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Yet ONE MORE wire size/ampacity question

Discussion in 'Model S: Battery & Charging' started by AudubonB, Mar 8, 2015.

  1. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    I'm almost apologetic (I'm never apologetic but I thought it would be politic to claim I almost am...) to begin yet another thread on wiring, but here we go with my own situation.

    What's slightly different is that the run I am considering dealing with is phenomenally short. We're looking at hardly two feet from the breaker location on the main service panel through to a perfect site for an HPWC on a garage interior wall. So I upped that by 50 percent to err on the safe side.

    As follows: from the ampacity tables I have then, under the conditions of:

    * 240 V single-phase
    * Cu wire
    * 3' one-way distance
    * 80 amps continuous/maximum draw

    we will get voltage drops and nominal end voltage (given perfect 240V starting conditions) of:

    3ga 0.04% or 239.903V
    4ga 0.05% or 239.877V
    6ga 0.08% or 239.805V

    Further consideration: the cross-sectional diameter of the three gauges differ as follows:

    6 ga = unity
    4 ga: 59% bigger than 6 ga
    3 ga: 100% bigger than 6ga; 26% larger than 4 ga

    So......can anyone come up with a reasonable answer as to why I shouldn't make use of the far, far more tractable 6 ga than the alternatives?
     
  2. dirkhh

    dirkhh Middle-aged Member

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    Because the NEC ampacity of 6ga is only 75A even at 90°C insulation?
    (not sure, is this a trick question?)
     
  3. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    Well, Dirk, I can't tell if it's a trick Q or not...and I made up the question!

    You see, from the NEC guidelines, your response is right: it allows only up to 75 amps....regardless, it appears, of the distance of the run. Yet from the data I posted, the loss (equals, more or less correctly, the heat build-up) over a three-foot run is absolutely negligible. At 0.195 volts, such loss is far less than noise in the system. So perhaps if I pose the question as follows, it would be more appropriate:

    * can there be any dangers brought upon solely by the choice of 6ga rather than 3ga in the set-up as proposed?
     
  4. deonb

    deonb Active Member

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    Maybe not, but let me ask you this way:

    Knowing that there have been previous Tesla fires in garages, how sound will you sleep at night with the knowledge that if something happens and you file an insurance claim, the first thing they'll look at is whether the wiring was rated for the load?

    Is that not worth the extra hassle and a few dollars?
     
  5. Griffinlair

    Griffinlair Member

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    THHN #3 is about $1.79/ft, THHN #6 is $0.87/ft. Even assuming #3 for the ground, the extra cost is less than $10 including tax. Stranded copper wire is not that hard to run.
     
  6. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    Have you tried to create with 3ga that S-curve inside the HPWC needed to cram those puppies into their home? Now, I haven't, but I've looked into it and my recollection is others haven't kind words about it. It's that - not the few dollars - that prompt the question.

    And in my own and probably NOBODY else's situation, Deonb, homeowner's insurance is rarer than white-and-gold (or is it blue-and-black) angels in our part of the world. It simply doesn't exist. 'Course, that means we haven't that useless premium to shell out, either. A dozen or so years ago we had a structure catch fire (guest in our sauna). Fire truck (Alaska State Forestry) showed up five hours later - long after I'd run my bulldozer through the blaze to move the conflagration away from some other buildings.

    It's different out here......
     
  7. Griffinlair

    Griffinlair Member

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    I didn't run the wire myself. My electrician did, so i don't know how difficult it is for the "S" in the HPWC. I'm not sure of the space inside the HPWC, would a loop be easier? Anyway, as I'm a bit more conservative and my run was significantly longer, i ended up over sizing the wire instead of under sizing. BE careful if you go down this path.
     
  8. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    You're right and I'm wrong: a loop, not an S-curve. Thanks.
     
  9. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    From American wire gauge - Wikipedia:






    Gauge Diameter (in) Ratio to #6
    6 0.1620 100%
    4 0.2043 126%
    3 0.2294 142%

    My guess is that you were looking at area rations and not diameter ratios.
     
  10. Moonwick

    Moonwick Member

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    IANAEE, but I'd be afraid of running 80A over any length of 6ga. The voltage drop isn't the only thing to take into account; you may only be dissipating (0.195v * 80a) = 15.6 watts over that run of 6ga, but that's 15.6 watts over only 3 feet. 5 watts per foot for 4-5 hours straight is quite a lot of heat, especially with much of that wire within an HPWC, which seem to have a reputation for getting pretty warm under normal installations.
     
  11. dirkhh

    dirkhh Middle-aged Member

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    IANAEE either, but it's my understanding that it's this calculation that drives the temperature rating / ampacity rating of cable to begin with...
     
  12. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    I was: cross-sectional area. But when you're trying to mash wires in and around, I suppose your diameter is the relevant number.
     
  13. kennybobby

    kennybobby Member

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    How about doubling up on the 6 or use a 6 and an 8 in parallel to make it easier to loop plus have enough copper for the amps? Maybe two 7 gage? Or three 8 gage? Plenty of ways to skin that cat...
     
  14. Lloyd

    Lloyd Active Member

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    Ding Ding Ding Ding!!!! Right answer!!! :tongue:

    - - - Updated - - -

    Parallel circuits are not allowed at 240V. Problem is if one connection is not made properly, heat can generate and not allow the breaker to release properly and your house burns.
     
  15. kennybobby

    kennybobby Member

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    Well what is a piece of 3 gage stranded wire anyway?--it's just a bunch of tiny gage (30?) wires in parallel. Or get some easy to loop welding cable, which is even smaller gage wires in parallel. Using parallel wires is not a parallel branch circuit, it's just a way to get sufficient copper in an easy-to-loop manner. Electric motors are sometimes wound using parallel conductors e.g. 5-in-hand, for just this very reason.
     
  16. billarnett

    billarnett Member

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    I have an almost identical situation. My "run" is less than a foot: the HPWC is mounted directly behind the breaker panel, the wire has only to run the distance thru the wall plus a little bit inside the HPWC and the breaker panel.

    I used #3. I was indeed a pain. But much less of a pain than the #2 I had for my Roadster's HPC.
     
  17. Lloyd

    Lloyd Active Member

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    From IAEC:

     
  18. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    Thanks, all!

    Last poll - for now - it appears that fewer headaches and colorful language have emanated from those who opted for the lhs knockout on the HPWC than from those who used the rear knockout. Any comments?
     
  19. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    No worries. I know how easy it is to think one thing and write another...



    As many others have said, it's the NEC that applies in most U.S. jurisdictions. Those Ampacity tables are determined for the worst of worst situations to be safe. Therefore, for a more typical situation, they have a lot of margin, but the NEC is effectively the law for home wiring.
     
  20. FlasherZ

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

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    For what it's worth, the HPWC cable itself uses a #6 conductor at 105 degC rating. However, it's an open-air cable and isn't enclosed within any raceways or enclosed walls. The NEC's articles take into account the ability to dissipate heat in raceways and risers, perhaps next to steam pipes and other stuff.

    Now, that said, you could try your hand at the formula in 310.15(C), but it's a pain in the ass and I have never seen anyone really use it...

    Capture.PNG

    As Cottonwood noted - it's not just effectively the law, in most jurisdictions it IS the law. Some jurisdictions actually call it a misdemeanor (more than just a minor cited penalty) to willingly and knowingly violate it. So a couple feet of #3 or #2 is probably best. I can't stop you from assuming the risk, but would just point out it's easier and greater peace of mind to follow the law.
     

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