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Constant voltage, variable current

  1. Jan 25, 2017 #1
    Will my described method allow me to achieve my objective? If so, could you recommend a rheostat that is rated for at least 20A @120VAC that will work for my application? If not, could you describe an alternate method?

    Objective:
    Determine the ampacity of various wires at 120VAC

    Method:
    Wire a rheostat in series to the wire to be tested. Attach this circuit to a power cord and plug it into the wall (120VAC power source). Use a multimeter to measure the current across the wire and adjust the dial of the rheostat until the wire fails.

    Thanks for reading.
     
  2. jcsd
  3. Jan 25, 2017 #2
    This thread will probably get shut down - this is too dangerous using 120VAC.

    Note -- Ampacity has nothing to do with the voltage - 120VAC?, you could do this with a car battery and be safer ( still not totally safe)

    A better test would be to attach a thermal sensor to the wire - and measure the time it takes to get to some critical temperature, or if it achieves steady state below that temp. There are many factors for this type of investigation - insulation type, ambient temp, airflow. Is the wire suspended or on a surface - etc.
     
  4. Jan 25, 2017 #3

    NascentOxygen

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    As a thought experiment, or the plot for a short Daffy Duck comic book, that sounds like a good attention-getting start. But there are some serious safety breaches in your method were it to be applied in real life.

    Is this a school exam question? Or how has it arisen?

    Can you identify some problems that might arise? How do you think we might be able to improve the procedure?
     
  5. Jan 25, 2017 #4
    Not a school question, this a proposal for an actual experiment. It seems I should be using a much lower voltage. Is windadct correct in implying that resistive heating is not effected by voltage, only current?
     
  6. Jan 25, 2017 #5

    NascentOxygen

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    Those are answers you'll be able to investigate by experimentation, providing you survive.

    Yes, only low voltage experimentation up to around 15 volts would be allowed. Using a car battery is not a safe procedure, either. Your school should be able to supply you with an overload protected variable power supply of 12 or 15 volts maximum, and which complies with your country's safety standards. Otherwise, you cannot perform experiments safely.
     
  7. Jan 25, 2017 #6

    CWatters

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    Resistive heating depends on..

    a) The current flowing in the wire (P=I2R) and/or
    b) The voltage drop down the wire (P=V2/R)

    The voltage at one end of the wire makes no difference.

    PS: I share the safety concerns that others have raised. Voltages over about 40V can kill. Even using lower voltages the wire may get red hot and an inappropriate power supply might be damaged. Car batteries can produce hydrogen (=Boooom). Lots of potential hazards. Hopefully your proposed experiment will be vetted/approved before you do it.
     
  8. Jan 25, 2017 #7

    NascentOxygen

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    So you won't be testing samples at 120 volts. You can investigate their current capacity at lower voltages, the results will be the same.

    The appearance of smoke is not a good measure of amp capacity. Smoke indicates destructive testing, and after that the cable is not fit for further use.

    Windadct's suggestion of a thermal sensor sounds like a better indicator, because you should be testing for gentle warming only. You may even be able to borrow an IR temperature probe, you point it at something and it reveals where heating is occurring.
     
  9. Jan 25, 2017 #8
    Thanks! Here is my revised method based on your feedback:

    Wire a rheostat in series to the wire to be tested. Attach this circuit to 12V constant voltage LED driver. Use a multimeter with a thermocouple to monitor the temperature of the wire. Use a second multimeter to measure the current across the wire and adjust the dial of the rheostat until the temperature rises to an unacceptable level, while making sure to stay at least 10% below the maximum current output of the LED driver.

    Any suggestions?
     
  10. Jan 25, 2017 #9

    CWatters

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    Thats better. Typically it is the temperature that limits how much current a wire can carry because high temperature affects the life of the insulation.

    Depending on the application the voltage drop may also limit the max current that the wire can carry.
     
  11. Jan 25, 2017 #10

    berkeman

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    No standard power supply is going to appreciate having its output shorted. You need a power source that is specifically designed to put out lots of current at a low voltage into a short circuit or heavy load.

    Why don't you just look up the ampacity in wire tables?
     
  12. Jan 25, 2017 #11

    CWatters

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    You should also do this to work out the how much current the experiment is likely to require.
     
  13. Jan 25, 2017 #12

    jim hardy

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    Have you figured how much power you are asking that Rheostat to handle? It's going to be an expensive industrial sized unit.

    An alternate , safer approach..............................
    ..... would be to

    find a "Variac" adjustable transformer
    find an old 250 watt soldering gun (thrift shop)
    find a clamp-around ammeter

    The soldering gun steps 120 volts down to just a few volts yet is capable of several tens of amps. Connect a short length of wire to be tested in place of the soldering gun's heating element. Plug the soldering gun into the Variac.
    The Variac will give you fine control of the current.
    To measure small currents, wrap several turns around the ammeter jaws and divide reading by Number of turns .

    j
     
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