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HPWC install - copper or aluminum

Discussion in 'Model S: Battery & Charging' started by pbleic, Feb 24, 2014.

  1. pbleic

    pbleic Member

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    I have a 70 ft run underground to a detached garage. Is copper much better than aluminum for 100 amp service? The price difference is very substantial.
     
  2. FlasherZ

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

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    Most large service conductors are aluminum. You don't want them for branch circuits because of expansion/contraction effects, but because you have a detached garage and I assume you have lights in it, you will require a subpanel in the building anyway. So if you're going with 100A to the detached garage, you'll use aluminum to feed it, then copper within the garage for the branch circuits.

    Conductor sizes at a certain rating are different - AL requires 1 AWG for 100A, while CU requires 3 AWG.

    CU is nice, but for underground feeders > 100A, it is much less expensive to go with AL as you've found out. No real downside because the cables aren't moved all that often.
     
  3. PhilBa

    PhilBa Active Member

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    For direct burial cable, Al can be a bad thing. Make sure it's in conduit. If you get a nick in the insulation during installation you will eventually wind up with a break in the cable due to electrolytic corrosion. This happened to me for an outbuilding. We lost one leg of the 240 service. The contractor that replaced the cable (in conduit) said he sees that sort of thing all the time as it's hard not to nick the insulation while manhandling the cable and it isn't very obvious when you look at it.
     
  4. buzzbar

    buzzbar Member

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    For that distance and size, I would stick with copper wire. You would only save around $100-125 if you went with aluminum. For me, I'd be happy to pay that amount to get a superior product.
     
  5. wycolo

    wycolo Active Member

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    Curious how one would put (pull) 300 ft of copper service thru conduit? I've considered cutting thinwall pvc in half and laying the cable in it, then covering with the other half, mostly for physical protection. Seems better than just laying it nude in clay/topsoil and almost easier than trying to imbed in pure sand. Thoughts appreciated.
    --
     
  6. FlasherZ

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

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    #6 FlasherZ, Feb 25, 2014
    Last edited: Feb 25, 2014
    Cutting the conduit in half pretty much removes any benefits the conduit produces, at least for direct-burial cable. The two halves can shift fairly easily, and then begin to break apart, creating shattered plastic around the cable that can potentially cut into the cable over time - ever see a PVC downspout extension after it's been run over by a vehicle? You don't want to manufacture that from the start.

    Pulling cable through conduit isn't all that hard. First, note that it is an NEC requirement that a raceway be fully assembled before any cable is pulled through it - it's not legal to assemble the raceway as you pull cable (although I've seen plenty of people do it). You start with a spool of twine connected to a ping pong ball (or other plastic ball or big cotton ball -- anything that becomes more of an obstacle to glide in the suction) that's roughly the size of the conduit. Put it in one end and use a shop-vac on the other to suck it through. If the cable is small enough, you can use that twine to pull the cable through. For heavier cables, however, that twine is used to pull a heavier rope back through the conduit, which in turn will be used to pull the cable through. Cable grippers that are effectively Chinese finger cuffs with a hook on the end are used to pull the cable through.

    EDIT: note that it is legal to feed the pull twine as you assemble the conduit as well, although I will note that I've seen cases where the twine gets wedged or glued into a joint and becomes completely ineffective. The ping pong ball with twine and a good vacuum works every time.

    In the case of my upgraded service, the old cable was used to pull the new conductors through the Carflex conduit beneath my driveway. They unhooked my service, tied the new conductors to the old ones, and used the hydraulics on the bucket truck to pull the cable through.
     
  7. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    With the assistance of lots of conduit lube, a capstan winch, good strong rope, etc, we did a 1200' pull of 4, #6 AL wires in 1.5" conduit for a solar install. It was a net 150 feet down hill and we pulled down, and with some careful coordination, we were able to do it. Oh, and also, we used metal for the last set of 90˚ sweeps to the surface, to deal with any abrasion from the rope. We optimized so that it was about the same price to buy more panels as use larger wire... We also left ourselves a place to put in a junction box half way and do two pulls, but that was not needed. :wink:
     
  8. FlasherZ

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

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    Oh yes, I probably should have stressed it too. When he says lots of conduit lube, he means LOTS of lube. Buy a gallon or two. You can never use too much.
     
  9. wycolo

    wycolo Active Member

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    Thanks guys, always ready with heroic tales! I've got 1-1-1-4 cu wrapped in black rubber casing 1.25 inch dia. Deeper it would be entirely in mesosoic clay but at 3 ft will be transitioning into topsoil hence my interest in extra protection. You've sold me on the ease and simplicity of ~6 inch sand encasement. Plus I'll lay rocks & boulders over the whole run (which follows a driveway). Mostly to remind me to keep the heavy equipment off of it.
    --
     
  10. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Seems to me, by using 2" or larger conduit, nice big (2' or more) sweeps, lube, rope, and a capstan winch, this 300' run should be pretty straight forward. Of course, if there is an elevation change, pull down hill. I like having stuff in conduit, and that is the way I would go. Have fun and good luck! :cool:
     
  11. tezco

    tezco Sig P85

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    When the electric utility had to reinstall the underground service at my place they drilled with a directional boring machine and pulled conduit back when withdrawing the bit. Then they blew tape thru the conduit with compressed air and wrapped the free end around the wheel of their backhoe! Powered up the hoe and pulled the cable using tons of lube. They pulled some very heavy cable in 750 to 1000' increments. (Don't try this at home.)

    If you install conduit, the bigger the better in my opinion. The sweeps are more gentle, particulary if you buy the long sweeps. Remember that the NEC limits the total degrees of bending between junction/pull boxes.
     
  12. wycolo

    wycolo Active Member

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    > I like having stuff in conduit, and that is the way I would go. [Cottonwood]

    Indeed. It is a straight run up a moderate slope. I'm thinking pull thru first with the conduit ends both held up at ground level and then attach the ells at each end. If I can rent a capstan winch they probably will also have rope to use with it.
    --
     
  13. pbleic

    pbleic Member

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    Follow up from OP: electrician came up with a great solution. The wires run behind the garage so we put in a new 200 amp service (capacity for a second ev in the future) just for the garage. Was expensive and costs $10 extra per month, but I get time of day pricing yielding .135 per kWh vs. standard .187. Lots of optionality.
     
  14. N4HHE

    N4HHE Member

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    With magnifying glass and bright light my HPWC's terminals are marked indicating use of either AL or CU is permitted. This means the material and thermal expansion properties of the terminals is equally safe with either. Is also marked 2-14 ga. I can't hardly imagine an HPWC being useful with only 14 ga feed but perhaps thats just what the standard off-the-shelf terminals support.

    There is nothing wrong with using aluminum when the end terminals are designed and rated for aluminum wiring. In fact there is much right with using aluminum. Aluminum costs less which means it consumed fewer resources in manufacture. 2-2-2 Aluminum Service Entry cable is currently $1.39/ft at my local Lowes and Home Depot. Am not comfortable running 80A continuous through it but then again I don't have the twin charger option and will be replacing my NEMA 14-50R.
     
  15. FlasherZ

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

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    #2 AL in conduit is good only to 90A (75 deg termination), so it would not be a legal install for 80A charging.

    I agree with you that there is nothing wrong with aluminum when properly installed and used, especially today's alloys that, mechanically, work nearly the same as copper.
     

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