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how much voltage drop do you see

Discussion in 'Model X: Battery & Charging' started by Birdcar, Jul 10, 2016.

  1. Birdcar

    Birdcar Member

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    When I plug in my car, I see an initial voltage of 240V at 0A, when the charge current climbs to 16A, it drops to 233V. That is a drop of 7 V, That means there is a resistance of 0.44 ohm. I know from this table(American wire gauge - Wikipedia, the free encyclopedia), #10 AWG has a resistance of 1 ohm/ 1000 ft. It is hard to visualize a 440 ft wire round trip to the panel. My guess is a 80 ft max routing distance from subpanel to the main panel. that leaves 0.28 ohm resistance unaccounted for. Is that the contact resistance in the system ?

    what is the voltage drop do you see ?
     
  2. Blurry_Eyed

    Blurry_Eyed MS Sig #267, MX Sig # 761

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    I use the Universal Mobile Connector plugged into a NEMA 14-50 outlet and when I charge either my S and X the voltage starts at 240 (or sometimes 241) and usually settles in at 236 volts at 40 amps. I don't know anything about how the electrician installed the plug so can't give you detail on wire size or length.
     
  3. ccutrer

    ccutrer Member

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    I see about a 6-7V drop at 40A. 6AWG wire on about a 100' circuit (so 200' round trip). I actually measured today, and from the UMC itself (plug to car), the voltage drop is about 2V (assuming the car and my multimeter are in close enough agreement).
     
  4. FlatSix911

    FlatSix911 918 Hybrid

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    Here are my stats with #10AWG to a HPWC:
    • 240V at 0A
    • 239V at 16A
    • 233V at 32A initial rate
    • 232V at 32A sustained
    IMG_5112.PNG
     
  5. Birdcar

    Birdcar Member

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    you guys have really good installations: 0.1 to 0.15 ohm resistance are great. that is what 6 AWG can do.

    Flatsix, your #10 line is only 0.25ohm at 8V drop. How long is your charger from the main panel ?

    I did additional measurement and found out that the voltage drop from the main to sub panel is 5.2V @ 16A, so it is 0.325ohm. It is still higher than a pure 80 ft x 2 round trip #10 wire resistance (0.16 ohm). still not sure where the problem is.

    The other 1.4V drop (ie. 0.09 ohm) is from the subpanel to the car, which is really short, may be 2 feet. I guess the multiple adapters are the culprit: 6-20r to 5-20r and 5-20p tesla adapter.
     
  6. FlatSix911

    FlatSix911 918 Hybrid

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    My run is about 80' from the main service panel.
    Measured 8V drop @ 32A is 0.25 ohm ... I can run a test at 16A this evening if it helps.
     
  7. ABCCBA

    ABCCBA Member

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    I use a HPWC with #2 copper and a 80A breaker on a 200A panel. I started out at 241VAC @ 0A the car ramped up the charging to 239VAC @ 40A
     
  8. Birdcar

    Birdcar Member

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    thanks, no need for 16A data point. Your #10 wire is very good. I am supposed to have the same wires and about the same length, yet 30% higher resistance.
     
  9. Birdcar

    Birdcar Member

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    That is really good setup. it is future proof too.
     
    • Like x 1
  10. K-MTG

    K-MTG Sunshade Captain of TMC

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    During the day mine charges around 242V on 48 amps at night it is around 232V, probably because of my solar
     
  11. Birdcar

    Birdcar Member

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    Definitely, part of the 48A flows from solar PV panels, which is likely attached to the garage and suffers very little voltage drops. if you have a large system like 10KW, then almost all will be from PV and you pull little from the grid.

    I saw the same on my PV system too.
     

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