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Older HPWC - settings & circuit breakers

Discussion in 'Model S: Battery & Charging' started by Pollux, Dec 20, 2016.

  1. Pollux

    Pollux Active Member

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    Dear TMC,

    What is the right thing to do with the DIP switch settings on an older HPWC to achieve 72A charging for a new P100DL?

    Details:
    Appropriate gauge wiring (in a trench) from basement main panel to garage -- 100A capable, protected with 100A breaker
    Sub-panel in garage
    Circuit from sub-panel to HPWC, protected with an 80A breaker
    HPWC is what I think of as a first-generation or maybe 1.5 HPWC, with DIP switches to select breaker setting
    I have set the HPWC to the 80A breaker setting (page 12 of its manual)
    Result: car achieves 64A charging but refuses to go beyond 64
    Note: I never tried this with my 2013 P85+, for which I only had a 40A charging circuit. But now that I've got the P100D, and even though its dual chargers only achieve 72A rather than the full 80A of my older car, I'd like to get every last charging droplet :)

    I am thinking that the car automatically limits itself to 80% of the selected breaker setting.

    I thought about changing the sub-panel's breaker to be 100A instead of 80A... but I've already got a 100A breaker set on the main panel for this one circuit to the garage.

    Is it reasonable instead to set the HPWC DIP switches for a 90A breaker? 80% of that is 72A, which is what the car will draw sustained. And then I've got the 100A breaker on the main panel to back that up.

    Or is there a better / more appropriate / safer way?

    Thanks,
    Alan
     
  2. Hammer@OR.US

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    If the breaker feeding the HPWC is 80 amps then 64amps is the max that it can feed (80% of 80). You can only increase the breaker size and the DIP settings if you have the appropriate sized wire to accommodate this. You can not just increase the DIP settings because you will pop the 80amp breaker.
     
  3. Pollux

    Pollux Active Member

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    Hi, @[email protected],

    Thanks for your quick response!

    I had the whole setup wired with appropriate gauge wire to handle 100A out to the sub panel and 80A out to the HPWC.

    Are you implying that it would be reasonable to change the breaker on the sub panel from 80A to 100A?

    Thanks,
    Alan
     
  4. davewill

    davewill Member

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    No, he's saying that since you wired the circuit to the HPWC for 80a, you have to set the HPWC for an 80a breaker, and be limited to 64a charging. Without rewiring the circuit, you can't use a bigger breaker, and without a bigger breaker, you can't go any higher.
     
    • Informative x 1
  5. Hammer@OR.US

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    No, exactly the opposite. You are limited by the smallest part of the circuit, the wiring of the 80 amp part. If the wiring from the subpanel to the HPWC is sized for only an 80 amp breaker that's what you are limited to.
     
    • Informative x 1
  6. Pollux

    Pollux Active Member

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    Ah.

    OK, but: I DID have the circuit wired (all the way from the house through the sub panel over to the HPWC) for 100A. Since I wasn't initially going to charge higher than 64A, I had the electricians put in an 80A breaker at the sub-panel (there's already a 100A breaker at the house). But if I follow you, all I have to do is change the sub-panel breaker from 80A to 100A and change the settings on the HPWC (given that I already have wiring in place capable of handling that load).

    Amiright? :)


    Thanks!

    Alan
     
  7. Pollux

    Pollux Active Member

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    Agreed.

    But: I had them wire to support 100A, only installing an 80A breaker because I didn't think I needed the higher amperage yet. (Of course, the wise man would revisit this discussion with his electrician, verifying that the sub-panel-to-HPWC segment really is rated to handle 100A as I originally specified.)

    Thanks!

    Alan
     
    • Like x 1
  8. Boatguy

    Boatguy Member

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    This subject creates a lot of confusion, which I will attempt to clarify, but probably only heap on more!

    - the circuit breaker should be sized to the weakest link it is to protect, usually the wire or an outlet, irrespective of loads.
    - the maximum sustained load, or current, is 80% of the circuit's weakest link, and thus usually it's circuit breaker.

    If the wiring to the HPWC is rated for 100a, then you can use a 100a breaker and set the HPWC to deliver 80a.

    In any case, the load (e.g., Tesla charger), will only draw what it wants, so plugging in the new P100D (congratulations!) will still only draw 72a and everything should be just fine.

    But as you wrote, the wise man will get the counsel of a licensed electrician, not the collected wisdom of fellow owners!
     
  9. Pollux

    Pollux Active Member

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    Hi, @Boatguy,

    I felt the need to have a better appreciation of the issues before round #17 with the electrician.

    For example, I had specified from the beginning that I wanted a 100A capable circuit all the way from the house through to the car. Confusingly for them, I said that I only initially would charge at 40A or maybe 60A. I made the mistake of saying that because I had a load test done on the house, and at peak I was using about 130A out of my 200A service. So, I figured I'd charge the car at, say, 60A so that NO MATTER WHEN I went to charge the car, I wouldn't have to worry about overburdening the entire house service. So, no problem: size everything for 100A, configure it to run at roughly 60A, revisit the issue when I had a better understanding of the loads and/or chose to upgrade the house service.

    The very day that they were about to lay the wire in the trench they had just dug, I happened to see a post on TMC about various wire gauges. I thought, well, that's interesting, let's go down and talk to the electricians about that. And lo and behold, I found out that they had decided they were installing a 60A circuit and had sized all the wires and other components accordingly. Thus, we stalled for two days while they re-calculated their equipment and price and then I signed off on the new, what-I-had-originally-requested, 100A all the way from main panel through to sub panel through to HPWC. With a 100A breaker at the house and an 80A breaker at the sub panel.

    So, worst case in front of me now, is that they screwed up on the sub-panel to HPWC wiring. That will be a much cheaper repair than digging up the cable from the house to the sub panel.

    But most likely at this point is that everything from the sub panel through to the HPWC is sized to handle 100A, and then the HPWC can feed 72A to my new S (once I have them swap the 80A sub panel breaker for a 100A breaker).

    So the upshot is that I appreciate your clarifications, as well as the contributions of the others in this thread, and feel ready to have this discussion again with the electrician.

    Thanks,
    Alan

    P.S. OK, worst case, I just continue to charge at 64A. But, hey, the extra 8A means that I can recharge just a little bit faster, which is so handy when I arrive home so late at night from visiting one of my mistresses, but have to get up early to do some work.
     

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