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Voltage Cliff (Electrical Wire Length Limitations)

Discussion in 'Charging Standards and Infrastructure' started by EVATL99, Oct 8, 2017.

  1. EVATL99

    EVATL99 New Member

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    I just ordered my MS and am expecting delivery in mid-November.

    I live in a high rise condominium which recently allowed the residents to set up their own charging stations in their private parking spot. My parking spot is about 210 feet away from the main power source. I went on Tesla's website and set up time with two certified electricians in my area to give me quotes.

    Electrician A mentioned that since my parking spot is over 150 feet away from the power source, they'll need to use a different (more expensive) wire. He mentioned a "voltage cliff" would occur if they used the standard 6 gauge wire. He quoted me $4,300.

    Electrician B did not seemed bothered by the length at all and said that a 6 gauge wire will work just fine, and that a voltage cliff doesn't really happen until 500 feet. This electrician quoted me $2,500.

    I'm obviously inclined to go with Electrician B (purely based on the fact that he's quoting me 40% less), but it's hard for me to tell which electrician is right. I've done some research online, but I, admittingly, don't fully understand what I'm reading.

    Does anyone here have any insight on electrical wire length limitations and whether I will be doing any damage to the battery if the specs aren't right?

    Thanks in advance!
     
  2. boaterva

    boaterva Supporting Member

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    Cliff? Lol... (Old Electrical Engineer here...)

    Haven’t looked it up recently but I or you can: Voltage drop tables are well known and the NEC site is free if you use it online.

    You may need to go up a size (drop is a function of resistance) but the important thing is how much current are you wanting to run? 40 vs 60 vs 80 will matter. 6 AWG sounds like perhaps 60 (breaker size)?
     
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  3. EVATL99

    EVATL99 New Member

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    Thanks for the reply. When I'm looking at the quote from Electrician B, it says "235' feet of number 6 wire for 50 amp 240 volt e/v charger." It sounds like he will install a 50 amp breaker. Should I instead ask for him to install a 60 amp breaker in order to use the 6 gauge wire? Or should I ask him to use a larger size wire with the 50 amp breaker?

    Thanks again!
     
  4. McRat

    McRat Active Member

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    #6 THWN-2 150' 3 power wires max, good for continuous duty, wet or dry, 60 amps.
     
  5. boaterva

    boaterva Supporting Member

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    You need to spec the run for the max current (the breaker size). If you do 60 Amp breaker, the HPWC will be set to 48, if 50, the HPWC will be set to 40 Amps. (80% of max for continuous load, etc.).

    The issue here is that there is supposed to be a 3% max voltage drop to be within code (if I recall correctly) That’s this ‘cliff’, which doesn’t exist strictly. It’s a continous decrease as you get farther and farther away. What they may mean is that at some point 240 VAC equipment won’t work (and that’s the cliff). But you should be using large enough wire so that it has a voltage drop of less than 3%, and they should be happy to tell you how they got their size calculation. Beware if they don’t want to.

    Rough estimates on 4 gauge vs 6 gauge wire is about 50% more expensive (back of envelope calcs at Home Depot looking at 500’ spool prices per foot).

    Without doing the actual voltage drop over your 200 foot run to see what it is, you can’t tell if it is over the 3% number. For a continuous load where it will be running every night all night, you do not want to heat up the wires excessively as an excessive drop would do for the current load in question.

    I’d definitely have them calc it out and get the added cost; it’s only parts cost, the labor is the same. You might be able to have a higher current limit with the larger wire and get that benefit for the added cost.

    Good luck!

    (Added comment: the quote for 50 Amp may have already been derating the run from 60 to 50 to have it work for the 6 AWG wire for the extra length and not have the large voltage drop. If you understand the lower max load (40 Amps at the HPWC) then that would work also without getting the larger wire. It’s all trade offs for long runs, unfortunately!)
     
  6. GaryW

    GaryW Supporting Member

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    In my current house, I had my electrician put in a 50 amp 240v plug. I get 28-30 miles of range per hour of charge. He used 6 gauge wire, per the instructions on the Tesla install document. The run was only 25’, and I paid him $360 to put it in. The car draws only 40 amps on the 50 amp circuit. The 20% is the recommended safety margin.

    I could have put in a higher amp plug, if I needed to charge at a faster rate. My car supports up to ~80 amps, which would charge at 55-60 miles of range per hour of charge. I can’t imagine needing to charge that fast, unless a car supported two drivers.

    I just bought a condo in Florida. I hope to gain agreement to run a charging plug there, so congratulations on already getting permission.

    Many folks on the forum have been offered very high prices for putting in these plugs. There is nothing really that different putting these plugs in compared to wiring a stove or 220v electric hot water heater. It may be that electricians think you are rich if you have a Tesla.

    If you wanted, you could ask a regular licensed electrician the price to run the line, and see if you get a better answer.

    Good luck, and please keep us posted.
     
