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HPWC wire gage

Discussion in 'Model S: Battery & Charging' started by Atebit, Sep 2, 2015.

  1. Atebit

    Atebit Member

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    I'm building a house & I had my electrician run a dedicated 100A circuit for my HPWC. In my case, the service entrance & breaker panel is on the side of the house opposite the garage. So the total run of the wiring is probably around 80 feet or so. I know he used copper 3 AWG as called for in the install guide, but for a run that long I'm concerned about voltage drop and heat buildup along the run.

    I looked up up the resistance/foot of 3 AWG and came up 0.016 ohms total assuming 80 feet and 0.020 ohms total for 100 feet. So worst case that works out to about a 1.6V drop assuming 100 feet and the HPWC drawing 80A. Doesn't sound bad, but that's still 128W of heat. But then again, given it's a straight run and not coiled up, I guess it's really only 1.28W/ft...
     
  2. Soolim

    Soolim Member

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    It is a one time cost, if it makes you feel more comfortable, why not just upsize it, and use #2 AWG copper conductor? Wires are cooler, lower voltage drop.
     
  3. KJD

    KJD Member

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    The Tesla home charging FAQ has a section on wire size for the HPWC and it has some good detail on the subject.
    FAQ: Home Tesla charging infrastructure QA

     
  4. wk057

    wk057 Senior Tinkerer

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    While I prefer #2 for the HPWC, #3 at 80 ft / 80A / 240V is only about 1.2% voltage drop (~3V). It should be fine, and meets code.
     
  5. llavalle

    llavalle Member

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    If you decide to go with #2, it's going to be a VERY tight fit in the HPWC. #3 is already pretty tight in there, I can't really imagine #2 fitting.

    But maybe you can run #2 up to your disconnect and #3 from the disconnect to the HPWC? That's what I did. I ran 00 gauge Aluminium wires from my main panel to a subpanel (125amp) and from the sub panel to the disconnect. Then #3 copper for the HPWC. Way cheaper than having 80ft of #2 or #3 copper.

    Here's what mine looked like during installation :

    IMG_20150221_105700.jpg
     
  6. wk057

    wk057 Senior Tinkerer

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    I couldn't find any pics, but I have two at my place wired with #2. It's not too bad and not really any harder to do than with #3.
     
  7. llavalle

    llavalle Member

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    I stand corrected.

    I just looked up the Standard size (diameter) for AWG :
    3 AWG : 0.2294in
    2 AWG : 0.2576in

    Difference : 0.0282inches, between 1/64in and 1/32in more... I was under the impression it was more than that.
     
  8. FlasherZ

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

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    #2 fits with a little elbow grease and cursing. Not the easiest in the world, but not the most difficult I've wired either.
     
  9. Soolim

    Soolim Member

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    Trust that when the installation was completed, the aluminum ground conductor was bonded to the disconnect switch enclosure just like the copper ground wire is.
     
  10. llavalle

    llavalle Member

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  11. Atebit

    Atebit Member

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    #11 Atebit, Sep 3, 2015
    Last edited: Sep 3, 2015
    Thanks for the replies & info! My electrician wired it similarly to llavalle's setup...2 AWG aluminum from the service entrance to the sub-panel in the garage & then 3 AWG copper from the sub-panel to the HPWC.
     
  12. Soolim

    Soolim Member

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    #12 Soolim, Sep 3, 2015
    Last edited: Sep 3, 2015
     
  13. llavalle

    llavalle Member

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    I was about to say the same. 00AWG or 2/0AWG is not the same as 2AWG. Are you sure he used 2AWG AL wires? not 2/0AWG ?

    In order of big to small :

    0000(or 4/0) > 000(or 3/0) > 00(or 2/0) > 0(or 1/0) > 1 > 2 > 3 > 4... and so on.

    Ref : American wire gauge - Wikipedia, the free encyclopedia
     
  14. FlasherZ

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

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    #2 AL is only good for 90A circuit rating. 80A charging requires 100A circuit & feeder rating. I hope you charge at no more than 72A (the limit with 90 degree rated feeders and circuits), or you have an illegal install.
     
  15. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Because there is no 90A CB/72A Charging DIP setting on the new HPWC's, #2 AL will require the HPWC to be set to 80A CB/64A Charging.

    I hope that your electrician used 2/0 or 00 AL which has twice the aluminum and half the resistance of #2 AL.
     
  16. Atebit

    Atebit Member

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    Thanks for the info, I'll check. The rough in is still exposed so I should be able to tell by looking at the jacket directly instead of getting second hand info.
     
  17. Atebit

    Atebit Member

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    So I was able to get over there today. The sheath states "Style R XHHW-2 3 CDRS 2 AWG 1 CDR 4 AWG". According to http://www.allenelectric.com/referencedata/ampacity.htm, XHHW-2 is rated for 100A, unless I'm reading it incorrectly, which is entirely possible since I'm not an electrician. So I think I'm ok?
     
  18. FlasherZ

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

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    That's the conductor temperature rating for the 90 degC column.

    There are two factors to consider:

    1 - the temperature of the conductor itself. As you noted, XHHW-2's insulation is rated at 90 degC. You're good there because of the rating to 100A.
    2 - the temperature at terminations - where wire is bolted to breakers, receptacles, junctions, etc. This is where you have a problem - most equipment that you'll find today is rated only at 75 degC for those terminations. All new breakers and receptacles that you buy today will likely be rated at 75 degC, with 90 degC limited to special applications (that you will pay $$$ for). This is what limits you in this case to 90A maximum circuit rating (NEC 110.14(C)).
     
  19. Atebit

    Atebit Member

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    Thanks for the detailed info, Flasher. I will pass this information on to my electrician. Here's my perspective: prior to pulling any wiring I provided him with the HPWC install guide that specified 3 AWG CU for 80A charging. He chose to use something else without consulting me. So if he now has to spend more money for the subpanel, breakers, etc. that's on him. I'm sure my builder will support me in this.

    I'll google around to see what I can find, but if you'd be able to fast-track me to the components that meet the required specifications that would be helpful.
     
  20. FlasherZ

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

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    #1 ALuminum is good to 100A with 75 degree terminations, and #3 CU (copper) likewise.

    Be careful - your electrician might try telling you that what he did is fine because the car will never charge at more than 80A, so 90A is plenty good. This is incorrect! NEC 210.19(A)(1)(a) says otherwise - the circuit conductor rating must not be less than 125% of the continuous load. NEC 625 says all EV loads are treated as continuous loads, 80A * 125% = 100A. The conductor and terminations must be rated at 100A.
     

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