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Multemeter - dc current warms wires.

  1. Jun 16, 2010 #1
    Just curious why the red & black leads from my multimeter get quite warm when measuring higher dc current under a 20A setting?

    First I measured 4-5amps for 20secs & the leads didn't get warm, then measured another system with 16-17amps draw & within 5 seconds it got quite warm. Not like its over 20A.

    Are you just meant to hold it on the circuit for a few seconds? Why is it you can hold the leads when measuring voltage for a long time, but not current?
  2. jcsd
  3. Jun 16, 2010 #2
    Your multimeter wires are too thin for the currents you measure. All wires have a resistance. Smaller wires have a higher resistance than thicker wires. When high current (which 16-17 amps are considered to be) pass through a wire with high resistance (a think wire) it causes heat. This is waste effect generated in the cable due to the high current and the high resistance.

    The effect is written as:

    P = U * I

    P = Effect (watts)
    U = Voltage (volts) (in this case, the voltage drop over the cable)
    I = Current (amperes)

    The formula can be rewritten as this for simplicity in this case:

    P = U2 / R

    P = Effect (watts)
    U = Voltage (volts over the cable)
    R = Resistance (cable resistance in this case)

    The R for the cable can be calculated as:

    R = Ro * L / A

    R = Cable resistance
    Ro = Conductivity of the material (cobber is 0.0175) (the symbol is probably not correct)
    L = Cable length in meters
    A = Cable thickness in square millimeters (mm2). Your cables are probably around 0.5 or 0.75.

    The voltage drop can be calculated by taking the current you measure and multiply it with the cable resistance you calculated. Then you should have all you need to calculate how many watts that your cable generates when you run 16-17 amps through it. It's probably quite a lot since it got hot fast.

    It's the same reason why its so important to use correct cable thickness in house installations and why fuse sizes are so important. Overloading a cable causing heat inside the wall or on the wall can cause fires.

    The measuring cables don't get heated when you measure voltage because when measuring voltage you don't run any current through the measuring cable. Only a slight amount of current will run through the measuring cables which the meter uses to determine the voltage. Only current running through a cable can generate heat.
  4. Jun 16, 2010 #3
    Ton of info, thanks.

    First time using it, I also tried measuring current directly from the 12v 12ah battery terminals (just curious what reading id get) & the wires got warm, is this actually measuring any current? As i did get a reading of up to 18amps. Wouldn't it be just shorting it out & damaging the multimeter?
  5. Jun 16, 2010 #4
    Yes, that would be the same as shorting it out using a wire directly from + to -. Except that you're trying to measure the current. Can damage the battery and possibly the multimeter if the battery is capable of outputting enough current. Do it on bigger batteries such as car batteries or similar batteries capable of outputting several hundred amperes you could also end up with burnt hands due to the hot flames such high current would produce when you short it.

    But you can safely measure the voltage from + to -. But when measuring currents you will have to disconnect the load from the battery and connect the load to one of the multimeter cables. Then connect the other multimeter cable to the batteri. That means connecting the multimeter and the load in serie. Then the current will pass through the multimeter and then the load and you will measure the current the load is using.
  6. Jun 16, 2010 #5
  7. Jun 16, 2010 #6
    Are you guys trying to put your eyes out? Explode your meter of both? Get a current probe for gosh sakes. The clamp goes around the current carrying wire and gauges current by the magnetic field strength. The prongs plug into the DC voltage receptacles and one millivolt indicates 1 amp. 10 millivolts is 10 amps. https://www.amazon.com/gp/product/B..._m=ATVPDKIKX0DER&pf_rd_r=0J9D5H4591AGA7YCH297

    You run your meter leads between a 350 amp hour battery reading amps thru the leads is a disaster of deepwater proportions waiting to happen. Or use a shunt and measure voltage drop across the two ends of the shunt and run your voltage splitting formula.

    And get a mentor to run these things by before you get hurt.
    Last edited by a moderator: May 4, 2017
  8. Jun 17, 2010 #7
    Figured he didn't need to know that yet as it looks like he's only working on small battery stuff. Working with big batteries or high amount of power/voltage obviously requires more knowledge of what you're doing and more equipment to maintain safety. I work with electricity every day.
    Last edited by a moderator: May 4, 2017
  9. Jun 17, 2010 #8
    I was an underground electrician for 30 of my 31 years at the coal mine. I haven't seen it all, thank goodness, but have seen that cheap meters don't have the fusings and protections of a good Fluke. And even a Fluke 77 doesn't take to measuring 2000 volts when its max rating is 1000.

    At annual electrical refresher training we view photos of exploded meters from improper use and sometimes the charred flesh resulting. It is law that we cover incidents at those classes.

    The man is working on small stuff and wondering why the leads heat up while flowing 18 amps indicated shows me he knows not what he is about. An accident waiting to happen seems to me. If something like what he was doing went bad, was doing it under my supervision, and he filed an accident report, I would probably be fired.

    I may have over reacted BUT, it has been drummed into us that we work in a hazardous place with lethal conditions powered by lethal energies. We are taught that though risk is unavoidable in these endeavors, risk is manageable. Risk Management had developed in my years underground and when I left it was far improved from when I started at the mine. It is the workers job to manage risk for himself AND HIS BUDDIES.
  10. Jun 17, 2010 #9
    Yep, I agree, safety is important. Here in Norway we have one of the strictest laws in the world for electric installations and safety. Clearly Dav333 should stick to small battery electronic stuff that can't go that bad as he clearly has no idea what he's doing :).
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