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  7. rabar10

    rabar10 FFE until Model 3

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    Are you using/installing a Tesla Wall Connector for this parking spot? That provides 48A for new Tesla MS/MX vehicles (ones without High Power Charger option). Need a 60A breaker to charge at max power (see earlier comments on 80% derating of continuous loads).

    Online voltage drop calculators show you needing 4AWG conductors to stay below 3% voltage drop at the rated capacity of the circuit. This is based solely on copper wire diameter and conductivity calculations.
    Voltage Drop Calculator

    I would be hesitant to use the cheaper electrician if he said he would run up to 500’ of 6AWG wire for this application — that would not only be out of spec, but (in the hypothetical case that you actually needed a 500’ run to your charger) would likely trip the Tesla voltage-drop detection while charging, and it would artificially lower your charging current in response...

    I second the suggestion to get more quotes/estimates from other licensed electricians.
     
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  8. EVATL99

    EVATL99 New Member

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    Thanks for all of your responses. This has been incredibly insightful and helpful.

    My plan is not to use a Tesla-branded wall connector, and instead use a vanilla NEMA 14-50 plug. That essentially removes one less variable, as I know I'll only be able to effectively use 40 amps when charging.

    That was my thought exactly. I will schedule a few more licensed electricians for this week. I've read on this forum that electricians need to send photos to Tesla to validate the install. If I use a non-Tesla certified electrician, will they still need to do this? What is the purpose of this step? Are there any risks of using a non-Tesla electrician?

    Now that I think about it, Electrician A added a $175 line item to the quote which says "Pull permit, schedule inspection, complete electric company rebate form." I'm not sure which permits they need to pull and who they schedule the inspection with. Is this related to 'registering' the install with Tesla?

    Thanks!
     
  9. BrettS

    BrettS Member

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    There are lots of reasons to just go with the 14-50 plug vs the Tesla wall connector, but I wouldn’t count that one. The wall connector can be connected to circuits all the way up to 100 amps and your electrician can simply set a switch inside the unit to match it to whatever circuit it is connected to.

    It sounds like you’ll be charging in a public space, so I might have some concern about leaving your UMC charger just plugged into the outlet all the time. Someone could come by and unplug it and steal it. I suspect that unplugging it from the wall and storing it in the car every day would get old pretty quickly. The Tesla wall connector being permanently attached to the wall might be less easy to steal. But this may not be a concern in your parking garage.

    That definitely shouldn’t be necessary for just a 14-50 plug. I think Tesla has a some sort of program where they will train and certify electricians to install their wall connectors and it’s possible that an electrician who is part of that program may need to send pictures of the installed wall connector to Tesla to show that he’s complying with the program, but the 14-50 plug (and frankly even the wall connector) are pretty straight forward installs and the sort of thing that electricians do every day, so I wouldn’t worry about not using a Tesla electrician.

    I’m sure this is to get a permit and inspection from your city, which should be necessary for any electrician that does work in your house.
     
  10. rabar10

    rabar10 FFE until Model 3

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    Looking further on the NEC voltage drop stuff — appears that the 3% rule is a “recommendation” and not a hard-and-fast code-enforceable rule. See Mike Holt Enterprises - the leader in electrical training.

    So sizing for a 40A actual load per the NEMA 14-50 install comment, 6AWG wire has a just slightly higher than 3% drop (using earlier calculator link), and is likely fine in this application.

    Disclaimer: IANAE (I am not an electrician) :)

    But now securing your UMC is a potential issue. A Tesla Wall Connector can be adjusted down to 40A max, to work in this application, then removed and swapped for a 14-50 when you move. Or, there are wedge devices that you can lay the UMC cord underneath then pull (summon?) your car tire over to hold the UMC cable secure.
     
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  11. GSP

    GSP Member

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    @EVATL99,

    Since you have a long run, I recommend not installing a 14-50, since it requires an extra conductor for the neutral pin. That conductor is expensive for long lengths, and not used for charging a Tesla, or any other EV.

    Tesla's HPWC would be a good choice. If you want to save money by using your UMC, have a 6-50 outlet installed. The 6-50 does not have the unused neutral pin.

    The 6-50 UMC adapter may be hard to find. If you can't get the UMC adapter, you can make or buy a 6-50 to 14-50 adapter to use with your UMC.

    Another option that might work is to cut off the neutral blade from your 14-50 UMC adapter. This might fit in the 6-50 outlet, if the remaining "hot" blades are not too wide.

    Good Luck with your install,

    GSP
     
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  12. wycolo

    wycolo Active Member

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    > My parking spot is about 210 feet away from the main power source.

    Is this a random spot they have given you, or all the 'charge spots' located in a general area? If the latter then bldg mgt should bring in a heavy power feed for each EV to source from.
    --
     
  13. Moonwick

    Moonwick Member

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    Congratulations on your imminent new arrival, and from another Atlanta resident, welcome to the club! Hope you enjoy your car as much as I've enjoyed mine. :)

    I didn't see anyone else address it yet but you explicitly asked about possible damage to the battery pack caused by using the thinner #6 wire. You can rest assured that the battery will not be harmed at all from any sort of voltage drop caused by a long electrical run. The only possible problem that you could have (and it's really more an inconvenience than anything) is that if the car detects a voltage drop beyond a certain threshold as it's ramping up its charge, it will scale back the charging amperage by 25%, which will result in slower charging.

