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12V thru house 110V wiring

Discussion in 'Electronic Design' started by Mark Fisher, Mar 2, 2011.

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  1. Mark Fisher

    Mark Fisher Guest

    I have a cabin in my backyard that has been wired to the 110V grid. I
    want to remove the feed cable and install solar panels.

    Rather than re-wire for 12V, is there amy technical problem with using
    the existing in-wall 110V wiring?

    Of course, for safety, sockets and lighti fistures will need to be
    replaced with 12V versions, or at least relabelled.

    Mark Fisher
  2. Martin Brown

    Martin Brown Guest

    Depends whether or not you mind if your shed burns down.

    The current flow in the wires for a fixed power load will be roughly 10x
    larger when running at low voltage 12v compared to 110v. The resistive
    heat dissipation in the wires I^2R will be 100x more.

    A 100W lamp at 110v draws less than an amp, at 12v it draws 8A.

    I would do the sums again *very* carefully for solar power if I were
    you. It generally is not at all economic if you are already on grid. How
    many Ah of batteries needed and what panels & charge controller.

    Most times it is a lot cheaper to hump a heavy battery around and charge
    it at home than to charge it in situ by solar (or wind) power.
    Provided you don't exceed the total current rating it might be OK. But
    remember to use a low voltage automotive fuse in the circuit to protect
    against accidental short circuits causing red hot cables.

    I saw a very nasty fan heater failure at the weekend - one of those
    oscillating ones and the power cable failed by stress fracture on the
    wrong side of the thermal cutout protection. It was a filled plastic
    chassis construction and well alight when the owner found it.

    Martin Brown
  3. Mark Fisher

    Mark Fisher Guest

    Hmm... maybe I should consider gas lamps and a Sterling engine.

    Mark Fisher
  4. hamilton

    hamilton Guest

    Wouldn't running Natural Gas through the Romex be just as problematic ??

    Just asking.

  5. PeterD

    PeterD Guest

    I can't imagine you'll be producing much more than 20 amps per circuit
    so it should work OK. I'd use a breaker panel (or fuses) to limit the
    current to an acceptable level. I'm assuming you will 'buffer' with a
    deep cycle LA battery, right?

    Personally I'd replace the connectors/fixtures with 12V ones, for no
    other reason other than you'd be able to control polarity and anything
    plugged in won't accidentally be plugged into a 120 V circuit.
  6. nospam

    nospam Guest

    If you are not technically competent to judge the suitability of a bit of
    wire you are not technically competent to judge the suitability and cost vs
    benefit of grid vs any other form of power generation.

    My advice would be to leave the grid connection and wiring intact because
    he will almost certainly want to use them in the future.

    Batteries alone cost more than the amount of electricity at grid prices
    they can charge and discharge in their lifetime.
  7. mike

    mike Guest

    There aren't many reasons to be off grid.
    1) there is no grid available.
    2) you derive some personal non-economic benefit, well being,
    "hey look what I can do", from being off grid.

    You can live in a tent...but unless you are willing to dramatically
    decrease your needs for energy, there is no economically feasible
    solar system, today. No, the schemes whereby you transfer the cost
    to ME thru government subsidy or force the utility to buy electricity
    at inflated rates raising MY bill don't count. I don't want to pay for
    your system!

    The current you can get thru the wiring is the rating on the breaker.
    So you get 1/10th the power. Unless your peak load on the circuit
    was less than 1/10th the rating, you're gonna have a problem.
    See "living in a tent" above.

    You don't want to put 12VDC on 110VAC wiring. Plug in a device
    with a transformer and sparks will fly. Polarity is important.
    Lotsa devices have non-polarized plugs. Call up the local electrical
    inspector. If he's on his toes, he won't let you do it.

    I can hear the villagers lighting torches to come after me.
    "No that can never happen", "I won't tell the inspector", "he
    has no jurisdiction", "I'll never have any visitors who might
    compromise the system"...Use common sense. Stuff happens.

    Put separate 12V wiring in the place, or use an inverter and
    the existing AC wiring.

    Go visit newsgroups that address these issues for details, but...
    Use more than 12V for your DC storage system into the inverter.

    Here's how you talk yourself out of the project.
    These are round numbers, but the message is the same.

    Turn on everything you want to power simultaneously and spend
    some quality time with the utility meter. You need at least that
    much peak power plus any additional for motor starting peaks etc.

    Measure consumption over an average day in the season when you
    use the most energy. Multiply that by
    how many consecutive cloudy days you want to survive. Times some
    efficiency factor. That's how big your batteries need
    to be.

    Google for insolation charts for where you live. That'll tell you
    how much solar energy you can expect to get from a typical winter day.
    Divide that by 10 for the efficiencies of the solar system and charging
    system and and and...
    Divide that number into the average daily energy requirement to see
    how many square meters of panel you need.

    A popular number is 1KW/square meter. Depending on the efficiency of the
    panels and other system components you can afford, you're looking
    at 10% of that on a sunny summer day with no buildings or trees
    to block any of it. But averaged over the day, it's
    a lot less. And in winter it can be near zero depending on where you live.

    Add up the installed cost of all that stuff.

    Bottom line is that solar power is a fool's errand.
    There's no way it can pencil out on an individual basis with current
    technology. Unless you can get someone else to pay for it...that would be
    me...and I object to my tax dollars fueling your errand.

    Wiring is the least of your worries.
    Find a different convincing the tree-huggers that we
    need to get some nukes on line BEFORE we kill off all the trees.
  8. Sjouke Burry

    Sjouke Burry Guest

    Or buy a 12v->120v ac invertor, they have about 70-80% efficiency,
    and your wiring gets not loaded with unreasonable currents.
    Then put in high efficiency lamps, etc.
    But you will need about an acre of sun panels, and 10 or twenty
    deep discharge lead battery's.
    If you go for an ac invertor, higher battery voltage will be better,
    use 24 or 48 volts, and choose an invertor accordingly.
    That will provide better battery efficiency and less losses.
  9. Sylvia Else

    Sylvia Else Guest

    What happens when you put 12v DC across a transformer winding designed
    for 110V AC?

  10. I thought large solar panels were 16 to 48 volts. I think it would be
    easiest to get a mini inverter that produces 120VAC rather than 12VDC.
    Some will hook up to storage batteries.

    12V is difficult to move any distance because the wire losses are so
    high. It wouldn't be practical for more than a few feet.
  11. Jasen Betts

    Jasen Betts Guest

    As long as the wiring can handle the current you should be alright.

    if you don't replace the outlets label the appliances too. plugging a
    110AC appliance into 12DC could burn out motors etc, but plugging a 12V
    appliance into 110 is almost certain to be spectacular.
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