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Electrical conductivity in steel wire ?

Discussion in 'Electrical Engineering' started by grumpyoldhori, Apr 26, 2008.

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  1. Could I have some advice please.
    I need to get a twelve volt, 1/2 amp supply five
    hundred metres up a hill.
    I do have a fence a metre high made from wooden posts and
    seven runs of Num eight (4 mil) galvanised steel
    wire.
    Is it feasible to use two of these wires to carry twelve
    volts that distance ?
    Thank you.


    --
     
  2. Don Kelly

    Don Kelly Guest

    ----------------------------
    -----------------------
    If you use # 8 copper, the voltage drop at 1/2 A will be about 1V. However
    the resistivity of iron which would be approximately that of steel would be
    about 5 times as much giving a 5V drop so for 12V at the sending end, and
    1/2 A load, the load voltage would be about 6-7V.
    Chances are highly likely that this would not be satisfactory.
     
  3. Samuel Chan

    Samuel Chan Guest

    500m by steel? Volt Drop in Cu is still a question. Thinking setp up and
    step down the voltage in transmission?
     
  4. Porky Taylor

    Porky Taylor Guest

    try it and see, might work.
     
  5. ehsjr

    ehsjr Guest

    You have some options open to you, which could be
    used in combination.
    1) Use all 7 wires - one group of 3 in parallel
    for 1 side of the supply, and the remaing 4 in
    parallel for the other side of the supply. That
    will reduce losses in the wire.
    2) Reduce the current drawn by the device(s)
    at the top of the hill, if possible.
    3) Use higher voltage AC as the source and step it
    down, rectify and regulate at the top of the hill.
    4) Use batteries at the top of the hill and charge
    them from the source at the end at the bottom.
    #4 is viable only if the load at the top of the hill
    draws current intermittently. Even then, you would
    need to determine if on time versus off time allows
    the battery to recharge at whatever charge rate the
    setup would allow.

    Ed
     
  6. Dean Hoffman

    Dean Hoffman Guest

    What about adding a solar panel to option 4? Solar powered fencers
    are getting to be fairly common. Even if the panel can't keep up the
    battery wouldn't have to be exchanged as often for a fully charged one.

    Dean
     
  7. Don Kelly

    Don Kelly Guest

    ----------------------------
    If he isn't using the Code, he could try jacking the voltage up and
    down and use the steel wire. Maybe, the electric fence idea would
    work. It sure would beat buying enough copper wire to go 3200 feet
    with the price of copper being so high.
    Cheap fence wire insulators and transformers can be purchased at a
    food lot store, the kind farmers use. An ignition transformer might
    also work.
    A used oil burner ignition transformer could raise the voltage to
    10,000 volts then at the other end back feed another transformer to
    get 120 volts. Then feed the 120 volts into a common Class 2 50 va
    bell transformer to get 12 volts. I have never seen this done, and
    do not know if it would work.
    At 10,000 volts:
    Power at 12 volts:
    P= EI =12 x 0.5 = 6 watts
    At 10,000 volts:
    I = P/E = 6 / 10,000 = 0.0006 amperes

    VD = VD=2(95.8)(1640)(0.0006)/16512
    VD = 0.01 volts
    Percent voltage drop = insignificant

    I would place signs around the fence warning people, and tell the
    inspector that the fence is electrified to keep the wildlife out.
    Electric Fences are not covered by the NEC for obvious reasons. The
    NEC has a purpose that is the practical safeguarding of persons and
    property from the hazards arising from the use of electricity while an
    electric fence has the prupose of shocking. When I was an inspector,
    the foreman at the Atigun camp for the Trans Alaska Pipeline repair
    job in about 1990 asked me about this. They installed an electric
    fence around the camp to keep the grizzlies out and he wanted to know
    where the code rules were. There are none! Atigun Pass is in the
    Brooks Range in Alaska and is a very beautiful place. It is also the
    highest point on the pipeline. I flew in there from Fairbanks in a
    Cessna 150 with a bush pilot. He couldn't make it over the pass
    because of fog so we landed at Chandalar field and I hitch hiked over
    to Atigun. Those were the days, my friend, those were the days.


    And what are the voltage regulation and exciting current requirements of the
    two (or 3) (high impedance) transformers needed as well as leakage on the
    crappy little fence insulators? They might be such that the scheme still
    wouldn't work. In addition, aren't most electric fences pulsed?
    At least 24VAC makes sense from a safety point of view.
     
  8. Stephen B.

    Stephen B. Guest

    "Gerald Newton" wrote

    Why would a square wave be allowed to run at a higher voltage than a
    sinusoidal wave?

    It seems to me that a true sine wave would have more energy and
    potential to do damage than a sin wave.
     
