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FAQ: Home Tesla charging infrastructure Q&A

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Thanks for the great Q&A! This whole thread is filled with really interesting stuff that I don't understand at all, lol.

I'd like to make sure I understand what you were saying about extension cords.

My dad and I visit my grandmother's house once or twice a year, and if our family got a Tesla we'd want to be able to do the trip with the Tesla. It'd be nice if we could charge, even just a little, during the days we stay at her house. We can't rely on having access to any specialized connectors or anything like that, only the standard 3-pronged plug, which I guess is called a NEMA 5-15. And I guess that's at 120 V? But I always learned that home power stuff was at 110 V, is the difference important?

Anyways, there's usually stuff in the garage, so we'd need to plug into the garage or the house and run the cable out to the car, which is definitely gonna require an extension cable. You say that:

FlasherZ said:
For 120V, to keep the voltage drop within range, you can use a 14 AWG cord if your TOTAL one-way circuit length from the car to the breaker panel is < 50 ft., 12 AWG if your total is < 100 ft., and you'd need to go to 10 AWG if you're at 150 ft. or greater.

So then it sounds like to do this we would need to know how far the the cable travels in the walls of the house between where we the house's breaker box and the receptacle on the wall, and then add the length of the extension cable? If we need to get, say, a 10 AWG extension cable because of the distance, should we be concerned that the cables inside the walls of the house might not be thick enough? Also, what happens if we use a cable that's too thin? You say that this is necessary "to keep the voltage drop within range," which doesn't sound terribly important—shouldn't the UMC be able to handle that at a lower charging rate, or at worst, just not send power? Basically, if we screw up and use a cable that's way too thin, are we risking a fire (or other significant danger)?

Thank you so much and sorry about the barrage of questions!
 
I want to give a special shout-out to FlasherZ for this FAQ. It might have saved serious property damage or even a life.

I live on a rural property, total electric, and our house is the only one on this power stub, or whatever you want to call it. We built about ten years ago, and there's always bit a bit of a power dip when my HVAC kicked on or some other big draw. Because it had always been so, I just assumed it was normal.

Well, Flasher's post included this:

There are signs that your current service is being strained: if lights dim significantly when your air conditioning or heating unit starts or your electric range or oven is turned on, you may need to have the power company assess your service – in the US, they should do this for free.
So I called.

Technician came out, listened to me, didn't think there was a real problem, but I talked him into looking at stuff. We checked my panel, my meter and line in, and finally walked out to the transformer which feeds my property. He started that sick, horrified-amused laughter as we got close. I said, "That doesn't sound like that's what you expected it to look like?" and he started a lot of invective on whichever moron had done that work.

I don't know the field well enough to follow all of his outraged jargon, but the short version was, it was badly done. Bad enough that he had a team of a half-dozen guys there with a new transformer and all-new lines that same day, working overtime in the dark to finish at 10:30 pm. Apparently it was a serious safety issue, and I'd never known it should be different, and I would never have asked about it if not for FlasherZ's FAQ.

AND I now have a charger installed as well, so the FAQ served its intended purpose as well! Thanks!
 
The standard voltage is North America is 120/240 and has been so for several decades. I think it used to be 110/220 about a hundred years ago, old habits seem to die hard :)

For 120V extension cords, I would look for a 12 gauge one. 14 gauge will work for the regular Tesla 5-15 adapter, but it's at the limit. 16 gauge, no way. The cord will get too hot, outside its design limit.

I would suggest also buying the Tesla 5-20 adapter in case the garage you are in has a 20 amp receptacle. Will result in about 42% faster charging.
 
I want to give a special shout-out to FlasherZ for this FAQ. It might have saved serious property damage or even a life.

I live on a rural property, total electric, and our house is the only one on this power stub, or whatever you want to call it. We built about ten years ago, and there's always bit a bit of a power dip when my HVAC kicked on or some other big draw. Because it had always been so, I just assumed it was normal.

Well, Flasher's post included this:


So I called.

Technician came out, listened to me, didn't think there was a real problem, but I talked him into looking at stuff. We checked my panel, my meter and line in, and finally walked out to the transformer which feeds my property. He started that sick, horrified-amused laughter as we got close. I said, "That doesn't sound like that's what you expected it to look like?" and he started a lot of invective on whichever moron had done that work.

I don't know the field well enough to follow all of his outraged jargon, but the short version was, it was badly done. Bad enough that he had a team of a half-dozen guys there with a new transformer and all-new lines that same day, working overtime in the dark to finish at 10:30 pm. Apparently it was a serious safety issue, and I'd never known it should be different, and I would never have asked about it if not for FlasherZ's FAQ.

