stumbling on ac amp draw

Discussion in 'General Electronics Chat' started by woogerboy21, Sep 22, 2008.

  1. woogerboy21

    Thread Starter New Member

    Sep 22, 2008
    sorry for the book but i REALLY need others input...

    alright so here's the scenario. lately I have been looking into pulling a couple of circuits off my main breaker onto a secondary load center and running those circuits on an inverter. which the battery bank will be charged by solar cells. I have been trying to calculate the total amp draw required to run the circuits. after firing up all the items on the circuit i pull out my handy dandy clamp meter and attach the meter to the wire. set it for 20~ setting and proceed to check the draw. i then go around and check into all the bulbs fans etc on the circuit and check the ratings there estimated at and compare the draw that i am getting from the wire to the estimated draw that should occur given what number i have from the devices. comparing the two its pretty close. the draw is slightly less than the estimated number (.5 - 1amp) difference. which tells me my meter should be working properly.

    so far so good....

    so then i get a wild hair up and decide for fun i will try and check the load on other circuits so that if later on i want to move things like my fridge over to the inverter i could. so i go and look up online what the average watt rating a fridge should run at (which from what i found is around 700 watt) and then plug the wire clamp up to the circuit and run in and open the doors. check the load. then wait till the freezer / fridge kicks on and test the load during that as well. To my suprise I find the meter telling me my fridge is pulling 1.5 amps which should equate to 165 watts. so now I am puzzled. is there something wrong with my meter or is it just a co-incidence that my fridge (being energy star compliant) runs at a MUCH MUCH lower rate?

    to try something larger in an effort to determin if my meter is wrong i test my microwave. i havent the slightest clue what wattage rating my microwave is so i set it to high and kick it on and test the circuit using the meter. the meter jumps to almost 6 amps. which equates to 650-660 watts. seeing as its a cheap-e microwave that sounds about right.

    So long story short am I going crazy to think my meter is wrong or could it be possible that things are really drawing that low of a draw (running rate obvisouly I am looking for)? Seeing as usually my fridge and microwave are running on the same circuit if I go off the estimates of usage online the 600 watt microwave (according to online should be 1100 watt real power) and fridge rating 700 watt which would indicate a draw of 16+ amps and the circuit is only a 15amp circuit.

    using the online estimates the circuit should go POP....
    can someone anyone tell give some input. is things that really low of a draw?

    any input would help.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    My experience with clamp ammeters was that you could only have one of the conductors running through the current transformer. Your readings could be far off if you simply clamped onto the appliance cord.

    You normally never place any other appliance on the same line with a reefer. That way, the breaker doesn't pop and let the food spoil.

    15 amp circuits are never supposed to have greater than a 12 amp continuous draw.
  3. theamber

    AAC Fanatic!

    Jun 13, 2008
    Some current clamps may have an error margen of up to 10%. Fluke up to 5%.
    The best way to measure electrical power is with a Power Quality analyzer.
    With respect to the other user, no a 15 amps rated circuit can handle safely up to 15 amps, a 12 amps up to 12 amps and so on.
  4. woogerboy21

    Thread Starter New Member

    Sep 22, 2008

    Yea I noticed when plugging the clamp meter onto the appliance cord there was a far weaker reading due to the cancelation that occures on the appliance cord. In the scenario that I wrote above I am not plugging into the appliance cord. I am connecting my clamp meter to the actual hot wire coming out of the circuit breaker in the load center (breaker box) so there should be little to no cancelation occuring.

    Any other idea's?
  5. woogerboy21

    Thread Starter New Member

    Sep 22, 2008
    Even with an error margin of up to 10% would it be safe to assume I could multiply my readings from the clamp by say 1.1 (or even 1.2 or 1.3) and come up with safe estimate for the correct current of the circuit?

    To your point what leads me to believe the meter is correct is the first reading on the circuits i mentioned being close. If the readings I am recieving from the meter is 5.5 amps and after running the numbers for all the bulbs etc on the circuit and the number comes up to alittle over 6 amps that would be a 10% error margin as you stated. I am just having trouble believing my fridge is running such a low draw.
    Last edited: Sep 23, 2008