  9. James Sweet

    James Sweet Guest

    The peak of a square wave is the same as the RMS, the peak of a sine wave is
    higher than the RMS.
     
  10. Stephen B.

    Stephen B. Guest

    "James Sweet" wrote
    Thanks, I forgot about RMS, since I tend to play with nominal 1/4"
    rods not electrons.
     
  11. Don Kelly

    Don Kelly Guest

    ----------------------------
    24 vac does not make sense because the voltage drop will be too high.
    Do the math.

    0.5 amperes at 12 volts equals 6 watts.
    I for 24 volts is 6/24 = 0.25 amperes
    VD=2(95.8)(1640)(0.25)/16512
    VD = 4.75 volts
    4.75/24 x 100 = 19.8 percent
    If an igniton transformer is used the output is not pulsed and losses
    are minimum.
    I think some electric fence power supplies are electronic and would
    not work.

    Additonally, 24 volts is not safe in a wet environment. That is why
    12 volts is used for bell transformers. Read note 2 to Table 11A and
    B in Chapter 9 of the NEC quoted below.
    This note is based on finding from the original work by Charles
    Dalziel, who, by the way, invented the GFCI.
    2. For nonsinusoidal ac, Vmax shall not be greater than 42.4 volts
    peak. Where wet contact (immersion not included) is likely to occur,
    Class 3 wiring methods shall be used or
    Vmax shall not be greater than 15 volts for sinusoidal ac and 21.2
    volts peak for nonsinusoidal ac.
    -------------------------
    Some time ago, I did the math at 12V and I'll generously take your math as
    correct even though the numbers that you have used are undefined: 95.8
    what? 1640 what? 16512 what? You appear to be using a cookbook expression
    that I am not familiar with -without giving the units so I have no way to
    check your data/calculation except that the result appears to be reasonable.
    Yes the voltage drop is high but I wasn't considering DC and neither were
    you. Now using a 24/12V transformer with a 19.2 V input at the receiving end
    and converting to DC gives a peak voltage of 13.5V and a fat capacitor will
    leave you close enough to 12VDC average. There will be some voltage drop in
    the transformer impedances but this should be fairly small as the smallest
    12/24 or 120/24 transformer that one can get will be rated at a fair amount
    more than 6 Watts. In any case dropping from 19+VAC to 12VDC at 0.5A for DC
    is not a big deal, even using a resistor, is actually cheaper than the
    warning signs .

    If 24V (42.4V peak) is not safe in wet environments and the "safe" limit is
    21.2 peak( the peak voltage of a 15V sinusoid)_- then what about 10KV?
    Sure- we both know fencing units are can be safe ( not according to the
    code that you quote) but that is due to their high impedance and that
    impedance along with wet condition leakage can have a considerable effect on
    your scenario-which is not according to any code. There is no use in
    considering code in one situation and ignoring it in another. If code is to
    be ignored- then I would rather go with the 24V setup.

    In particular, the 24V can be floating with respect to ground so contact
    between both wires is needed for a hazard and this can be limited by some
    planning as to which wires are "hot" and which are grounded "shield" wires.

    It isn't damnfoolproof but neither are signs- in either case, the hope is
    that there are no damnfools around .

    I note that you did not consider the impedance of the transformers in your
    proposal. It will be high- by design.

    Actually, there are other alternatives and these are based on location,
    purpose, etc. Paralleling conductors in a 3/4 grouping as suggested by
    others will bring the voltage drop down to less than 6% so the problem,
    after a 24/12V transformer is too high a voltage at the load.

    My point is that 12V wont do the job so what is the lowest standard voltage
    that will do the job. 24V appears to be OK.
     
  12. Don Kelly

    Don Kelly Guest

    ----------------------------
    Few people can argue with Charles Dalziel because for one thing it
    would be difficult to duplicate his experiments. He actually used
    live humans for some of them. For others he used animals. Today
    there be too many litigating special interest groups to do what
    Dalziel did. I read one AIEE paper by him dated in the 1940's. I
    wrote a little clip on him about 10 years ago. It is at:
    http://www.electrician2.com/articles/dal.htm if you are interested.

    ----------
    And much of this work as well as more data than is listed in the article is
    also given in the EPRI EHV book and other references. Note that all these
    references do consider current- not voltage as the criterion and also
    consider the 5% statistical levels of let-go current, fibrillation current
    etc.
    I can dig some of these up although I don't have the exact reference on
    hand.
     
  13. Roy

    Roy Guest

    I don't get it....

    If all you need is 12 DCvolts/.5 amps and if you can't run a household
    circuit up there & step it down with a wall wart - why don't you just
    use a battery ?

    Roy Q.T. ~ US/NCU ~ E.E. Technician
    [have tools, will travel]
     
  14. Don Kelly

    Don Kelly Guest

    ----------------------------
    K=95.8 circular mil ohms per foot. It is the resistance of a
    conductor 0.001 inch in diameter one foot long at 20 degrees C in this
    case.
    L=1640 is one way circuit length in feet
    CMA=16512 is the circular mil area of the No. 8 steel wire that is the
    diameter in thousandths of an inch squared
    I is amperes
    VD is voltage drop
    VD=2KLI/CMA is a standard formual for finding voltage drop for 60
    hertz single phase or Direct Current
    This is the standard formula used by electricians to find voltage drop
    for the last 50 years or so..
    We multiply VD by 0.866 for three phase three wire and by 0.5 for
    three phase four wire
    If you use 24 volts the NEC rules apply and bare conductors cannot be
    used, but by using the electric fence model the NEC rules do not
    apply.
    --------------------
    Thanks for the definition of terms. I have not used the "standard formula"
    and generally use metric so I simply go back to basics: Resistance
    =resistivity*length/area for conductors and the rest follows. This is, as I
    suspected, part of the basis for your standard formula but I wanted to be
    sure as I suspect any "formula" until I see the basis for it. The
    remaining part consists of assumptions of unity pf and negligable
    inductance- both reasonable in commercial and domestic applications as well
    as some industrial applications. They are reasonable in this case as
    well-given negligable transformer impedances (and exciting currents) which
    may not be the case.

    However the reason for your proposal is to get around NEC rules Fair
    enough. It is a situation that code doesn't cover or prohibits (whether, in
    a specific case such as this, the code is an ass is another matter ).

    Basically, a 24V scheme will work. Will it violate code? Yes. Will it work
    better than a HV system- I think so. Will it be safer? Maybe, maybe not- but
    it can be, in spite of the code.
     
  15. Don Kelly

    Don Kelly Guest

    In our local paper someone stole a 3500 foot spool of No. 4 copper
    wire that was worth $8500. No wonder people want to use steel fence
    wire.
    On the KLI/CMA this is the method used by the Neher McGrath paper to
    find DC resistance. Multipliers are used to convert to AC resistance
    or impedance. The paper was written in 1957 using feet and not
    meters.
    K for steel at 20 degrees C is 95.8 ohms per foot. To find the
    resistance at other temperatures the inferred temperature at zero
    resistance is required and the point slope equation is used to find
    the equation of the straight line. This is not entirely correct
    because the actual measured values of resistance are slightly off the
    straight line, but for most field applications it is good enough.
    Th N-M paper in PDF format is at:
    http://www.electriciancalculators.com/ampacity/ampacity.htm
    -----

    I have no problem with the calculator per se. You have clarified the terms
    satisfactorily.
    It is based on knowledge that dates back to at least the time of Ohm (well
    before 1900). The 1957 reference apparently uses an older cookbook
    expression as part of dealing with ampacities of underground
    cables.-Unfortunately, I could not bring up the reference from your site but
    it is an AIEE paper dealing with ampacities of underground cables- a much
    more complex problem than simply finding resistance and voltage drops which
    can be found in any decent introductory circuits text.
    The resistance/ temperature relationship has also been around for a long
    time. Actual measured data do have errors which cause wobbles in the curve
    so least squares fitting is used.

    The AC corrections do depend on frequency and wire size and the typical
    corrections for copper wire will be incorrect for iron wire (and for
    closely wound coils) because of enhanced magnetic effects leading to what is
    called "skin effect" (Rudenberg tackled this messy problem about 80 years
    ago (+/-)) .
     
  16. Roy

    Roy Guest

    From:
    In our local paper someone stole a 3500 foot spool of No. 4 copper |
    wire that was worth $8500. No wonder people want to use steel fence
    wire.

    Do we see a pattern here?
    http://www.local6.com/news/9841302/detail.html
    http://www.nbcsandiego.com/news/15516726/detail.html
    http://www.komotv.com/news/archive/4163491.html
    http://www.wftv.com/news/9842266/detail.html
    http://www.wyff4.com/news/9484796/detail.html
    http://www.highbeam.com/doc/1G1-163833841.html
    http://www.wmur.com/news/11425057/detail.html
    http://www.pasadenastarnews.com/ci_8764005
    http://www.herald-dispatch.com/homepage/x1657949065
    http://www.msnbc.msn.com/id/19791644/
    http://www.dailymail.com/News/200803310220
    http://news.sky.com/skynews/article/0,,30200-1287665,00.html
    ---------

    Thats messed up., they probably strip it and cut it up in small sizes to
    sell it to the local recyclers .....You should see the scavengers we
    have all around tearing up air conditioners for the copper, carelessly
    releasing freon gas into the atmosphere just for a few bux....

    Roy Q.T. ~ US/NCU ~ E.E. Technician
    [have tools, will travel]
     
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