multiple conductors vise large conductor

Discussion in 'General Electronics Chat' started by strantor, Oct 25, 2011.

  1. strantor

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    I am thinking (watch out). If I want to move lots of amps (think, hundreds of amps) then I need a big fat wire. But I don't have a big fat wire. I have a lots of 12 & 14 AWG wire though. Could I not run multiple smaller wire to achieve the same current capacity? NEC says I can (see attached) but anything over 4 wires has to be derated according to the table. I want to carry 500A, but NEC says max amps for 14AWG is 25A, so I would need 20 in parallel. The derating is 50% for 20 in parallel, so I need 40 in parallel. But, 40 in parallel puts me into the next level of derating - not sure if I'm supposed to take it a step further. That seems ludicrous to me. 40 wires in parallel? really? I need to know how to calculate how much a wire can carry, vise what NEC thinks is safe to carry in a residential installation. And where is this derating coming from? What is the reason for that? I fancy that the wires would share the current quite well without derating. How many 14AWG wire would I need in parallel to carry 500A? How do I calculate that? and what's a good rule of thumb for buffer room on the that?
     
  2. mcgyvr

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    The derating is coming from the fact that when you bundle wires you trap heat.

    Regardless...
    See NEC 310.4 (A)
    Only cables 1/0 and larger are allowed to be run in parallel.
     
  3. bertus

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  4. mcgyvr

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  5. shortbus

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    How about welding cable? The kind used for the stinger or ground of an arc(stick) welder.
     
  6. strantor

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    phew, when I saw the graph I thought "ah crap, maybe 14AWG really can only carry 25A" I didn't realize it based on NEC
     
  7. mcgyvr

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    14 AWG can only safely carry 25A (or 35A in free air based on 90deg C insulation and terminations)
    The terminations is an important part.. Example I have 90 deg C 14 AWG wire run in a conduit. According to NEC that wire can carry 25A. However if you hook it to a circuit breaker or other device that is only rated to 75 deg C at the termination point now you wire can only legally carry 20 Amps on that wire because you are now forced into the 75 deg C column of 310.16. (A chain is only as strong as its weakest link)
     
  8. strantor

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    I should point out that this is not residential wiring. It is for an electric go cart I am building. So "safe amps to carry", in the residential "up to code", house burning down sense (the way NEC thinks of it) doesn't really apply. I find it hard to beleive that 14AWG wire is literally only capable of carrying 25A when a mosfet leg, which is 1/2 or 1/3 the size of 14AWG wire can carry 75A. The LiPo batteries I am going to use are the type used in the big electric model airplanes. they are rated @ 200A continuous discharge and they have wire leads coming out which are only 12 or 10AWG.
     
  9. gerty

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    Don't forget, when they rate the wire They're talking about putting it in a wall, through holes drilled in studs. Any heat generated would cause a fire , or at least degrade the insulation.
    I have a Weller soldering gun that I put a piece of 14 ga solid wire (approx 8" long) in place of the standard tip. You can place a clamp on amprobe around the wire and pull the trigger and read 100 amps or so. If you keep the trigger pulled the wire will glow red hot. Try it some time, I do it to show my students why you need to use the proper size breaker/fuse. I agree with use welding cable, as shortbus said, for something like this. It's extremely flexible.
     
  10. strantor

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    also looking into that...
     
  11. Lundwall_Paul

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    Got some questions:

    Is this continuous current?
    What frequency?
    What is the load?
    How long is your run?
     
  12. strantor

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    A. that's still up in the air at the moment. probably 300A max
    B. DC
    C. a brushed motor/controller
    D. as short as possible. probably <2 or 3 ft
     
  13. nsaspook

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    If the wiring is fixed, (battery interconnects that don't move much) soft copper tubing (AC compressor piping) flattened on the ends and drilled out for bolts will work as well without the need for special terminals or crimps for large wires.

    http://www.stormcopper.com/copper-bus-tubing-specs.htm
    http://flic.kr/p/7FY1u8
    http://flic.kr/p/7FY1ur
    I covered the copper sections with slip-over pipe insulation.
    http://flic.kr/p/9Mdh1A

    General ampacity tables for high current copper components can be found here.
    http://www.stormcopper.com/
     
    Last edited: Oct 26, 2011
  14. #12

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    I used 1/2 inch copper pipe for the final terminals on my 200 amp battery charger. No problem! The jumper cables don't bung up the pipe like they would on nice, soft, copper wire and the problem of attaching an end to the wire feeds is solved by this method.
     
  15. strantor

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    Brilliant! any idea about the inductance of flattened cooper tubing? would it be higher, lower, the same as multiple wires (or welding cable)? I'm trying to keep inductance as low as possible.
     
  16. mcgyvr

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    Disclaimer...
    Nothing in this response should be deemed safe and is just for informational purpose...

    14 awg wire "can" carry up to approx. 166 Amps (not safely).. Of course the wire insulation would have already melted off. 166 Amps is the fusing current for 14 awg wire. Fusing current is the ampacity where a wire will actually melt like a fuse element. Of course at that current the wire would be glowing and 1900 degrees or so.

    Also the stated ampacities in NEC ensure that the wire will never exceed the temp of the insulation when carrying that current plus safety factor. (hence the ambient derating factors at the bottom of the chart). So based on 25 deg C ambient, 25 Amps through 14 AWG wire will ensure that a fire or overheating situation does not occur (no point in that wire will be more than 65 degrees hotter than ambient). Now put that same wire in the hot sun and the safe current is less due to the increased ambient.
    A pin of a mosfet is/will be much hotter when carrying that same current. You also have to factor in heat sinking from the attached traces, package, etc...

    Now for your actual situation. I would simply purchase a few pieces of copper bar and bolt them together (put shrink tube around for insulation). Safe ampacities of copper bar can be found here http://www.stormcopper.com/design/ampacity.htm

    Now welding wire (or machine tool wire).. Its simply a different amount/size of strands to allow greater flexibility. Unlike residential wire, welding wire will be exposed to repeated movement/flexing,etc.. by the operator so a more flexible wire is chosen/used.

    I don't care if its a go-cart or a house.. Safety MUST still apply and the NEC is a great source of information for situations like this.
     
  17. strantor

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    Thanks mcgyvr; looking at the copper bar ampacity puts things in perspective. due to the design of my mosfet module, I'm limited to 3/4" wide bus bar. in order to stay below 30C rise, I have to go all the way up to 3/8" thick (ideally 1/2" X 3/4", but they don't make it) - That's a hefty bus bar. And my battery & motor leads need to be the same.
     
  18. mcgyvr

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    I use up to 750kcmil wire at work and pass up to 475 Amps (NEC 310.16 75 deg C column) through it all the time. Its like a friggin baseball bat in diameter and SO tough to bend/run around a room (and I even use the really flexible stuff with 1850 strands of .020 dia wire make up a single wire versus only 61 strands of .110 dia wire for the stiff stuff and it makes a HUGE difference). I get to play with all the fun stuff. I have 2 x 1200 Amp 48VDC rectifiers, 2 x 750Amp (37,500Watts) Miller welding DC resistive load banks , 30 or so 1000W Acme DC resistive load banks,etc..
    You wouldn't believe how running 1200 Amps through something and all the heat the load banks put out just sucks the moisture out of the air. It gets really hot/dry after an hour or so of testing.
     
  19. mcgyvr

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    Just parallel thinner bars.
     
  20. strantor

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    dang man, what industry are you in?
     
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