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Thread: Voltage Drop

  1. #1
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    Icon5 Voltage Drop

    A question for all electricity expert out there.

    I am basically parking in my appartment building, in the basement. The landlord was kind enough to let an electrician install a dedicated circuit up to my car.

    Basically the cord is 228 feet long (I know it is long) and 8 gauge.

    I check the Voltage at my plug, when nothing is connected, using a Voltmeter, and it is around 210-211 Volt (so I am assuming it is a 208 V circuit?).

    So here goes:

    I plug to a NEMA 14-30.

    When the charging it set at 15 Amps, the Voltage on the Tesla dashboard reads 202 V ish.

    When it is 18 Amps, it reads about 200-201 V

    At 20 Amps, it reads 198-200 V

    At 24 amps (the max), it reads 195 - 197 Volts.

    Are these acceptable loss? Can I charge over a long time at 24 amps without fearing that something wrong will happen?

    What is the lowest "acceptable" volts reading?

  2. #2
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    Your voltage drop is a bit higher than I would expect based purely on a 230 ft #8 run, but I suspect there's more to that run than you know.

    Voltage drop across a conductor is V=IR (Ohm's law). Typical resistance of #8 wire is 0.6282 mOhm/ft, so a 230-ft one-way (460-ft round-trip) conductor path would be 290 mOhm. I=24A, so 24A * .290Ohm = 6.96V. Now, that just takes into account that part of the circuit, it may be attached to a subpanel in the garage that is connected to a main panel via #6 or #4 or something that would also contribute to the drop.

    There are only 2 things you need to worry about with voltage drop:

    1. The drop is wasted power in the form of heat dissipation across the conductor. Joule heating law is P=I^2*R. If I assume (based on your ranges above) that you're seeing a 15 volt drop at 24A, that means the resistance of the path is 0.625Ohm. 24A*24A*0.625Ohm = 360W dissipated as heat, which means every 2.78 hours charging is wasting 1 kWh in heat dissipation across the circuit. That's about $0.05 per hour of charging at a ~$0.14/kWh rate. An extra nickel an hour? Probably not too worrysome.

    2. Significant voltage drop (far beyond what you have) is typically representative of a high-resistance path somewhere, which would likely generate A LOT of heat somewhere. If you see a very high voltage drop (as I understand it the car will recognize this and give you an extension cord warning after failing the charge), you want to have the circuit inspected while charging to ensure there's not a loose connection somewhere. I don't think your situation qualifies here for that.

    Based on what you're telling us, I believe that you are seeing the effects of a very, very long circuit. The electrician already had some idea that you'd see voltage loss, because he used #8 wire and is only required by code to use #10. The upsize is an attempt to reduce effect of voltage drop.

    The lowest "acceptable" voltage that the car will take is 85VAC. Of course, at that voltage you'll only be charging at 2 kW, and it'll take you nearly 40+ hours or so to charge the car from empty.

    I typically select conductor sizes to keep voltage drop below 5%. NEC suggests we use 3% as a tolerance. You're at about 7%. It's reasonable, IMO. The worst part is that your voltage loss represents only 4.68 kW going to the car instead of 5 kW, and so you'll be charging a bit longer.

  3. #3
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    What voltage does Tesla read before it draws current? There are a few seconds before it starts, where the dash will tell you the voltage at 0 amps.

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    Quote Originally Posted by Cosmacelf View Post
    What voltage does Tesla read before it draws current? There are a few seconds before it starts, where the dash will tell you the voltage at 0 amps.
    I checked it today: 210 Volts.

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    Industrial 3 phase is about 208v here, and I see some drop (looking at the S dashboard) once I plug into a commercial charger (sometimes down to 197v), so no surprise if the voltage drops at both the source and also at the other end of the cable.
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    Thanks for the replies so far, very useful information.

    One thing that worries me is that the outlet becomes unusable.

    Let me explain: last year with my Volt I was plugging it on a 120 V outlet, using quite a long (60 feet It hnk) AWG 12 extension cord. At first it worked well, then after a few months the Volt would stop charging and the EVSE would start to blink. After posting my problem on the Volt forums some people said that because of the heat generated, over time the outlet / cord had more and more degradation / bad connection and the EVSE detected this and stoped charging.

    Can it happen if the Voltage drop to 195-198 ish? (Is the heat generated too much, or even dangerous to not only degrade the wire but also, say, start a fire?)

    (P.S. I know the simple solution would be to lower the amp to say 15 amps, but sometime I would use the faster charge rate of 18-24 amps...)
    Last edited by punkchip; 2013-09-21 at 06:30 PM.

  7. #7
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    Quote Originally Posted by punkchip View Post
    One thing that worries me is that the outlet becomes unusable.
    The wall receptacle has a number-of-plugins life. Replacing it is cheap and is what I'd try first. Heating is generally caused by bad contacts.
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    Quote Originally Posted by punkchip View Post
    One thing that worries me is that the outlet becomes unusable.
    In theory, yes, but very unlikely. The problem with 120V extension cords is that 99% of them are very cheaply made and the plugs can get very hot (not due to voltage drop, but rather due to continuous current).

    Most 14-30 receptacles and plugs are very well built compared to your typical household plugs. Copper wire should not degrade from heat at legal current levels to create a problem. Bad connections can cause a problem, but that can happen regardless of your length of circuit.

    Do you have problems with the Tesla stopping its charge and turning the LED ring red? If the car detects a significant voltage drop (we don't know the threshold, but we have seen photos of the warning) it will stop charging.

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    Quote Originally Posted by FlasherZ View Post
    In theory, yes, but very unlikely. The problem with 120V extension cords is that 99% of them are very cheaply made and the plugs can get very hot (not due to voltage drop, but rather due to continuous current).

    Most 14-30 receptacles and plugs are very well built compared to your typical household plugs. Copper wire should not degrade from heat at legal current levels to create a problem. Bad connections can cause a problem, but that can happen regardless of your length of circuit.

    Do you have problems with the Tesla stopping its charge and turning the LED ring red? If the car detects a significant voltage drop (we don't know the threshold, but we have seen photos of the warning) it will stop charging.
    Ok thank you. No I never had problems so far with the Tesla stopping its charge (on 15 Amps, anyway).

  10. #10
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    Quote Originally Posted by punkchip View Post
    Ok thank you. No I never had problems so far with the Tesla stopping its charge (on 15 Amps, anyway).
    I think you'll be fine. Enjoy the car! And welcome to TMC!

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