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Thread: Three phase / Single phase

  1. #1
    Gold Member Martinco's Avatar
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    Three phase / Single phase

    I guess the gurus should be able to answer my question.

    In SA we have 380 volt 3 phase supply to e.g. a factory. From phase to neutral we have 220/230 volt.
    Why then in USA is the supply 220 volt three phase and 110 volt from phase to neutral ?
    I am specifically referring to the difference from 380 to 220 ( 58 % ) and in the case of USA ( 50 % )
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    Whenever electrical designs are done in South Africa for LV systems 400V(3) & 230V(1) are used as the base operating voltage, and not 380V & 220V.

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    the limited knowledge i have with usa electrical installations...it works on a similar basis as the generator...they have a 220/110 system because in theory it is safer to have 110volt feeding circuits in a house ...and the 220 volt circuit will only feed the heavier loads...the usa has a very old system.

    a standard generator purchased in sa "can" also be connected in this manner...so when you go camping you can buy camping stuff from the usa and not worry about the voltage issues...provided you get someone to wire the generator in this manner.

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    Gold Member Singhms's Avatar
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    I cannot remember exactly, but I also think it has to do with the operating frequency of the actual generators in SA vs USA. i.e. SA 50Hz & USA 60Hz if i remember correctly.

    I am sure someone will verify this soon
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    you fellas are too sharp on a sunday morning...you are 100 % correct the frequency in the usa is 60 hz and sa the frequency 50 hz

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    Gold Member Martinco's Avatar
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    Ok, but I still do not understand why the difference from 400/380 to 230 and in the case of USA 220 to 110.

    Surely the frequency does not influence the voltage generated, provided the windings/lamination core are designed for the specific frequency.

    Looking at how the center tap of a 3 phase alternator/motor is determined, I cannot understand how a different "formula" can be used for the two different systems.
    Martin Coetzee
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    380/1.732 (sq.rootof3) = 220
    220/1.732 = 127 volts not sure why its a bit higher.

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    i think its actually 110/208 which will work out correctly.

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    Bronze Member mikilianis's Avatar
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    Hi guys a shot in the dark could it be economics I mean why do the mines operate on 660Volts 3 phase the higher the voltage the less the current the thinner the conducters

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    Diamond Member AndyD's Avatar
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    All the voltages being discussed are just nominal values and are rounded up, down and any other which way plus there's large allowable tolerances as well. The actual formula is as Bergie stated. There's also a difference of up to 10% between 'supply voltages' and 'equipment running voltages' because of voltdrop across the supply system cabling.
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