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another voltage drop question

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  • 16-02-2011 11:10pm
    #1
    grousedogtom
    Registered Users Posts: 463 ✭✭


    Hi
    I need to run an swa to a pump house 55mts from consumer unit buried underground , the submersible pump in well will be 1.5 hp or 1.1kw, now i have checked several online VD calculators and the results show 1.5 cable , this seems a little small , are these online calculators not accurate and is there a basic VD formula to calculate this?


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    #2
    pieface_ie
    Closed Accounts Posts: 301 ✭✭pieface_ie


    Yeah, a very quick calculation going by the information you supplied a single phase pump @ powerfactor of 1 (as not specified)

    1.5 mm = 7.3 v drop over 55 meters

    2.5 mm = 4.5 v drop over 55 meters

    so a 1.5 mm is fine.


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    #3
    sparcocars
    Registered Users Posts: 148 ✭✭sparcocars


    grousedogtom wrote: »
    Hi
    I need to run an swa to a pump house 55mts from consumer unit buried underground , the submersible pump in well will be 1.5 hp or 1.1kw, now i have checked several online VD calculators and the results show 1.5 cable , this seems a little small , are these online calculators not accurate and is there a basic VD formula to calculate this?

    By my calculations you would only have a 1.3% volt drop which is 3 volts on that cable if it was a 1.5 square. Well below the 4% regulation. Maybe a 2.5 square cable would be better for the simple reason that the pump might be uprated at a later date and this would accommodate that.

    Formula : Resistivity of cable x Length of cable
    CSA of cable

    Resistivity of annealed copper : .0172 micro ohms per mm

    .0172 x 55
    1.5 = .630 ohms


    1100 watts
    230 volts = 4.78 amps


    .630 ohm x 4.78 amps = 3 volts


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    #4
    pieface_ie
    Closed Accounts Posts: 301 ✭✭pieface_ie


    By my calculations you would only have a 1.3% volt drop which is 3 volts on that cable if it was a 1.5 square. Well below the 4% regulation. Maybe a 2.5 square cable would be better for the simple reason that the pump might be uprated at a later date and this would accommodate that.

    I think you made a mistake somewhere in your calculation mate, that cant be correct.


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    #5
    Randyleprechaun
    Registered Users Posts: 876 ✭✭✭Randyleprechaun


    I'd say you should be using 2.5mm2:

    You are allowed 2.5% volt drop for 230V which is 5.75V

    FOR 1.5mm2 29(VD)X55(mtr)X4.8(A)/1000=7.65V

    For 2.5mm2 18(VD)X55(mtr)X4.8(A)/1000=4.75V

    Figures are approximate, but even at that, in my opinion it has to be 2.5mm2


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    #6
    M cebee
    Closed Accounts Posts: 4,431 ✭✭✭M cebee


    i think your FLA for a 1.5hp is up around 10amps inc. efficiency and power factor

    3*4 swa by my reckoning to be safe

    that's about 6.6v running and about 30-40 at startup


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    #7
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    the overload fitted in a 1.5 hp control box is a 10 amp so probably runs a couple of amps less than this ,I was thinking at least 2.5 myself so i put in 4 sq today to allow for future changes etc , its amazing the difference in replys from 1.5 to 4 sq is the full formula for VD that complex or is there a simplified formula that would give you a good indication?Many thanks


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    #8
    M cebee
    Closed Accounts Posts: 4,431 ✭✭✭M cebee


    1.5HP at 230v 10amp FLA

    according to most charts
    http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm

    55 *12mV*10 is approx 6.6 from memory

    being 12mv for 4sq

    you have to allow at bit extra with motors for startup

    sometimes 125% of FLA is used or you can check terminal voltage for LRA or startup

    1.5 and 2.5 are undersized imo


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    #9
    M cebee
    Closed Accounts Posts: 4,431 ✭✭✭M cebee


    during startup you might be allowing 15% or so

    or 85% of nominal voltage


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    #10
    pieface_ie
    Closed Accounts Posts: 301 ✭✭pieface_ie


    The are numerous factors to be taken into consideration when sizing a cable.

    Ambient temperature
    Cable type
    voltdrop per meter of the cable
    No of other circuits run with it
    Installation method
    ...etc

    When sizing a cable for a given application there are many different installation scenarios. They all do not fall under the one bracket/calculation.


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    #11
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    grousedogtom wrote: »
    its amazing the difference in replys from 1.5 to 4 sq is the full formula for VD that complex or is there a simplified formula that would give you a good indication?Many thanks

    2.5 would cover it. A 1.1kw resistive load and 1.5 would possibly cover it. But with a motor 4 square will give you better coverage and is better for the starting current, and would likely improve starting performance. A 1.1kw load would take 5 amps for the 1.1kw output, but with a small single phase motor it could be double this current with efficiency and power factor as said. So for the 55 meter run and close to 10 amp current a 4 square cable would be more than enough and probably allow a bigger pump also if ever required.


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