. I traced the voltage back, and found it was only 228 volts at the main lugs now. At least my 60 feet of 6 gauge wasn't losing more than predicted.
I noticed my main panel was very gently sizzling at this point. More than a bit unsettling. I tightened the screws on the 60 amp breaker feeding the subpanel right next to it where the HPWC is connected(via a 50 amp breaker), which while not loose, were not as tight as the specs suggest. I found this stopped the sizzling, but wasn't real sure whether it was really the cause, or just my touching the breakers moved something a bit. I had intended to(and have the wire for) an upgrade of the subpanel feed breaker to 80 amps, but I might return those if I can't get any more power out of the mains.
I put the cover back on the main panel, and found the sizzling returned. It appears to be the main breaker making the noise... I've made a note to replace that ASAP.
I went out to the 3 and turned down the amperage using the touchscreen. At 35 amps, the voltage rises to 233, and I get a stable 30mi/hr of charge. I shudder to think what I really might see if I asked for 48(will do that tomorrow, or maybe after the main breaker has been replaced). I assume the 3 would stop asking for more amps way before it actually reaches 48, because the voltage will likely drop too much, not unlike when it only takes 9 of 12 amps when the 120V hits 105.
I did notice that the 6-2, while not at all warm to the touch, was detectably not as cold as when the power was off(this was checked after 10 minutes of 40Amps) I'll pull out the infrared measurer in a little while.
I'll probably give the electric company a call, I don't think that my home which is probably taking about 20 amps in addition to the HPWC is taking, should be drawing down the input voltage this much.
Sizzling of your main panel is NOT OK. I highly recommend you stop charging altogether right now and immediately isolate the issue. Sizzling (especially combined with unexpected voltage drop) could be arcing or an overheated terminal. What might be a simple bolt tightening could become a full panel replacement if you overheat things (not to mention we are in serious "burn your house down" danger mode). Note that while adding the EV charging might have finally triggered some failure mode, just stopping EV charging might not cause it to go away since the joint or connection that has an issue may be more weakened.
You need to isolate where the noise is coming from exactly - a flir camera (if you know anyone with one like an iPhone attached one) or an infrared thermometer may be the best way to figure this out. Once you find the location, the question is can a simple tightening fix it? Or will it require a breaker or bus or whatever to be replaced to fix it? If you get lucky, it is just the main breaker needing tightened to the bus. Maybe that could be done by just shutting off the main breaker, but you may also need to shut the main breaker, cut the seal on the meter base, pull the meter, then do your work, put the meter back, and re-energize. This would HAVE to be done with permission of the utility. I think they will let you cut the tag in some cases yourself (and come back and replace it later) if you get permission upfront (otherwise it is considered power theft).
I have mine hooked up this way but dialed it down to 50 amps rather than 60 on the HPWC. Probably not necessary, but I can’t see myself ever needing that extra 7 miles/hour and now I know I’m code compliant. As others have said, I imagine the wire ratings are incredibly conservative and keeping it dialed to 60 amps wouldn’t be an issue if the extra power is important to you, but it’s not compliant either.
50 amps (40a continuous) is the NEC approved way to make use of 6 AWG NM-B. I actually have not read the details on the next size up breaker rule to be sure, but I see no reason to use a 60a breaker on 6 AWG NM-B if you are hooking it to a HPWC since there is no 55a/44a setting that could make use of those extra five amps.
When I installed my HCWC I deliberately ran the car down a few days then began charging soon as I got home so I could check on it repeatedly over the course of hours. To think 10minutes is enough time to plateau wire temp is absurd.
Sizzling I would suspect is anti-oxidation grease getting hot at bad connections.
Yeah, sizzling is no bueno. That is a stop everything and figure out what is going on kind of an issue.
