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Trying to figure out amperage

Discussion in 'Model S: Battery & Charging' started by Cr8it, Jun 1, 2016.

  1. Cr8it

    Cr8it Member

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    My P90DL will be delivered Monday. I love everything about the vehicle, but find the amps confusing. My electrician came today and said h could put in an 80 amp circuit which I will have him hook up to a Tesla charging station. Am I overamping? Since I did not buy the dual charger, does 80 amps help me charge faster. I know this may be a basic question, but most f my adult years dealing with my vehicle fuel was easy. Pull into a station, swipe a card, select high octane, stand for a while and get back in the car with the faint smell of gasoline on my hands. I know the EV route is better, and my hands won
    T smell bad. I just find understanding electrical current complex.

    Any help appreciated.
     
  2. Chris TX

    Chris TX Active Member

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    Which car are you getting? There are four options, now:
    1. Facelift Model S with 72A onboard charger?
    2. Facelift Model S with 48A onboard charger?
    3. Non-facelift Model S with single 40A onboard charger?
    4. Non-facelift Model S with Twin Chargers, which is up to 80A?

    For the MAXIMUM charge rate for these four options, here's what you'll need:
    1. 100A circuit run to a HPWC (one of the newer ones, preferred)
    2. 60A circuit run to a HPWC (one of the newer ones, preferred)
    3. 100A circuit run to a HPWC (one of the newer ones, preferred)
    4. 50A circuit run to a HPWC or a NEMA 14-50 outlet (to plug in the UMC)

    ALL options will work on the solution #4, but would be ~30MPH of charge rate. If your car can only handle 40A, installing and 80 or 100A circuit will not help you charge faster. You might gain a tiny bit of charging efficiency due to the oversized wire, though ;)
     
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  3. aesculus

    aesculus Still Trying to Figure this All Out

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    Note that Chris TX flipped his 3 and 4 answers. 100A for the twin charger Model S.

    BTW a good rule of thumb is the circuit and breaker need to be 120% of the largest amperage draw. That is why he is calling out a 100A breaker even though the car can only draw 80A. Your electrician needs to be reminded of this because the car can draw this load continuously, not just for a few minutes at a time like a oven or dryer would do.
     
  4. davewill

    davewill Member

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    It's not 120%, it's 125%, or the way it's usually put, the charge rate can only be 80% of the circuit size. And it's not a rule of thumb, it's an electrical code requirement.
     
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  5. Boatguy

    Boatguy Member

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    Summarizing some of the good comments made here already, here are the basics for someone not familiar with electricity.

    Amperage measures the flow of current through a wire, irrespective of voltage
    The Tesla references to 40a/48a/72a/80a all assume voltage in the range of 200v - 250v, commonly referred to as "220".

    Voltage measures the push behind the flow of current, like water pressure a water pipe (aka PSI).
    Voltage x Amperage = Watts, the total power being delivered instantaneously
    Watt hours (Wh) is the total power delivered over a period of time. Like gallons / minute in a water pipe, it tells you how much will be coming out the end of the pipe over a period of time to fill the battery (or water barrel).

    For safety reasons, a circuit is wired to be capable of 125% more than the highest expected sustained current through the wire.
    A circuit breaker protects the wire from overheating and is sized to the wire installed.

    If you have a 40a or 48a charger, then you need a 50a or 60a circuit. A larger circuit will not provide any advantage or faster charging, but it will provide capacity for a larger charger in the future.

    To support a 78a charger you would need a 100a circuit, as an 80a circuit would only support a 64a charger. Your electrician may in fact be proposing to wire a 100a circuit, which would support an 80a charger, and the cost increase over a 50a or 60a circuit should be minimal. But you should clarify with him what he is actually proposing to install.

    Best of luck!
     
  6. Cr8it

    Cr8it Member

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    Thanks all of these answers have been helpful.
     
  7. aesculus

    aesculus Still Trying to Figure this All Out

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    Thanks for pointing out my error. Was coming at it the wrong way. :(
     
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  8. roblab

    roblab Active Member

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    It just takes practice and time. You've been pumping gas for most of your life. And never worrying about range, barely thinking about miles per gallon. Now you have new gallons: Kilowatt hours. About 30 make a gallon of gas equivalent, so a 90D carries about 3 gallons.

    Filling a 90D gas tank takes 90 kW. Pumping 10 kW per hour means it would take 9 hours. But how do you know how fast you are pumping? Well, you might as well learn it now, since it will never change. Volts x Amps = Watts. 220 volts at 40 amps. I do it in my head. 8800, or 8.8 kW. I get 225 volts at my house, so I charge at 9 kW. A full tank takes TEN hours. I don't care. I'm asleep. And it is pretty unnecessary to worry about a few percent loss here and there. Round it off.

