Confused about wiregauge

Discussion in 'General Electronics Chat' started by Chalma, Dec 18, 2013.

  1. Chalma

    Thread Starter Member

    May 19, 2013
    54
    2
    We had a project we were working on that is basically a UPS. A lithium battery is supplying current to whatever load we want. My question is we are drawing 100A through the wiring. I wanted to save money and use the smallest gauge of wire but on an automotive wire site it's telling me 12 AWG is sufficient, but googling for awg sheet I see that gauge 4 (doubled) up would be a better fit. Am I reading the charts wrong or what is the deal? Am I maybe looking at a chart that is comparing current in small doses rather than constant? My wire is not that long, maybe 3'. I havn't bought the 4 gauge wire yet, but 10Amp wire (and I'm bad I know) gets really toasty at that load. Thanks [EDITED*]My voltage range is like a standard car battery from 10-14.5(overcharged) so in reality 13.8 would be a better value
     
    Last edited: Dec 18, 2013
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    Ampacity at 60°c is #1 AWG for 110amps and #2 for 95amps, in a conduit, cable or raceway.
    In an enclosure or open air, then #4 is good for 105amps.
    Max.
     
    Chalma likes this.
  3. Chalma

    Thread Starter Member

    May 19, 2013
    54
    2
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    I use NFPA79 which is basically the same as the NEC.
    Max.
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
    496
    The difference in current rating is most likely due to the different codes/regulations. Most "sites" use code for home construction, the building code. And while it is useful... it is often useless in industrial setting because we are not trying to wire a house.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    The NFPA79 and NEC ratings apply to both industrial and domestic installation applications, in N.A. anyway.
    The links in the post #3 do not apply to either.
    Max.
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Test 3' with #8. IMO
    All depends on details of course.
    If it's critical application, go with the overkill.:)

    #10 is often used for short jumpers @ 100 amps.

    It's all about temperature rise, voltage drop tolerance, and good termination practice.

    Most failures I see are not melted insulation on the "run" but hot terminals.

    ps.
    I know!
    I'm an electrician. #2 or #3 cu for 100 amps.
     
  8. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    True. No sense using insulation rated over 100C to cheap out on the cost of copper when the connections are going to fry.

    I doubt 12 gauge wire would survive 100 amps in the long term, no matter what the insulation and connectors are, and can't figure out why anybody would recommend that.
     
    Chalma and inwo like this.
  9. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Perfect fusible link.:D

    Expected more heat than this!
    What I meant by my post, is test it.
    With all the wild speculation, who ya gonna believe?
     
    Chalma likes this.
  10. slight_return

    Member

    Mar 4, 2009
    12
    1
    Agreed. 12 gauge is way too small. What's the application? Commercial? Industrial? Automotive? Sounds industrial at 12V so I'm assuming you need to be adhering to industrial standards? In that environment, considering temperature, enclosure, venting, what-not...You are looking at least at 8# but better 4# to be on the safe side. If this really is a UPS and it's meant for a server room, you have to consider a power outage as well (the reason for the UPS in the first place...). If that's the case, what's the room's temp when the power is out! Can get very high very fast with servers and no ventilation (i.e. no working AC). If this is something else, like automotive, you'll need to check with standards agencies for the requirements... SAE.org handles automotive and I believe someone else gave you links for commercial and industrial.

    Br, mb
     
    Chalma likes this.
  11. Austin Clark

    Member

    Dec 28, 2011
    409
    44
    Coppers resistance goes up with temperature, I wonder how much this will affect things? It could lead to thermal runaway, not that it would have survived otherwise.
    I wouldn't want to use such thin wire even if I could dissipate all of the heat, as it'll waste a LOT of power and cause a voltage drop of about .16V per foot if my calculations are correct. 0.16V * 3 = 0.48V, or about .5V drop off of a 12 volt battery, which is nasty, 50W (4-5%) @ 100A.
    When in doubt, go big or go home.
     
    Chalma likes this.
  12. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    #12AWG wire has a resistance of 1.588mΩ/ft. For 100A a 6' round trip the voltage drop will be 0.95V and the power dissipated in the wire would be 95.3W. Obviously the wire would get pretty warm under those conditions.
     
    Chalma likes this.
  13. Chalma

    Thread Starter Member

    May 19, 2013
    54
    2
    Thanks for everyones excellent responses. Yes, I have #4 wire with some ring terminals ordered as well as crimping. I just don't have the expendature to buy super expensive wiring to test out ideas. I originally quoted my #'s wrong and I was using 10awg between wiring of the battery and the fuse, it held ok but EXTREMELY HOT. I would like to eventually move this to be a usable/sellable product so I did go with the appropriate gauge. almost $1k later and I believe we are ready to go =P. Thanks again
     
  14. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    You can add solder to the crimp connectors. I would do that for 100 amps. I actually have a soldering tip attachment for a propane torch so I can solder 100 amp (and up) crimp connectors without an open flame wrecking the nearby insulation.
     
    Chalma likes this.
  15. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Wow jumping from 10 to 4, skipping 8 and 6.

    Not that there's anything wrong with that.

    I never seem satisfied until I try all the wrong ways first.:)
    Guess I should not recommend my method!:eek:
     
  16. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Were you thinking about running 100 amps through the wire and feeling it to see if it's hot?:D
     
  17. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Actually, yes!:D
    That's the extent of my engineering.
     
  18. Chalma

    Thread Starter Member

    May 19, 2013
    54
    2

    yeah, 8 still wasn't appropriate for me and the 6 was pricier ring terminals for me believe it or not.
     
Loading...