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Available capacity with 125amp breaker at home

Discussion in 'Model X: Battery & Charging' started by noahsw, Aug 22, 2017.

  1. noahsw

    noahsw Member

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    I've had two electricians come to scope out work on my townhome installation. The first claimed I only had 20amps available but the other claimed I had 50. My heating and stove are gas-powered.

    How do I know which to trust?

    If I use the UMC with a NEMA 14-50, will the car automatically use what's available without tripping the circuit breaker? Any risk of damaging the car, charger, or breaker?

    thx!
     
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  2. kort677

    kort677 Banned

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    no, wrong! you need a dedicated 50 amp breaker for this. a good electrician is necessary for this job. if your system does not have the capacity there is little that can be done other than expanding your capacity
     
  3. CmdrThor

    CmdrThor Active Member

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    Ask them to show you their load calculations. Post both here and someone can probably weigh in on which is more accurate.
     
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  4. boaterva

    boaterva Supporting Member

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    This... they should be doing the same load calc and getting the same answers. Very suspect what's going on here. A third one would be interesting to see if it's even close to one of the others!

    The first question is, what's your overall capacity? See the FAQ in my sig for tons of info, but if you have 150 or 200 Amp service, and no electric stove or heating (and you don't have airconditioning, most likely, there, right?), I would think you have tons of capacity). Hard to see where you are using all of your Amps.


    Get a good electrician to do a full load calc and show you what you have available.
     
  5. noahsw

    noahsw Member

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    Thx for the input guys. And great FAQ @boaterva ! That homeowners insurance stuff was eye-opening.

    A couple other notes:
    1. The guy who said I'd be fine charging at 50amp was found using Tesla's Find an Electrician. He's done several Tesla home charger installs per week for a couple years. The other guy is also an EV-specialist who's done many of these as well.
    2. Neither electrician provided a load report. Lots of hand-waving their numbers but it sounds like I need to push them on something more concrete.
    3. My overall capacity is 125amps. That's what my breaker says so hopefully I'm answering your question correctly @boaterva.
    4. I actually do have a portable AC unit that I run at night in the master bedroom.
    5. I am getting a 3rd company coming on Thursday so will be interesting to see what they say.
    6. The 50amp guy says that the vast majority of his customers use the UMC. He said, "only get the wall charger if you want the Tesla logo". I was surprised by this given what I've read on the forums.
    7. Not sure if it matters but my electricity comes through my connected neighbor's townhome. His unit has all the power meters.
     
  6. boaterva

    boaterva Supporting Member

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    Uh... power comes from a neighbor? Okay, that's sorta weird... And a wall AC can't be too large since it's running on a 15 or 20 amp circuit.

    Definitely need a load calc to see what you really have available. I can't believe a Tesla-suggested electrician didn't do one, mine sure did. Took a week of back and forth before we decided a 100 Amp circuit was okay for my two (load shared) HPWCs.

    As for the HPWC(s), people want them for the higher output (faster recharge for the maybe rare times you need to 'refill' and go), for sharing the circuit with two cars (or more), and for having the permanent installation. The ' Tesla logo' is on the UMC also. :D
     
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  7. CmdrThor

    CmdrThor Active Member

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  8. noahsw

    noahsw Member

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    I'm in a 4-unit townhome. I share a wall with one of the other units, which is also the unit that has all of our power meters. It wouldn't be the first weird design decision that the builders made ;-)

    You're right on the AC unit. I looked it up and it draws 9.5A.

    I don't drive enough to need the fast recharging and I'm probably a ways away from having two EVs.

    I've pushed these electricians for a load calculation so we'll see what they say.
     
  9. brucet999

    brucet999 Active Member

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    There is nothing weird about the meters being installed on the wall of one unit of a 4-unit building with main feeds passing through walls to the other units. They had to mount them somewhere, didn't they? It is cheaper for the poco to bring all four lines into one place.

    Since you have no electric stove, heating nor central AC, your panel may have only single breakers in it, no 240V circuits, unless there is a 30A dryer outlet somewhere. The first electrician may have meant to say that you had only one slot unused in the panel, which enables adding one 20A 120V breaker, but that would just be a lazy response. By replacing two single breakers with a tandem one, he could free up slots for a 240V 50A breaker.

    125A service sounds quite ample for a townhouse and I would think it unlikely that you would ever be using more than 2/3 of that at any one time. My 2400 sq ft house has run on a 100A panel quite nicely for the past half century, including power tools in the garage and charging my MS on the 30A dryer outlet. Even if you were charging at peak load time instead of the more usual middle of the night, at 80% of capacity there would be 60A available for the rest of the house loads.
     
  10. boaterva

    boaterva Supporting Member

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    The way it was phrased, you were sharing the meter with someone. Having all the meters at one end is, as was said, a normal practice so they can all be read at once. You still have your own feed, breakers, etc.
     
