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Do I need 400 amp service?

Discussion in 'North America' started by mspohr, Oct 24, 2015.

  1. mspohr

    mspohr Active Member

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    I'm in a bit of a quandary here and I was hoping to get some advice.
    I'm building a new detached garage for my Tesla. The plans (approved by the building department and the power company) call for a sub-panel in the garage.
    The house has 200 amp service with a 200 amp breaker panel. I figure my maximum electrical use if everything (hot tub, electric dryer, Tesla charging plus other misc use) is turned on at once is 20kW (80 amps). (Hot water and heat are natural gas.)
    Garage is to the point where we call in the electrician. He says no way he can put a sub-panel in the garage since I already have a 200 amp panel in the house. Says I have to have 400 amp service.
    Consult the electric company about 400 amp service. They say I have to buy a new transformer ($7000) plus underground wiring to get 400 amp service. I should note that my current electric feed comes from a power pole mounted 25 kW transformer which also feeds 6 other houses plus an ATT DSLAM... seems a bit dodgy.

    Do I need 400 amp service?
    Can I put a sub-panel in the garage on my current 200 amp service?
     
  2. MyJoule

    MyJoule Member

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    I'm sure you'll get a better answer from others, (Flaseherz comes to mind) but the main thing is you need to follow the NEC codes and do a proper calculation of the loads. There are some specific rules in the NEC ( National Electric Codes) for doing these calculations Once you have the calls completed, you'll know how much if any power you have left over for the detached garage. If you don't have enough available power, then yes, you'll need to do the upgrade, since the power company has so many loads on the transformer, maybe you can negotiate a discount, since they should be splitting up those loads anyway. Good luck and let us know what you find out.
     
  3. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Check out the calculations with a few sources, but here is a web page that claims to do the load calcs per the NEC. Be sure to read the instructions.

    Service and Panel Size Calculator
     
  4. CHG-ON

    CHG-ON Still in love after all these miles

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    Your electrician is on drugs. I only have 100amp service and have a 50amp sub panel in the garage. I time my power hungry appliances (hot tub, etc) to run when the car is not charging and I have absolutely no problem at all. With 200amps, you will be fine.
     
  5. aesculus

    aesculus Still Trying to Figure this All Out

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    If you are going to put in another panel, I would suggest you look at your rate plan options first. In my case I was bumping the 200 amps but could probably have gotten by. However the biggest reason to get another panel was to dedicate 100 amps to the car so it could be on its own rate plan and not cause me to use energy at higher tiers on my regular plan. The second panel (100 amps) gets a separate meter and your can have a different plan optimized for your car if your provider (mine is PG&E) has such a plan. Only the car(s) is on that meter, nothing else can be on it.
     
  6. mspohr

    mspohr Active Member

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    Thanks.
    I ended up using the calculator here:
    Residential Load Calculation Spreadsheet ~ Electrical Knowhow

    If I separate out the house and the garage, it tells me that I can get by with 100 amp service for each.
    The electrician's problem seems to be that he doesn't want to wire a 200 amp panel (house) and a 100 amp panel (garage) to my 200 amp service. Each would have their own breaker and there would be a main 200 amp breaker. I've found a bunch of people who say that is just fine.
    I may need a new electrician. I think I'll check with the building inspector and if he says it will pass, then I'm good to go. That may convince the electrician.
     
  7. Tuan

    Tuan Member

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    I have 200amp service panel and 100amp subpanel. It works fine so far. I only charge at 30 amps anyway to keep the heat low, but I have charged 80amps with no problems using HPWC
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    You're probably fine with 200A service, based on your description and the work you did on the load calc. Make the garage a subpanel off your main panel, then you won't have an issue.

    I could have kept the 200A service I had when installing my HPWC, but the power company had to do work anyway because I only had a 10 kVA rated transformer (40A sustained) serving my home, with 2/0 service cable (NEC calls this good for ~135A). Thye had to replace the transformer and suggested I put in a 400 meter base while they did the work to replace cable and transformer. Difference was that my co-op does everything up to the meter pan for free.
     
  9. Theshadows

    Theshadows Active Member

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    Get a new electrician. Running a 100a sub to your garage with a 50a breaker should be no problem.

    Also the utility's price for the transformer upgrade seems high. I have done two transformer upgrades for solar one was on a primary line up to a 25kva and it cost $2300 and another was a 50kva on a secondary line and it cost $1800.

