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Best plug type for EV charger?
- Thread starter h2ofun
- Start date
I wonder if it has to do with expected ambient temps and heat dissipation. I mean, the US specs being different for Romex vs THHN, the conducting part of the wire is the same size and material, it must be that the insulating jacket on Romex doesn't dissipate heat as well as having the wires in conduit, or something like that. But the delta-T from ambient temps will also play a factor in heat dissipation, and the U.S. trying to create a consistent spec for all of the U.S. has to account for say summer in Arizona in the wall of an unconditioned garage or attic - what is the typical worst case ambient temp scenario for Canada?
Midnightsun
Active Member
95f summer and -30f winter is not unusual however it can get hotter or colder. Lots of people think we do not get warm up here but we do. Summers around 90f and 100% humidity almost feel like the worst of summer in Florida sometimes. Not as often or as long but yes it gets uncomfortably hot. Almost every home has a pool and AC. Remember, almost all population in Canada is very close to the border of the US. In my case I am 45 minutes away so might as well say it is the same location as northern US.
roblab
Active Member
A good many of us Tesla owners have been charging on 220v 50-amp circuits for years, from an outlet, and have never experienced any problems with that setup. An EVSE is a waste of money, and higher charge rates are unnecessary since we charge overnight. I bought and wired my own outlet in my garage. The outlet cost me (ten years ago) about $10, the wire not a whole lot more for #6 romex. I cannot understand why people need some fancy junk on the wall just to charge their car, when this set up has done the job for me since 2012.
The NEC simply doesn't trust NM cable as much as other wiring methods and imposes a tighter limit on its use. I don't think there's a technical justification for it.
There's also another overly conservative corner case in the NEC that happens to impact installing 48A continuous EVSEs using NM cable. Namely, as 55A is not a standard breaker size, it's fine to install a non-continuous load up to 55A on wiring with a 55A ampacity, protected by a 60A breaker. However, the continuous load on that circuit is limited to 80% * 55A = 44A, rather than the 80% * 60A = 48A. The latter would be more rational.
Cheers, Wayne
To be clear, you are charging using an EVSE, the portable one that came with your EV and plugs into a receptacle. So your rant is against higher amperage hardwired EVSEs, not against EVSEs.
Cheers, Wayne
Yes, but northern US is unlike Arizona or southern U.S., and humidity affects comfort but not heat dissipation. So 120F ambient days in southwest desert, often dropping to "only 90F" at night doesn't allow buildings to cool off much. So say a wire at 140F, only a delta-T of 20F, vs delta-T of 45F for 95F. Twice the delta-T for much shorter periods of time.
But anyways, just speculating, I have no actual idea about the code
Ampacity values are based on a 30C ambient and an insulation rating of 60C, 75C, or 90C. Ampacity is adjusted for ambient temperature in excess of 30C, and final ampacity is limited to the termination temperature limits (which will be 60C or 75C in practice) as well as any specific limits like the 60C ampacity limit for NM (which does have 90C conductors inside).
So for the case of #6 Cu, with ampacities of 55A, 65A, and 75A at 60C, 75C, and 90C respectively, as you are limited to 55A in the end but can start derating for temperature based on the 75A rating, you can afford a temperature factor of 55A/75A = 0.73 with no change in the final ampacity. Ampacity varies as the sqrt of delta T (since heat loss is proportional to delta T and resistive heating is proportional to the square of current, I^2*R heating), so the maximum delta T ratio that would allow a final ampacity of 55A would be (55/75)^2 = 0.54. Meaning that ambient could be up to 0.54 * 30C + (1 - 0.54) * 90 C = 58C = 135F while maintaining a 55A ampacity.
But just the idea that NM cable may be used in high ambient is not a justification for limiting it to the 60C ampacity, by itself. Since there is after all a procedure for accounting for high ambient. The idea that the less qualified, who would install wiring without considering derating for ambient temperature, are more likely to use NM cable than other wiring methods is perhaps a reason to penalize NM. Or, if you prefer, to impose a simpler measure for dealing with ambient temperature for NM cable.
Cheers, Wayne
I just looked at the wiring that was used in my solar/ battery install.
The approved plans from engineer say thhn #6 in 3/4 conduit. But, they used NM #6 instead. I am about 100 feet from inverters to generation panels.
The first installer of solar used a 75 amp breaker with #6 NM.
But when the second company installed, they used #6 NM rather than the Thhn and conduit on the plans. Further, the NM the second install used
is "smaller" over than the first. So, since learning, I am assuming the first used 90c, for the 75 amp breaker, but the second used 60c, which
would only be good for 55A.
So, with the above said, do I have an issue? I paid to have it done to code, but seems what they installed is not what is on the engineer plans?
And with the length and NM cable, concerned? And how does one tell if the NM is 60C, or 75C or 90C?
when is h2ofun’s annual NEM true up? I hope it’s in the summer. I think he can host an EV-chargeathon to blow through his NEM credits with some juicy suds.
If you see a thick cable with a plastic sheath, you need to read the text on the plastic sheath to find out the cable type: NM, SER or SEU, UF.
Modern NM (the printing will say NM-B) always has 90C conductors inside, but the NEC always restricts the final ampacity to the 60C value. Similarly for UF.
SE cable should have 90C conductors inside, and there is no 60C restriction, so depending on the terminations, the 75C ampacity may be useable.
If you're concerned something is wrong in your install, make up a one line diagram showing the feeders, the breakers protecting each feeder, and the exact details on the feeder cable.
