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Anonymous asked in Science & MathematicsEngineering · 2 months ago

If transformers keep the same power on both sides, what happens to the amperage at my house?

A power line can be carrying 400,000 Volts along with 250 Amps of current.

If transformers keep the same wattage on both sides, this would mean that stepping it down to 250 Volts would produce 400,000 Amps of current.

Obviously this is not the case, so how can transformers be keeping an equal amount of power on it's primary and secondary coils, if that kind of amperage is not going through?

5 Answers

  • Dixon
    Lv 7
    2 months ago
    Favourite answer

    Your house draws the current it needs (Ih) at 250V depending on what you turned on, which includes the possibility of zero current (Ih = 0). Back at 400,000V, the increased current demand (ΔI) due to your house is, 

    ΔI = Ih x (250/400,000)

    ΔI = Ih / 1600

  • Anonymous
    1 month ago

    In my area, the primary voltage through the neighborhood is 7500 volts.  It is stepped down to 120-0-120 volts (240 volts centertapped) throughout the neighborhood. 

    The individual step down transformers are additive.  They DO give the higher current collectively.

  • 2 months ago

    Basic Ideal Transformer (xfmr) equations:

    Power in = Power out:

    VpIp = VsIs where Vp is the primary voltage and Vs is the secondary etc.

    Vp/Vs = Np/Ns and Ip/Is = Ns/Np

    250A is not realistic at 400kV.  It isn't impossible but not likely 

  • Anonymous
    2 months ago


    those 400.000 Volts don't just serve YOUR house. It serves maybe 1600 houses

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  • 2 months ago

    Ant electronic load pulls the power it needs. The rating of a load represents MAXimum power usually during start up.

    In a transformer power in= Power out.

    Assume you have a transformer with a load connected. It is a 10:1 step down transformer.

    The input is 120v, the output is 12V. The load is pulling 4 A during startup and then settles down to 1A.  4*12=48W and 1*12=12W.

    The primary will be pulling .4A (4A/10)during startup. and .1A (1A/10)after settling.

    120*.4=48W & 120*.1=12W



    When you read a spec of a 60VA transformer they are referring to absolute maximum Voltage times Max current.

    A transformer very rarely works at max power in or out unless it is a very poor design.

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