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Thread: Motor amp draw

  1. #27
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    Oct 2007
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    Midwest
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    The difference in current between hot and neutral is not important in a 120 volt circuit (current in = current out). If you suspect leakage to ground check with a megger or put circuit on gfi and see if it trips. They trip at 6ma which most meters can't read accurately. All components (including caps) of an energized circuit have magnetic fields unless they are sheilded. Cheap, and non Rms meters can not be counted on to be totally accurate, and will pick up readings from these fields. You can peg many analog meters just by transmitting on a 900 mhz walkie talkie, even if the meter is shut off. The voltage reading between the neutral and the ground is created by the voltage drop in the neutral wire going back to the panel where it is bonded to the ground. This voltage drop will increase as the load increases. There is no voltage drop on the ground since there is no current flow. This is how inspectors test new wiring to see if it is within the 3% or 5% NEC rules when VD is critical. AirMechanical is right, don't waste your time on duplicate or non important readings. If you have excessive low voltage at the load though, neutral/hot to ground readings can be important, otherwise don't bother.

  2. #28
    Join Date
    Aug 2007
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    28
    Thanks for the replies everyone. The theory on current in being equal to current out was the thing that made me wonder about the readings I was getting. I'm kind of funny about wanting to know exactly what's happening to something when I'm trying to figure out what is or could be wrong with it, especially when I run into intermittent problems and can't really pinpoint anything.

    Most of the readings I get have been very close to each other but once in awhile I can get larger differences (like 0.6A vs 1.1A). I am using a Filedpiece meter though, so maybe that's the problem?

  3. #29
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    I've learned that some of the smallest things in life have the most difficult answers, like "Why did the dryer eat my sock?"

  4. #30
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    Feb 2008
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    Angry

    Quote Originally Posted by sciencefreak614 View Post
    Thanks for the replies everyone. The theory on current in being equal to current out was the thing that made me wonder about the readings I was getting. I'm kind of funny about wanting to know exactly what's happening to something when I'm trying to figure out what is or could be wrong with it, especially when I run into intermittent problems and can't really pinpoint anything.

    Most of the readings I get have been very close to each other but once in awhile I can get larger differences (like 0.6A vs 1.1A). I am using a Filedpiece meter though, so maybe that's the problem?
    yeah i am very unhappy with my Filedpiece meters!!! i spent way too much for them, i think
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  5. #31
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    Dec 2007
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    Quote Originally Posted by Texas-Tech View Post
    color me stupid I guess, but in all my years of service I have never taken an amp draw on a neutral line to a 120v motor.

    How do you know that you have never taken an amp draw on the neutral line? What about the times when both lines were black (as is often the case with ID motors, etc)?

    Looks like another licensed tech who doesn't believe that the neutral carries current.

  6. #32
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    Jan 2004
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    They're partially correct.

    On a balanced system, in the panel box, the neutral doesn't carry current.

    No system is balanced 100% of the time though.
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  7. #33
    Join Date
    Oct 2006
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    So. NH
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    Is the meter you are using a "true RMS" meter? With many electronic devices today using power supplies that clip the sine waves a standard reading meter can read low on the neutral. Not sure why that might apply in this situation but just a thought.

  8. #34
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    Dec 2007
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    East central Indiana
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    Quote Originally Posted by beenthere View Post
    They're partially correct.

    On a balanced system, in the panel box, the neutral doesn't carry current.

    No system is balanced 100% of the time though.
    Indeed, the current from each phase to the neutral would theoretically cancel each other out at the panel box.

    Is this part of where the (often dangerous) misconceptions about neutrals come from?

  9. #35
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    Also, there's the hypothetical panel box where everything is on 240

  10. #36
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    [QUOTE=ECIndHVAC;1985951

    Is this part of where the (often dangerous) misconceptions about neutrals come from?[/QUOTE]

    Probably.
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  11. #37
    Throw away the digital meter and get a good old Amprobe analog and you can't read the difference between 1.2 and 1.5..... grumbling.... dang teenagers!

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