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How to calculate transformer KVA

Kelly Marie Angel
If you have a transformer with let's say 3x 400 amp fuses protecting its LV outgoing cables how do I calculate its KVA? Do you just time amps by the single phase voltage or by the volts across 2 phases
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  • Former Community Member
    0 Former Community Member
    Rated current of the LV winding, Rated voltage of the LV winding and Apparent power of three phase transformers are components of the following formula: S= √3 x Ur x Ir

    Usually everything happens the other way around: active power [P in kW] and power factor [cos] of the summarized load is known, it allows to find apparent power of the transformer [S in kVA] and then we can calculate the rated current of the LV winding. [when choosing a real  transformer, power is always taken with a reserve]

    But in you case, if you know the rated voltage of the LV winding [for example 415V phase-to-phase] then the question is - how do the rated current of the transformer and the main protection device on LV side correlate?
    I propose to assume that the rated current of the LV winding is not less than nominal current protection device, so   S =  1.732 x 415V x 400A =~ 300kVA
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  • Former Community Member
    0 Former Community Member
    Rated current of the LV winding, Rated voltage of the LV winding and Apparent power of three phase transformers are components of the following formula: S= √3 x Ur x Ir

    Usually everything happens the other way around: active power [P in kW] and power factor [cos] of the summarized load is known, it allows to find apparent power of the transformer [S in kVA] and then we can calculate the rated current of the LV winding. [when choosing a real  transformer, power is always taken with a reserve]

    But in you case, if you know the rated voltage of the LV winding [for example 415V phase-to-phase] then the question is - how do the rated current of the transformer and the main protection device on LV side correlate?
    I propose to assume that the rated current of the LV winding is not less than nominal current protection device, so   S =  1.732 x 415V x 400A =~ 300kVA
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