    Did you get the price quote for the utility upgrade by calling the poco and they gave it to you right a way or did they give you a request number and you had to wait a few days for them to give you the price. A legit price would have to come from the engineering dept running calcs then them sending that info to get a price quote. This will take a few days at best. Usually a few weeks.
     
  10. hcsharp

    hcsharp Active Member

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    Get a new electrician. I have 200A service with a 125A sub panel in the garage and 80A charging. No problems with the inspection. I'm a little worried about your undersized transformer though. The power co put in a 25 KVA transformer just for my house.
     
  11. MorrisonHiker

    MorrisonHiker Beta Tester

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    I originally thought I was going to need to upgrade my 200 amp service and got similar quotes for $7000. I ran some numbers and calculated I could get by with 200 amps to the house and then feed 100 amps to the garage. I ended up putting in two 14-50 outlets and "saved" over $5000.

    I recommend you get a few more estimates.
     
  12. cottylowry

    cottylowry 2013 Model S

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    We charge our Tesla - single charger car - and Volt at the same time. WE have a 200 amp main panel in the house and only a 60amp sub panel in our detached garage -- had to string a separate overhead line with new mast on the garage. We seem to have NO problem simultaneously charging the Tesla and our Volt. I generally have the Tesla charge at around 1am
     
  13. mspohr

    mspohr Active Member

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    The utility quoted me the price at my first conversation without even looking at the property or consulting their engineers. I assume that $7000 is their standard price and from what you say, it's way too high.
    I don't think I need 400 amp service but I'm going to warn the power company that their 25kW transformer for 8 customers is woefully inadequate and that "they might want to do something about it" but I'm not paying for a new transformer.
    I've noticed that sometimes when I charge the voltage goes down to about 225 volts.
     
  14. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    It may fall on deaf ears, depending upon who you talk to. If you can ask them to get you someone in engineering, they might listen to you. And you may want to approach them with a different tack - tell them that you will be charging an electric vehicle and wanted to notify them in case they want to check their capacity.

    Otherwise, you may need to go this route:
    Incoming utility cable capacity - A cautionary tale - Page 2
     
  15. mspohr

    mspohr Active Member

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    That's quite an extreme measure to intentionally overload the transformer and watch it boil. I don't think I'd have the balls to do that.

    I've satisfied myself with load calcs that I only need 200 amp service. Now I just need to convince the electrician and the building inspector to install the proper panels and breakers.

    The power company is coming out Thursday to take a look and I will talk with them about my concerns about their 25kW transformer serving 8 customers. It should be their responsibility to size it properly but if not, I guess there is always the "boil" option.
     
  16. mspohr

    mspohr Active Member

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    Update.
    Today had the electrician and building inspector both here at the same time.
    I didn't want to confront the electrician and tell him he was wrong so I asked the building inspector "How can we do this?".
    His answer was that we can, indeed, have 200 amp service with two 200 amp panels, each connected with it's own breaker.
    The load calcs I did show that I'm well within 200 amps (100 + 84 demand load) for both buildings so 200 amp service should be fine.

    Everybody is happy now.

    Power company is coming tomorrow and I'll talk to them about the 25 kW transformer serving eight customers.
     
  17. AMPd

    AMPd Active Member

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    If the utility still decides not to upgrade the transformer then the only solution is to convince the other 7 customers into purchasing a Tesla
    8 teslas charging at the same time will surely make the utility see the error of their ways.
     
  18. mspohr

    mspohr Active Member

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    There is a certain clarity in seeing your transformer fry.
     
  19. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    I would say that you could tell the power company that it's either in the day time, or on overtime, because it will get overloaded; however, it's likely that'll fall on certain ears who would want the overtime.

    I am blessed that I'm served by a co-op and I personally know a good number of the staff who work for it.
     
  20. dmckinstry

    dmckinstry Model S - U.S. P - #1649

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    You may not need to put in a sub-panel in the garage, if you have available space on you panel in the house for an extra 50A breaker.
    Of course, you would then have to run a line/conduit from you main panel to the garage. That's what I had done. And the house in which this was done had electric heat and hot water heater.

    Talk to you electrician about the cost of running the line. Unless the garage is very far from the house, it has to be a lot less expensive than the new transformer.
     

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