AND I now have a charger installed as well, so the FAQ served its intended purpose as well! Thanks!

Before and after pictures??
 
And I guess that's at 120 V? But I always learned that home power stuff was at 110 V, is the difference important?

Funny enough, the history here involves Edison and Tesla. It addressed a number of factors - from the best voltage to light filaments in light bulbs, to accounting for transmission voltage drop, voltage divisibility for multi-phase power, etc. Edison's DC system was specified at 110V to match the optimal voltage associated with light bulbs. Eventually, we ended up with the 240V split-phase standard in the US (and therefore 120V per leg).

(For what it's worth, the standard is +/- 5%, so anything from 228V to 252V is considered acceptable in the US. The power at my home is around 249-250V nominal.)


So then it sounds like to do this we would need to know how far the the cable travels in the walls of the house between where we the house's breaker box and the receptacle on the wall, and then add the length of the extension cable? If we need to get, say, a 10 AWG extension cable because of the distance, should we be concerned that the cables inside the walls of the house might not be thick enough? Also, what happens if we use a cable that's too thin? You say that this is necessary "to keep the voltage drop within range," which doesn't sound terribly important—shouldn't the UMC be able to handle that at a lower charging rate, or at worst, just not send power? Basically, if we screw up and use a cable that's way too thin, are we risking a fire (or other significant danger)?

Well, there are a few things that will protect you. If you use too small of a cord, the Tesla will detect too much of a voltage drop and either a) cut the charging current back by 25%, or b) refuse to charge at all. These are safety mechanisms built into the car.

With regard to knowing the distance to your panel, you really don't need to be exact. You just need to know that the longer the run is within the wall, the more sensitive the Tesla is to the voltage drop that will be seen.

As noted by Cosmacelf, go with a minimum of 12 AWG - some contractors' cords use 10 AWG.

- - - Updated - - -

I want to give a special shout-out to FlasherZ for this FAQ. It might have saved serious property damage or even a life.

Happy to help. Glad everything was ok.

This is unfortunately a common story. In older neighborhoods, it's common to find 2-3 homes connected to a 10 kVA transformer. The investor-owned utilities will generally wait until it blows up to replace it with something bigger, or they'll try to charge the consumer.
 
In older neighborhoods, it's common to find 2-3 homes connected to a 10 kVA transformer. The investor-owned utilities will generally wait until it blows up to replace it with something bigger, or they'll try to charge the consumer.

Just for giggles I took some pictures of the transformer feeding the houses around me. It's one transformer for four houses. The only numbers I could find on it are below.

Building Our House Dec 27, 2013, 1-42 PM.JPG
Building Our House Dec 27, 2013, 1-43 PM.JPG


Does that "100" painted on mean 100 kVA? could they have actually planned for 25 kVA power per house?
And while we're at it, what does kVA mean? I see UPS units rated at 1500VA but can only protect 865 watts of load. What am I missing?
 
Does that "100" painted on mean 100 kVA? could they have actually planned for 25 kVA power per house?
And while we're at it, what does kVA mean? I see UPS units rated at 1500VA but can only protect 865 watts of load. What am I missing?

kVA is just kilo VA. VA is what the industry uses because if they used Watts (which is what V * A is) they might be held to it. VA is more flexible so the marketing department likes is (similar to HP, but that's already on another thread). Someplace there should be a rating plate, although it might be inside the cover.
 
kVA is just kilo VA. VA is what the industry uses because if they used Watts (which is what V * A is) they might be held to it. VA is more flexible so the marketing department likes is (similar to HP, but that's already on another thread). Someplace there should be a rating plate, although it might be inside the cover.

It's not just marketing. You're ignoring the power factor (PF).
 
Just for giggles I took some pictures of the transformer feeding the houses around me. It's one transformer for four houses...
This is my transformer:
23596913412_704672b648.jpg

23077317274_f20392a52b_n.jpg

It isn't apparent in the picture but it is 15 kva (the "1" doesn't show well in the picture). Serves just my house in this very remote, rural neighborhood. As I recall I paid about $300 for the transformer installation seventeen years ago when my house was built; I wanted to have my meter on the house rather than four hundred feet away at the road, so that I wouldn't have to pay for the potential drop of that long cable run.

15,000 VA seems to be not all that much for one house much less several, as seems to be common elsewhere. If I used 40 amp level 2 charging that would be 9600 VA just for that, right? And 80 amp charging would be 19,200 VA and exceed the transformer capacity. Perhaps I don't understand how transformers are rated — definitely not my field of expertise!
 
