Max amps in a wire? Any charts or

  1. anything out there that could help me with this? or is there no limit? or does it depend on gauge?
     
  2. jcsd
  3. Integral

    Integral 7,345
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    Yes, it depends on gauge. Did you try google?
     
  4. it does depend on the gauge indirectly
    but directly it depends on the voltage across the wire and the resistance of the wire's length
    the bigger the diameter(gauge) the less the resistance of the wire per unit length and this yeilds more current per unit voltage
     
  5. the formula for finding out how much current is through a wire is
    current = voltage / resistance of wire per unit length
    to find out the resistance of wire per unit length
    use an ohm meter to measure a length's ends
     
  6. Another way to find out the resistance of a wire per unit length of a particular wire diameter (gauge) is to get some charts showing resistance per unit length of wire qauge and multiply or divide that to your choosing length
    you can get such charts at electrical/wire manufacturers/retailers
     
  7. Cliff_J

    Cliff_J 789
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    That's a poor application of ohm's law. Typically the wire is used to deliver power to a load and is not the load itself so therefore the exclusion of the load makes this calculation worthless.

    A method to determine how much current a wire can safely handle is a matter of temperature control. Even a small wire can pass large amounts of current for brief periods of time but will heat up while doing so. To find the amount of heat, find the resistance and current and use the following:

    resistance*current^2 = power loss on wire in watts = heat

    Now the thermal mass of the wire (how long it takes x watts to reach y temperature) and the safe level for the insulation and the ability of the wire to dissapate the heat to reach a state of equilibrium are other factors.

    The cross-sectional area of the wire is like a pipe or hose for delivering water - a firehose can deliver much more water than a garden hose at the same pressure. Its called circular mils.

    As a safety factor, the amount of current allowed to flow over a certain number of circular mils is used and is conservative. If the wire were in a bundle of other wires and not able to dissapate the heat, we wouldn't want it to build up enough heat to melt the insulation and cause a short. So some charts use 300 circular mils per amp, and some use 200 circular mils per amp. Using only 200 nets a higher number because it allows more current per unit of cross sectional area but reduces the safety factor.

    Unless you have a lab grade low resistance meter this is highly inaccurate and the resistance is tyipcally well within the tolerance of the meter in cases where current flow is of concern. And it doesn't matter anyways, see above for the reason why.

    Another consideration is the power delivered to the load and losses in the transmission. The heat calc above shows how many watts are lost just in the wire. This means less power is available to the intended load because the series resistance of the wire (like a series resistor) drops some voltage and thus reduces the available power.

    Most charts you will find online are conservative enough where this isn't much of a concern but you should be aware of the variables.

    Cliff
     
  8. Cliff_J

    Cliff_J 789
    Science Advisor

    Oh, and here's a link if all you wanted was a quick answer. This author spends more time on the explanation of terms further down the page and uses 300 mils/amp in his second chart from the top in recommending fuse sizes.

    http://www.bcae1.com/wire.htm

    Cliff
     
  9. Gokul43201

    Gokul43201 11,141
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    Also check out the American Wire Gauge tables, or one of the millions of links to their tables online.
     
  10. Cliff_J

    Cliff_J 789
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    Hey, nothing personal is meant here - this is about answering someone's question with useful and accurate information.

    I can follow your post I quoted above, I'll take my Fluke 87 and measure a wire and it shows .03 ohms. I've got a 12 volt battery in my car, and using your equation I get 400 amps. So my wire can take 400 amps then? For how many microseconds?

    Using circular mils to determine current capacity or referencing charts that have already factored this into account is far safer than understanding the basics of ohm's law and applying them out of context and in a way that could lead to fires!

    Averagesupernova, this is your cue to add the last word. :smile:
     
  11. :approve: Since I agree with you Cliff, I'll let you have it.
     
  12. Whoa! lot more response than i expected. Thanks everyone!
     
  13. oh. one more thing, how would i find the thermal mass?
     
  14. Gokul43201

    Gokul43201 11,141
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    philo, do you understand why your suggestion doesn't work ? There's no harm in learning !
     
  15. Umm, I'm an electrician and i can tell you that the national electrical code has a chart of american wire gauge sizes and their max amperage (ampacity), although it is probably a bit on the conservative side. also these figures are based on (given) ambient temps.
     
    Last edited by a moderator: Jan 17, 2005
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