  11. Rocky_H

    Rocky_H Active Member

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    I also have a 125A main service at my house. You said your heat and stove are gas, so no issue there. You mentioned a plug-in air conditioner, so no central A/C? Other possible large electrical loads I could think of would be water heater or clothes dryer outlet.

    No, none of this will damage the car, charger, or breaker, but you do need to find out what size circuit you can really put in there. Load calculations are pretty standardized, so they should be able to show their work, or you can run one and check it. A load calculation does have an option called "non-coincidental loads", where you're pretty sure that two appliances will not run at the same time, so you don't have to count them both at the same time toward the total. I have an electric dryer and an electric stove, but I'm not going to be cooking or doing laundry after 1AM when the car is charging, so I don't have to add all of those together toward the total.

    As to the wall connector, it does have another good use besides just being for high power. There are more common 240V outlet types for some amp sizes, like 30A or 50A, but if you are limited to something small, like a 20A or 40A outlet, Tesla does not sell a plug for the mobile connector for things like that. The wall connector is great for being flexible with settings for 15A and up, so if all you can fit is a 20 or 40 amp circuit, I would recommend to just get a wall connector to have it done right instead of trying to use pigtail adapters and turning down the current in the car and junk like that.
     
  12. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Quick question - when Americans describe mains service, what exactly is being discussed? On my land I have "100A" service, but this means 3-phase 100A @ 230V, which is something like 35kW. In the US if they say "125A service", what is meant by that? I'm more familiar with what you get on the other side of the breaker box.
     
  13. brucet999

    brucet999 Active Member

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    #13 brucet999, Aug 23, 2017
    Last edited: Aug 23, 2017
    North America electric power for residences is 240V single phase 60Hz with a center tap on the power company's transformer connected to a neutral conductor. We have three conductors coming to the house main service meter, two sides from the 240V transformer and a neutral. Potential between the two "hot " leads is 240V; between either of the hot leads and neutral is 120V.

    After the meter we have a local distribution panel (nowadays mostly with the meter socket built in, but electrically after the meter) with two hot buses and a neutral bus, which is bonded to ground (earth). Capacities of these service entrance panels has grown over the years from the early 20th century standard two circuits of 15 or 20 amp 120V fuses, to mid-century circuit breaker panels with a main breaker feeding bus bars commonly ranging from 60A to 100A capacity. Connected to the bus bars are various branch circuit breakers ranging from 15A 120V for lighting circuits and 20A 120V for convenience outlets on every wall, to various sizes of 240V circuits for clothes dryers, heating devices, water heaters, stoves, ovens, etc. ranging from 20A up to 50A. Nowadays, service entrance panels below 150A are rare, most new houses have 200A or larger service and some go to 400A.

    Commercial and industrial buildings commonly have 3-phase 480V main service.
     
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  14. Rocky_H

    Rocky_H Active Member

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    Well, that is more accurately called "split phase". The use of the term single phase is kind of sloppy but is used frequently. That would be if you only had access to the + and - hot lines but not the neutral. It's not used for a whole house, though, because you can't make a 120V circuit with just that.
     
  15. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Okay, so 125A is only 30kW in the best of circumstances, and with more realistic circumstances - say 230V actual, power factor of 0.9 - 26kW? Oh, and I know US sockets are named for peak loads, but for sustained loads it's only 80% of the peak. Is the 125A sustained or peak?

    For my lot that's 230V * 100A * 0.9 power factor * 1,73 (3 phase) = ~36kW. So 125A looks pretty low. Then again, a lot of people here only install 50A 3-phase which is ~18kW with a 0.9 power factor, so I guess it's not that weird.
     
  16. Rocky_H

    Rocky_H Active Member

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    Right, in the U.S., the named circuit rating is for peak, and sustained long-term loads can only be at 80% of that. So the 125A would be a peak level.
     
  17. noahsw

    noahsw Member

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    Attached are the photos of my circuit breaker. I do have a 30amp dryer which you can see by the tandem circuit.
    IMG_6698.JPG

    FullSizeRender 2.jpg
     
  18. Fiver

    Fiver Member

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    Gosh I wish my panel was labeled that clearly. I have no idea where half my circuit breakers feed to.
     
  19. ShockOnT

    ShockOnT ⚡️⚡️⚡️⚡️⚡️

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    Just FYI, if you connect the car to a line that can't take the load the car usually can detect this by the voltage drop as the wire heats up, then will reduce amps accordingly.
    This is NOT recommended!!!
    Get your electrician to explain their load calculations. If your heating and stove are gas, there shouldn't be much else that's particularly high load. Clothes drier? Water heater? Pool heater?
     
  20. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Shut off the 20-foot tesla coil on the roof, then you should be fine. ;)
     
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