Cheers, Wayne
Cool, knew someone here would have the technical details. So not sure I followed along with the exact formula, but I think you're saying US 55A limit works up to 135F, which is probably enough to cover an Arizona summer. Now if I plug in 65A, I think it comes out to 0.75* 30C + (1 - 0.54) * 90C = 45C = 113F. And 75A comes out to 1 * 30C + (1-1) * 90C = 30C = 86F.
So 65A is 113F which is plenty good for Canada summers that max out at 95F, but 75A at 86F is not per US code. So part of the looser Canada spec of 75A is based on their lower ambient temps, because still within US code; but part is based on them trusting NM more?
I can not understand why anyone would need to have more than a 1 bedroom apartment. I lived in one of those for a decade, and I am still here, so why in the world would anyone ever need more than that? /s
At least you are consistent in continuing to attempt tell people that what you chose is correct for everyone.
(note for anyone who saw this before I edited it. I will admit to editing the above statement a bit, because after I read it, I realized I would have had to report myself as a regular poster, to myself as a mod, and if I did, I would have likely moved my own post to quarantine (lol). I edited it to tone it down some.)
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Yes, except you have a typo, it should be 0.75* 30C + (1 - 0.75) * 90C = 45C. And 0.75 comes from 0.75 = (65/75)^2.
I would say it is all about trusting NM users not to misuse it and to apply the temperature corrections if required.
Cheers, Wayne
I cannot understand why someone who has owned Teslas for 10 years (and let's keep in mind that 10 years ago, Teslas were nowhere near mainstream and cost 6 figures) for some reason isn't willing to buy a high current EVSE. It's not like you really needed a Tesla in 2012 in the first place. Even if you haven't owned Teslas for 10 years and initially had another type of EV, EVs 10 years ago in general were not a mainstream item either, and were very expensive.
If you can afford to spend $50-150k on a Tesla, why is it such a big deal to spend another $500 (less than 1% of the cost of the car, mind you) or so on a fast EVSE to make sure you can take full advantage of its charging capabilities?
Cool! Hey, just an entirely theoretical question, but if a NM-B wire is entirely within a conditioned space, and thus ambient temps are mild, from the same engineering formulas, it should be able to handle larger ampacity. I don't want to use the term "up-rating", and I'm not saying code allows it, I'm just curious.
When we remodeled over a decade ago, we didn't consider the need for EV charging, there were no EV's and barely a PHEV at the time. However, when later installed a 240V outlet, and later a HPWC, in the garage, we were able to repurpose unused 240V circuits and breakers (for laundry or HVAC) that were run from the main panel through the space between the 1st and 2nd stories across the middle of the house to the garage. I have photos and know exactly where the wires run through the studs and interior gaps (surrounded by air, not insulation).
If the inside of the house were always kept below 86F, and so the gap between stories also below that, then #6 AWG should be able to handle 75A safely rather than 55A, based on temperature corrections? Again not saying code allows it, just what the engineering says. I'm entirely satisfied with my overnight charging speeds, so just curious...
#6 Cu with 90C insulation in 30C ambient with no more than 3 current carrying conductors in the same cable or conduit has an ampacity of 75A. Which means that under those conditions there's an adequate safety margin for the insulation temperature not to exceed its 90C rating and damage the insulation.
But the end to end circuit includes more than the conductor, and actually using #6 Cu to carry 75A in practice runs into a bunch of other limits. One is that if the #6 Cu is in the form of NM cable, there is an additional limit to the 60C ampacity of 55A. That's a code rule, not based on engineering to my knowledge.
Another is that the connections at each end of the #6 Cu typically have a 75C limit, so they are not supposed to get hotter than 75C. That is certainly true if one end is connected directly to a breaker. That will limit the circuit to the 75C ampacity, or 65A. If the other end is a receptacle, then you will have to check the receptacle rating to confirm that it is marked for 75C terminations. Otherwise you are limited to the 60C ampacity of 55A. The rule is that equipment rated at 100A or under, or for conductors up #1 AWG, is assumed to have 60C terminations unless marked otherwise.
So rarely does the 90C ampacity actually matter, other than as a starting point for elevated temperature or for more than 3 current carrying conductors in the same cable of conduit. Here's about the only plausible scenario in which you could use #6 Cu on a 75A circuit:
You have an existing detached garage supplied by a feeder in underground PVC rated for 90C conductors that has #6 THWN-2 conductors in it. It is currently supplied by a 60A breaker in the house and connected to a 100A main breaker in a panel at the garage (that breaker size is immaterial, the 60A at the supply end protects the feeder, and the load end breaker is just to provide the required disconnect.) You add load and need more than 60A at the garage. But the conduit is not large enough to pull in larger conductors. And the landscaping in between is well established, so you don't want to disrupt it to bury a larger conduit.
So you can intercept the PVC conduit near both ends and set junction boxes. At the supply end you junction to short #4 Cu conductors going to a new 80A supply breaker. At the load end you similarly splice to #4 Cu going to the existing 100A disconnect breaker. In each junction box you splice using a 90C connector (most wire connectors, but you need to confirm).
That lets you use the #6 Cu at its 90C ampacity of 75A. And as long as the calculate load on the feeder is 75A or under, you get to round up to the next higher standard size breaker and protect it at 80A. The breakers typically have 75C rated terminations, and #4 Cu's 75C ampacity is 85A, so that connection is fine. [Otherwise, #4 Cu's 60C ampacity is 70A, so you'd be limited to a 70A breaker.]
Cheers, Wayne
Some people might need more, but we have 2 Teslas and are fine on a 50A circuit.
For me, it is a question of hassle vs. want. I am pretty close on loads on my panel and the electrician did not want to sign off on going above 50A. And it turns out that 40A is fine.
Also, there are so many superchargers by us, we just Supercharge while shopping, getting bagels, going into Home Depot, etc. more often than charging at home.
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