Just for giggles I took some pictures of the transformer feeding the houses around me. It's one transformer for four houses. The only numbers I could find on it are below.

...

Does that "100" painted on mean 100 kVA? could they have actually planned for 25 kVA power per house?
And while we're at it, what does kVA mean? I see UPS units rated at 1500VA but can only protect 865 watts of load. What am I missing?

The 100 is 100 kVA. With resistive loads (heaters, ovens) or other loads with a power factor* of close to 1.0, such as modern EV chargers, kVA is very close to kW.

100 kVA for 4 houses should work well. Utilities work under different rules. They can run this transformer at 100% load continuously, and even more, for short periods of time.

As an example, I share a 50 kVA transformer with two other houses (three total) in Boulder. I have a 200 Amp panel in my garage exclusively for EV charging. As a test with multiple EV's, I loaded it up with 160 Amps total. 240V*160A is 38.4 kVA. The 240 V feed dropped a few Volts and the 200 Amp panel had a nice 60 Hz buzz going, but all was fine. This was while all other normal loads were going in the three houses.


*Power Factor - Wikipedia
 
Most power companies will tell you that you can run a transformer at 200% of rated capacity and not see major problems, except perhaps in the high heat of the summer desert. Just prior to getting the Model S, my home was served by a 10 kVA transformer and the power company saw times where I was bursting to 100A. I now have a 37.5 kVA transformer dedicated to my home.

I hope the kVA/kW explanation helps. Bottom line is that transformers must be able to handle both true power and reactive power, which is why they're rated in kVA. As noted, most homes have a relatively high PF, and so it's pretty close.
 
Yes... It's not the easiest subject to explain in layman's terms. It was once taught to me this way: imagine a handle connected to a shaft on one of those souvenir penny-smashing machines... The energy you put into that crank is real power because it all goes to the load (smashing the penny).

Now attach a heavy weight to the crank such that you need to work harder in the upstroke, but on the downstroke, the weight makes it easier and so you have to use less energy. That's reactive power, you've stored it in the weight. It hasn't been applied until the crank makes its full revolution.

Still not perfect but is a start to understanding it.
 
Yes... It's not the easiest subject to explain in layman's terms. It was once taught to me this way: imagine a handle connected to a shaft on one of those souvenir penny-smashing machines... The energy you put into that crank is real power because it all goes to the load (smashing the penny).

Now attach a heavy weight to the crank such that you need to work harder in the upstroke, but on the downstroke, the weight makes it easier and so you have to use less energy. That's reactive power, you've stored it in the weight. It hasn't been applied until the crank makes its full revolution.

Still not perfect but is a start to understanding it.

I always found this DOE paper with a horse pulling a railway car from the side of the tracks was a good analogy...
 
I always found this DOE paper with a horse pulling a railway car from the side of the tracks was a good analogy...

That's a close analogy to the actual calculations, but it can cause a layperson to think incorrectly about it. In the horse and railcar analogy, pulling at that angle is going to consume the horse's energy because of the friction of trying to pull the cars off the rail, whereas that doesn't really happen with reactive power. In the reactive power scenario, you're "lending" power to the reactive load, but eventually the magnetic field (inductor) or power field (capacitor) is going to give most of the power back to the circuit -- that's why I use the weight-attached-to-crank analogy myself.

I do agree that the rail analogy works for describing the size of the distribution network required (size of chains in the analogy) but in terms of actual energy consumed, it doesn't mean anything. This is how companies scam homeowners with PFC devices - unless your power company is charging you for low PFC, they will save you very, very little.
 
In the reactive power scenario, you're "lending" power to the reactive load, but eventually the magnetic field (inductor) or power field (capacitor) is going to give most of the power back to the circuit -- that's why I use the weight-attached-to-crank analogy myself.

For sure, but I just liked how it showed the angles comparable to the X (real) and Y (reactive/inductive) power diagram. We have a guy here that uses beer and foam in a glass as his analogy, but that never seemed clear to me. Probably because I had to keep re-setting the experiment by draining the beer :tongue:
 
For sure, but I just liked how it showed the angles comparable to the X (real) and Y (reactive/inductive) power diagram. We have a guy here that uses beer and foam in a glass as his analogy, but that never seemed clear to me. Probably because I had to keep re-setting the experiment by draining the beer :tongue:

On come on, how hard is the integral of the product of two sinusoids with varying phase angle offsets, going from power consumed to no power transfer to power produced...just kidding. :biggrin: