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Transformer Wire Current Capacity

Discussion in 'Electronic Design' started by Rich Grise, May 5, 2006.

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  1. Rich Grise

    Rich Grise Guest

    I was musing about my MOT, with 143 turns of either skinny #18 or fat #19
    - it's probably some metric size - anyway, I go googling for "wire table"
    and this is one of the hits: . And I get
    to #18, and it says, "Max Amps: 0.5390".

    ?????!?!?!?!??!!!!!! This MOT draws about 2.5 amps while IDLING! I read
    somewhere in some electronics hobbyist mag decades ago, in an article
    about transformer rewinding, that #22 was good for an amp in a
    transformer. Proportionally (proportionately?), #18 should be good for
    FIVE AMPS! So, I'm wondering where the guy got his figures, but I got so
    het up I haven't even checked any of the other google hits, so I'll
    probably be enlightened in no uncertain terms. ;-) Anyway, my interest
    was I've got a potload of #24 wire from scrounged telephone trunk scraps,
    and was wondering how many #24's I'd have to put in parallel to equal a
    #18. ... Hmmm, according to the calculator, if I use these ludicrously low
    numbers from the chart, 4. I'll have to see if I have that kind of
    ambition[1] tomorrow or so. ;-)

    [1] and patience! ;-)
  2. Pooh Bear

    Pooh Bear Guest

    That table looks wrong. According to them 10 gauge is only good for 3.4 amps.

    What does ampacity mean btw ?

  3. James Waldby

    James Waldby Guest

    Although the ampere numbers are wrong, they might be internally
    consistent; ie, 4 strands of #24 is right, to equal #18. Wire
    size doubles (fairly closely) when guage decreases by 3, so #18
    has 4 times the area of #24.
    ampere capacity, I think

  4. ehsjr

    ehsjr Guest

    According to the NEC:
    "The current, in amperes, that a conductor can carry continuously
    under the conditions of use without exceeding its temperature

    Don't know if "ampacity" means precisely that as used in
    the table at the url Rich posted.

  5. Be skeptical of any tables or calculators that give wire ampacity
    without specifying ambient temperature, maximum conductor/insulation
    temperature, thermal resistance or at least specifications of the
    conductor's installation, etc. Be doubly skeptical of a table that
    begins with the word 'Gauge' misspelled.
  6. Pooh Bear

    Pooh Bear Guest

    Yes. In a transformer, the rating is going to be heavily dependent on cooling
    which depends on construction.

  7. Roy L. Fuchs

    Roy L. Fuchs Guest

    Ampacity of mag wire doubles every three gauges since circular area
    does as well.
  8. Roy L. Fuchs

    Roy L. Fuchs Guest

    You're kidding right?
  9. Roy L. Fuchs

    Roy L. Fuchs Guest

    Yes. EACH wire out there is not alike.

    A "single strength" mag wire cannot handle the same temp as a "double
    strength" mag wire, or a High temp" mag wire.

    There is a raw copper ampacity table, and from there on, as you have
    said, there are a LOT of factors that go into a given wire
    installation that determine what it can or should pass through it on a
    continuous duty basis.
  10. Pooh Bear

    Pooh Bear Guest

    It's purely a US term.

  11. Roy L. Fuchs

    Roy L. Fuchs Guest

    Mmmm... OK...

  12. The chart on my website doesn't list current ratings because there
    are too many variables like insulation type, and how its used. For
    transformers you have to decide how many circular mils per amp,
    depending on how its to be used. for the web page. for a PDF you can print

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  13. Tom Bruhns

    Tom Bruhns Guest

    My rule of thumb for line-frequency power transformers is "about" 500
    circular mils per amp. As someone else posted, the rating depends on
    transformer construction, wire insulation temperature rating, maximum
    operating temperature, cooling options, ... But 500CM/A should be a
    reasonable starting point. 18AWG is 1624CM; 19 is 1288. So 2.5A
    sounds at least reasonable.

