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Measuring amps

Discussion in 'General Electronics Discussion' started by floridachipper, Sep 15, 2010.

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  1. floridachipper

    floridachipper

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    Sep 15, 2010
    I have a 240 volt X 1000 watt device that runs from a 240 volt single phase power source. This device has 2 wires and the power source has L1, L2, & a ground (not neutral). The device is connected to the 2 hot wires. The device should pull 4.16 amps. When checking the amps with a clamp meter is the readout I’m seeing how many amps are going thru each wire or the entire circuit?

    I want to put a fuse on this circuit but I don’t know if I should put a single 5 amp fuse on one leg to cover not going too far over the entire 4.16 amps or can I put a single 3 amp fuse on the leg to ensure not going too far over the 2.08 amps of that leg.

    An understanding of this type of info would be great to know.

    Thank you,
     
  2. Zander

    Zander

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    Sep 11, 2010
    Do you have a diagram?
     
  3. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Amps in each wire/leg = amps in entire circuit = 4.17A. Fuse(s) = 5A. There's no current of 2.08A flowing anywhere there.
    Clamp meter will show 4.17A if clamped over either L1, or L2, but will show 0A if clamped over both L1 & L2 simultaneously, or ground, or all.
    Zander; please; if you need a diagram for that then you're in no position to help, but you may find the issue interesting for your own education..
     
    Last edited: Sep 15, 2010
  4. floridachipper

    floridachipper

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    Sep 15, 2010
    You see now this is why I’m confused because we have machines right now that have a device that is rated to produce 3000 watts from a 240 volt single phase power source. The math adds up to say this device should be pulling 12.5 amps total draw of current. One hot leg of the power source is going right to one side of the device. The other hot leg for the device goes thru a 10 amp fuse before going on out to the device. The clamp meter says there are 12.3 amps going thru the hot leg with the fuse. Why is the 10 amp “Fast-Blow” fuse not blowing?

    This is the reason I thought that maybe the current was split up between the 2 power legs.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    No, there is a circuit. What "comes in" on one "power leg" goes out on the other.

    The 10A fuse won't blow immediately if 12.5A are passing through it.

    Here is a spec sheet for a fast blow fuse. You'll see that the 10A fuse will last something between 1000 and 10000 seconds (15 minutes to over 2 hours) at that current.

    However it's a little odd that any appliance would have a fuse rated at 10A if it was ever supposed to draw more than about 5A to 7A. In most cases you leave some significant leeway between the expected current drawn and the fuse rating.

    What is this device? Is it expected to "produce" 3000W for any length of time?
     
  6. floridachipper

    floridachipper

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    Sep 15, 2010
    This is a melting pot. The 3000 watt heaters will run full power for an average of 20 to 40 minutes until the pot is up to the desired temperature then the power will just pulse on and off to keep the pot to the set point temperature.
     
  7. floridachipper

    floridachipper

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    Sep 15, 2010
    This pulsing of power will continue the entire time that the melting pot is turned on which quite often is 24hrs a day for 6 days at a time then the pot is turned off for cleaning or the one day that the end user is closed.
     
  8. floridachipper

    floridachipper

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    Sep 15, 2010
    I tried to open the PDF file for the fuse info but I got a file damaged error
     
  9. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    So it seems the fuse just barely gets by until the controller starts pulsing. The fuse ought to get hot to the touch I'd think.
    I once tested a 0.5A glass fuse at 1.2A (iIrc) and it simply refused to blow, it just sat there with a red-hot wire for minutes.
    At some current levels a fast fuse can even be slower at blowing than a slow fuse. Fusing is relative and non-accurate.
     
  10. floridachipper

    floridachipper

    6
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    Sep 15, 2010
    Thanks for all your input. I have a bit better understanding of things now and I know which way to go to further my self education on some of this stuff. You all be good and I’ll see you for the next brain teaser
     
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