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voltage drop at the meter box. excessive?

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  1. #1
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    Default voltage drop at the meter box. excessive?

    I've been doing some safety checks in an old house, and found the voltage drops higher than expected. e.g. oven plus grill dropping 12V when both on (20A), so barely in standard. Checking the voltage on another circuit (proper proxy for the meter box?) I see a 10V drop with 30A load. Is that normal for an old place? Cause for worry? Its about a 40m overhead run to the street pole.

    Looked in the roof, and got a shock at the ancient wiring before realising it was not in use. But much of the newer 2.5mm^2 cable was buried under the insulation. Pulled it all out on top and saw no sign of heat or other damage. No improper junctions.
    Have checked active/neutral allocation and earthing on outlets. Any other basic safety checks to do?

  2. #2
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    Default

    Use this table with Ohms law and see what you get.
    voltage-drop.jpg

  3. #3
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    Default

    Had a second look at your post and I assumed the voltage drop was at the appliance however from re reading it may be the mains so is it at the meter box or the appliance, if the appliance the table is relevant if the meter box it is not. One thing you could also answer is the open circuit voltage.

    Cable can go under the insulation but it has to be de rated. I bet the installers couldn't care less about this.

    Refer to this post on de rating http://www.renovateforum.com/f195/sa...lation-100883/

  4. #4
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    That table must be single conductor, so double for the round trip?

    My subcircuit voltage drop is as expected (~5V @20A).
    Assuming the remaining losses are mostly in the overhead line to the power poll, 7V/(20A x 45m x 2)= 3.9mV/Am.
    From your table, that is not too far off a 6mm˛ cable to the power pole.
    Looked again at the meter box, and the main switch is only rated at 36A , so that would be right. No air-con for me without an upgrade!

  5. #5
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    > I assumed the voltage drop was at the appliance

    Sorry for the confusion. I measured at the appliance first, then on an unloaded sub-circuit so as to get the meter box voltage.
    With just a few lights and low-power gadgets on, I get 240 to 250V, so it is never dropping enough to damage anything - still 220V at worst.
    I tried calling the power authority to find what was supplied to the house, but since the gov't has split SECWA to multiple privatised and corporatised entities, that got me nowhere.

    So I guess it is OK for now. Will ask about upgrading the line next time I need a sparkie.
    Meanwhile its their loss, not mine, ha ha.

  6. #6
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    Default

    Double check your cm cable size six seems too small but may be right.

    Also double check the length of the cm and the length of the proposed ac sub circuit and tell us how it will be run eg clipped to rafters or over the top of insulation whatever the more info the better and we can crunch the numbers.
    you should be able to put in an a ac with what you have you may just need to upsize the sub circuit cable which would be much cheaper and lees painless than an upgraded cm.

    Also buy a cheap ir thermostat of eBay and take a base reading of all your connections on a cool night then crank the grill and hot plates for ten to five teen minuites and look for hot spots.

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