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conductor rating

Discussion in 'Electrical Engineering' started by Warren Thai, Dec 6, 2006.

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  1. Warren Thai

    Warren Thai Guest

    given the required current rating of a conductor, how do you go about
    determining the required cross section of the conductor. Any documents would
    be great.
     
  2. TimPerry

    TimPerry Guest

    you can search "wire gauge size" and find any number of tables or
    calculators. for example http://en.wikipedia.org/wiki/American_wire_gauge

    different tables would apply if the conductor is in a metal conduit or
    subject to heat. aluminum wire uses a different table then copper.

    http://www.powerstream.com/Wire_Size.htm
     
  3. Guest

    | given the required current rating of a conductor, how do you go about
    | determining the required cross section of the conductor. Any documents would
    | be great.

    Cross section alone does not determine the current capacity rating of a
    given wire size. While you will see resistance being inversely proportional
    to the cross section area, consideration needs to be given to thermodynamics
    (the course that often makes or breaks engineers in school). Voltage drop
    across resistance means power dissipated as heat. But the heat must be
    removed. Unfortunately, wire cross section increases faster (squared) than
    the outer surface area (unless it is a hollow wire). At power frequencies
    for most cases, there is no skin effect, so the heat will come from the
    inside parts of the wire as well as the outside. Stranded wire will boost
    the outer surface area a bit. Then you have to consider the insulation.
    It will slow the heat dissipation rate (more so for thicker insulation
    required on higehr voltage wires), as well as being a limitation on the
    permissible heat rise. The ambient temperature is also an issue. And
    this gets more complicated with other heat dissipating conductors close by,
    especially if inside a cable sheath or conduit.

    If you are an engineer, this should all be recalled/researched from that
    thermodynamics perspective. If you are an electrician, your training in
    sizing and load calculations should give the information you need. But if
    you are a hobbyist, or do-it-yourself-er, your best bet is to be rather
    conservative and overrate for safety, after finding out the basics of what
    sizes can carry what (and don't stuff many wires together).

    It's simpler in most cases to just look up the capacity ratings in the
    tables in the National Electrical Code (for USA) or your Austrailian
    equivalent. There will also be some rules, particularly for smaller more
    common conductor sizes, that just spell out a maximum current that may be
    used. I can cite some of these from memory for US standard wire gauges,
    but since you are down under, you are likely using different wire size
    standards which would likely have different size points to consider.
     
  4. John G

    John G Guest

    The australian Equivelent--
    AS/NZS 3000:2000
    Electrical installations (known as the Australian/New Zealand
    Wiring Rules)
     
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