    Since you know you get about 250 miles off your tank, you can pretty easily see that one hour gives you 25 miles of range. And you probably will never start at zero on your tank, to charge to clear full. Generally you will be charging about a hundred miles, which will take four hours.

    The more you practice it, the easier it becomes.
     
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  9. valkeriefire

    valkeriefire Member

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    Thanks Roblab, you just made this photo make a lot more sense. 244x47=11.45kw per hour. My car has the 48 amp charger and I'm using a 60 amp breaker.

    [​IMG]
     
  10. FlatSix911

    FlatSix911 918 Hybrid

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    Good summary, but it's important to note that the Model S is rated at 100 e-mpg based on the 3 gallon analogy.

     
  11. AWDtsla

    AWDtsla Active Member

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    Run a line rated for 90A, then use a 50A or 90A breaker depending on if you just get a NEMA 14-50 or HPWC. This lets you upgrade to a HPWC later too without pulling more circuits.
     
  12. FlatSix911

    FlatSix911 918 Hybrid

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    Lots of good information and FAQs available at Tesla Charging | Tesla Motors

    Charging rates for classic Model S.

    Miles per charge.PNG
     
  13. Rocky_H

    Rocky_H Member

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    Normally I'm not nitpicky and correcting-y about this, but when you are explaining units, it's important to use them correctly. You basically don't ever say kw per hour. That's not right, because the unit of kilowatt is a rate of energy transfer, so it already means amount of energy per hour. The unit for an amount of energy is represented by taking that rate of kilowatt and multipying it by a number of hours, hence kilowatt hour (kwh).
     
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  14. Rocky_H

    Rocky_H Member

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    Here's another cool thing you can do relating to that picture of the charging screen. If you go into Controls -> Settings -> Units & Format, you can select for the energy to be represented in either distance or energy units. If you switch it over to energy, your main range display will go to a percent number, instead of rated miles, and in that charging screen, it will show on the left that 11.45kW value, rather than 34 rated miles per hour.
     
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  15. roblab

    roblab Active Member

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    So, I guess you're right, but the whole world says kWh, not kW/h. I understand what you're getting at, being a retired physics / chem / math / biology teacher. But believe me, it was hard for me, too, to get this into my head. Gasoline (or gasolene if you're really, really old) is automatic, but electric charge is definitely NOT. Miles per gallon, mpg (not m/g), is now miles per kWh, not miles/kW/h. I was trying for simplicity. Being simple myself, it works for me.
     
  16. Rocky_H

    Rocky_H Member

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    Granted, I usually don't remember to get the mixed uppper and lower case right, and almost no one remembers it for CHAdeMO. That thing is just a beast.
     
  17. valkeriefire

    valkeriefire Member

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    I found this discussion very interesting. It led me to Wikipedia.
    Gasoline gallon equivalent - Wikipedia, the free encyclopedia

    Which led me to the gallon of gas BTU to kWh conversion. This chart is particularly interesting. My rate is .11 cents so a filling my "3 gallon" tank takes around $10'minus whatever is lost in the conversion process.

    Does anyone know the efficiency of charging? For home computers it's very easy, a 90% platinum rating computer power supply basically wastes 10% of its energy (which is really good, some computers have 60% PSUs).


    image.jpeg
     
  18. Boatguy

    Boatguy Member

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    I measured my charging at about 95% efficiency.

    I pay $.14 fully loaded and the 90D is 290Wh/ Rated Mile. So...

    $4.67 / (33,400/290*.95) = $.043/RM

    My Mercedes diesel averaged 32mpg. At $2.90/gal = $.09/mile

    About half the fuel cost.
     
  19. roblab

    roblab Active Member

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    If all of you interested parties would "invest" in a solar panel every now and then, like maybe one or two a year, even, as they pay for themselves in five or six years, in a few years you would not be paying for electricity at all, any more, and your solar power generating station would have been paid off. This happening would make you all so very not interested in the prices of gasoline or diesel, and 'way more inclined to think of the world as suckers and yourself as genius.

    I don't care what gas or diesel sells for. I don't worry about the per cent losses that happen during charging. My panels far and away supply fuel for my house, my well, my septic, my A/C, and my car. I haven't paid an electric bill for twelve years and I get money back from PG&E. This is a far better investment than buying a larger TV or a case of Old Hickory or a new watch.

    Food for thought.
     
  20. Rocky_H

    Rocky_H Member

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    ...says the guy who lives in Napa Valley, home of exorbitant electricity rates. It's faster to get it to pay off with that. I keep looking at it, but it's still 10-15 years payback time. Our 7-8 cents rate and less than $100 a month bill just make it take a long time. But then again, a small system would cover most of that, so not too much upfront cost.
     

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