    It's possible to measure the average temperature of the wire in
    operation by measuring the wire DC resistance. You can either compare
    that with the resistance measured at some known temperature (e.g. room
    temp), applying the known temperature coefficient of resistance for
    copper, or put the transformer (non-operating) into an oven and raise
    the temperature until you reach the same resistance. The latter method
    is more difficult but more accurate. That won't tell you the peak
    temperature in the winding, though, which will be somewhat higher than
    the average.

    Some good things to know about AWG numbers:
    -- 3 gauges doubles (or halves) the cross-sectional area
    -- Therefore, 6 gauges doubles (or halves) the diameter
    (This is approx., so you'll be slightly off if you try to
    use it to go from AWG0000 to AWG48!)
    -- To a good approximation, copper wire resistance at room
    temperature is 10^(0.1*AWG# - 1) ohms per 1000 feet.
    (So 10AWG is 1 ohm per 1000 ft, 20AWG is 10 ohms, ...).
    The approx is almost right on at 10AWG, 5% low at 40AWG.
    -- Exactly, diameter(mils) = 5 * 92^((36-AWG#)/39)

    You'll notice that the gauge# behaves like dB: 3dB doubles the power;
    6dB doubles the voltage.

  14. Rich Grise

    Rich Grise Guest

    Probably not in this table - it starts increasing at #18, which doesn't
    make any sense at all.

  15. Ralph Mowery

    Ralph Mowery Guest

    From the charts I am seeing it looks like you are right on with the wires.
    The chart specifics 700 cm/amp, but states that from 500 to 1000 are being
    used. The #18 is listed for 2.3 amps using 700 cm/amp.

    I would guess that it depends on what the transformer is used for. If for
    long term use such as in a VCR or computer monitor probably the larger
    wire would be used and if for a short term use like a printer where it may
    use a small amount of current all the time but large amounts for a very
    short time the smaller wire may be used.
  16. joseph2k

    joseph2k Guest

    At 60 Hz only up to about #2/0 (assuming copper). At higher frequencies the
    transition gauge is smaller, above 1GHz it is very thin indeed.
  17. I did some calculations for wire size vs power dissipation and made an
    spreadsheet which has my estimation for ampacity of wires as well as bus
    bars. When wires are bundled together (as in a transformer or motor
    winding), ampacity is related to cross sectional area, but for single
    conductors, it is more related to the surface area for cooling effect. For
    this example, #22 wire should handle 1.9 amps, and #18 will handle 4.7
    amps, based on NEC guidelines for single conductors with ordinary PVC
    insulation. So, in a tight bundle, 2 or 3 amps should be fine, and even 5
    amps should be OK with high temperature insulation.

    For really large conductors, especially high frequencies, copper tubing is
    ideal. Run chilled liquid coolant through it and you can get a lot more
    current for the same amount of copper. This is being done in some large
    distribution transformers.

    My spreadsheet:

    Paul E. Schoen
  18. Tim Williams

    Tim Williams Guest

    I've ran oh, 200 or 300 amperes RMS through some 1/4" copper tubing, er,
    about 8' of it). It stays as cool as, well, whatever the temperature of the
    water is you pump through it! ;o)

    After ten or twenty minutes at that power level, you could make tea with the
    water though...(it wasn't going through a radiator or anything, just dumping
    into, and pumping out of, a milk jug, so, yeah).

  19. Roy L. Fuchs

    Roy L. Fuchs Guest

    Do you wind transformers for use at GHz frequencies?
  20. Tom Bruhns

    Tom Bruhns Guest

    Why yes, I do sometimes. But even at the frequencies used by modern
    switchers the skin depth becomes an issue. At 1MHz in copper, it's
    about 2.6 mils. For a winding that needs to handle several amps (and
    therefore need to use wire with an effective diameter considerably more
    than 5 mils), it's worth thinking about. It's not unusual to see Litz
    wire being used in such transformers.

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