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Ampacity of AWG 12 Cable

Discussion in 'Lighting and Electrics' started by NFK, Feb 16, 2017.

  1. NFK

    NFK New Member

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    Hi, beginner's question here. I'm looking at 2 ampacity charts for 12 AWG cable. The first one puts it at 20 amps, which is what I had always thought it was. The second chart has variable ampacity depending on if the cable is rated for 75 C or 90 C. For instance, AWG 12 is rated at 32A at a temperature rating of 75 C and 35A at 90 C. Is the second table referencing higher grade cable than the first or something? Can anyone explain this to me? Thanks!
     
    BillConnerFASTC likes this.
  2. porkchop

    porkchop Well-Known Member

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    If you dig into the National Electric Code there are a variety of conditions that can change the acceptable current capacity for a given conductor in a given situation. Air flow, ambient temperature, number of conductors, and insulation type are all factors. There is also a whole section related to entertainment and some of the oddities that come up in our field. For specific questions @STEVETERRY is our resident expert, but for the normal stagehand (or home DIY electrician) 20 amps is a safe max for most situations.
     
  3. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    Are you speaking of single conductor or multi-conductor cables, in conduit or free air, approved for rugged theatrical use or, or, or? As Ivan Beaver puts it: It depends . . .
    From how you've phrased your query, I gather you're looking at cables approved for differing Ampacities based upon the temperature ratings of their various jackets. For example: Various types of cabtire vs fixture wire vs. wire for winding transformers and motor coils, etcetera and on and on. The higher the temperature ratings of the insulations, the higher temperature the conductors are allowed to operate at providing they're also being used within designed applications. Many of these higher temperature insulations are only approved for use within approved assemblies and wouldn't be permitted for wiring between wooden studs within the walls of your home for example. I'm sure @STEVETERRY and others will be along shortly with further guidance for you.
    Toodleoo!
    Ron Hebbard.
     
  4. Ric

    Ric Active Member

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  5. STEVETERRY

    STEVETERRY Well-Known Member

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    For plain vanilla 12/3 cable with standard connectors at each end and two current-carrying conductors, use NEC table 400.5(A)(1). This resolves to 12 AWG for 20A.
    There are many other applications that allow use of 75 or 90 degree C cable with increased ampacity, but none of those have connectors at each end. By default, connectors are rated at 60 degrees C, which forces the rating of the cable to which they are terminated.

    ST
     
  6. JonCarter

    JonCarter Active Member

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    Now that we've dispensed with temperature, don't forget conductor resistance and the resultant voltage drop. A L-O-N-G run of a 20 amp circuit (don't forget it's both ways) may require 10GA or even 8GA cable. If in doubt look up the resistance per ft. of copper wire and calculate the voltage drop for the current in your application.
     
  7. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    An excellent point. On the last new build I was working on, the electrical engineers were forced to de-rate most of the T-90 wiring leading to dimmer per circuit FOH circuits to 6 gauge due to a combination of distance and number of current carrying conductors in the various conduits. There actually are crimp on rings and forks designed to crimp on to 6 gauge stranded copper and properly mate with 6-32 machine screws but they look pretty silly and they're not easy to find in your Panduit or Burndy catalogs. Sure glad I was only pulling and dealing with A/V and DMX runs on that project.
    Toodleoo!
    Ron Hebbard.
     
  8. Jay Ashworth

    Jay Ashworth Well-Known Member

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    6AWG wire. 6/32 rings.

    I don't believe you. :)
     
  9. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    I saw them with my own eyes. I was working for the A/V subcontractor as his lead on the project but worked very closely with the electrical contractor's foreman. When I noticed his lads pulling reel after reel of green, white and black 6 gauge for the majority of FOH cove and box boom circuits I asked him how he intended to land the 6 gauge onto the 6-32 machine screws on the sides of most of the receptacles and his answer was crimp-on lugs which he showed me when he received them. I think they were initially cutting notches out of the rings so they could slip them under the screws without having to force them out and get them restarted until he sourced forks, non flanged forks. Mechanically they appeared far too weak and flimsy to be up to the job but they were what were used and passed inspection. I didn't dig any further but suspected they could have legally gotten away with 8 gauge for the grounds but they pulled virtually all of the FOH circuits with 6 gauge. It was a very strange project watched fairly closely by most of the unions as the project was spear headed by a GC who managed to build the entire project with non union labor in a strongly union town diagonally across the intersection from city hall and the building department. I've likely already said too much but let's just say it's not the project I'm proudest to have worked on. (That one's probably still Toronto's Four Seasons Opera and Ballet centre, the four balconied home of both our Canadian Opera Corporation and the National Ballet of Canada where you can't hear a hit of downtown noise and / or feel any trace of subway rumble.) I'll go away now and you can continue to think me a liar.
    Toodleoo!
    Ron Hebbard.
     
  10. Dionysus

    Dionysus Well-Known Member

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    Don't forget distance, length of wire is another huge factor in allowable ampacity. Rule of thumb is go up a size for every 100'.

    Not familiar with american codes, but as an electrician in Canada I am well versed in our CEC. Happily all the necessary tables are the very first ones in the code book. Tables 1 through 4 are free-air and non-free-air copper and aluminum depending on cable temperature rating, and the next several tables show derating factors for distance, ambient temprature, number of conductors in proximity, etc,; of course referencing the necessary code rules is important.

    @RonHebbard that sounds like quite the interesting project. Geez I haven't spent any time in Toronto in YEARS. Would LOVE to take a look at that building one of these days.... Wonder if I know anyone who works there now...
     
  11. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Dionysus, if you're referring to the Four Seasons Centre, you might begin by contacting the Canadian IA head office in Toronto, not local 58's office, and inquire if they'd extend you any options for a private tour with one of the department heads or whomever they might suggest. While the building was under construction (with three levels below grade and 11 above, 14 levels in total) It was comparatively easy for me to run a number of personally guided tours in my off hours but currently I'm blind, in bad health, retired from both the IBEW and IA and totally out of touch with everyone. The only person I'm in regular touch with is the retired Head LX of the O'Keefe (whatever they're calling it these days) I know he and his wife are presently vacationing in Hawaii when they're not living in a condo in the heart of downtown, using their seniority to "cherry pick" calls and walking through the underground to work whenever they choose to earn a little pocket change. Sorry but I'm now much too far out of touch to be of any use to you as far as lining up any tours goes.
    Toodleoo!
    Ron Hebbard
     

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