    This scaling back of amperage is just a safety precaution that the car takes. In your case it would be perfectly safe to charge at a full 40A over that length of #6 wire, but unfortunately the car only has limited data to go off of---it can only see the voltage that is being supplied by the car, and in some cases a significant voltage drop can indicate that there's a "hot spot" somewhere along the circuit. The car can't be sure if the voltage is dropping just because it's at the end of a very long run of wire or if it's due to a problem, though, so it does the scaling back as a safety measure regardless.

    In my experience (I charge my MS in our detached garage, which is at the end of a lengthy #6 run back into the main circuit breaker inside our house) I'm able to charge at the full 40A with my UMC connector nearly 100% of the time. The only times I tend to see the amperage scaled back to 30A is when the voltage starts out a bit low, typically on hot summer afternoons when the electric grid is loaded down with everyone's HVAC.

    When I do encounter this though, the car will show a message in the instrument panel and you can also see in the app that it's only charging at 30A instead of the expected 40A. Most of the time when this happens I'm not in a hurry to charge so I'll just let it carry on at 30A, but if it happens and you're in a hurry you can get it to go back to 40A by disconnecting the charger and reconnecting it, and then going into the charging screen on the main control panel and increasing the amperage limit back up to 40A.

    Hopefully this clears up any worry you have about the battery, too! In my opinion you should have no trouble going with the electrician that spec'd out the #6 wire, since I'm using a run of #6 nearly as long as yours here at my house and I've had very little trouble charging my car with it.
     
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  14. rhumbliner

    rhumbliner Member

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    I concur. In a similar situation I installed a NEMA 6-50 receptacle (1 less wire) and made my own adapter (very easy). I also used aluminum wire since it was much cheaper and easier to work with. The aforementioned calculator will tell you what size you need. Also, since labor far exceeded the cost of materials, I sized the wire for 100amps so that down the road I could add a cutoff switch and upgrade to a HPWC.
     
  15. brkaus

    brkaus Active Member

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    #15 brkaus, Oct 14, 2017
    Last edited: Oct 14, 2017
    Model S/X NEMA Adapters

    Tesla has a 6-50 adaptor available in their shop again.

    Someone probably knows the wire fill off the top of their head, but may also save conduit size.
     
  16. ShockOnT

    ShockOnT ⚡️⚡️⚡️⚡️⚡️

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    I second this. The car will protect itself by switching off charging if a voltage drop is too great.
    This also has the added benefit of protecting the circuit, because the voltage drop can be due to heating of the wire if the gauge is too small.
    You really just need to run the appropriate wire gauge and you can have your charger at any distance.
    Go with the electrician that knows how to calculate wire gauge to prevent a voltage drop over about 3%. The thicker wire will cost more, but you can’t beat physics.
     
  17. arnis

    arnis Member

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    3% is just random recommendation. EU has another recommendation.
    But that is actually wasted energy (multiply by expected kWh per year and
    you get the amount of wasted energy, multiply by cost, make conclusions).

    I would likely keep the minimal 6gauge wire gauge for 48A continuous load and actually, on
    daily basis, charge vehicle slower. 32A for example. 50% and above is absolutely fine.

    This will avoid cable heating up, lessen that voltage drop, prolong equipment lifetime.
    In case of actual need you could accept extra losses above magical 3%.
    Though you will soon find out that you hardly ever need to charge faster than "within sleeping period".

    Tesla HPWC will definitely not react to 3%. Nor 5%. These are low fluctuation normal for everyday grid.
    This happens all the time due to other heavy equipment on the same transformer.
    Majority of the drop will happen at the moment charging starts. Cable heat buildup voltage drop will be minimal
    (ideally you want solo buried cable without conduit for best heat extraction, depends on cable type).

    The worst thing to do is oversize and pay unreasonable money for extremely small gains. This is how upselling work:mad:
    Buy 2 get one for half price. Though you only need one, this trick still works and many still buy 3 for price of 2½.
    Makes customers feel like they actually saved on this purchase.

    Nobody asks you to max out vehicle charging speed on daily basis;)
     
  18. ShockOnT

    ShockOnT ⚡️⚡️⚡️⚡️⚡️

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    Agree on the charge speed never needing to be faster than what will recharge your car during a normal sleep.
    I have two chargers at home, one is close to the meterbox and can pull 3phase 16A 240V, so 12kW. The other is in the garage and I can't pull more than 25A (about 7kW) before the voltage drop is too great and the car drops the charging session (240V to about 224V, about 7%).
    The garage is a more convenient and in a year I've only used the faster charger twice.
     

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