# Equations?

OK...I know this is something I should know, but I don't. I have 4160 entering the site. I need a 480 3 phase at 200 amps, and a second
panel, 120/208 3 phase 125 amp.
How do I calcultae kVA on the 4160/480 xfmr? And how do I calculate the kVa on the 480/208 xfmr?
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try volt-amps =voltage*current* root(3) in both cases. 1000volt-amps =1KVA
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Don Kelly snipped-for-privacy@shawcross.ca
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Huh?
Why the "root(3)?"
VA can be determine by taking each phase voltage to neutral and multiplying by the current. If the phase currents are the same you just take one current and multiply by 3.

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If you have individual phase voltages and currents, that is perfectly true. But if all you have is line-line voltage, then you'll find that phase voltage is line-line/sqrt(3).
So...
3*Vphase*Iphase = 3*(Vline/sqrt(3))*Iphase = Vline*Iphase*sqrt(3)
That's for a wye connected load. You can do a similar calculation for delta-connected loads but its the Iphase = Iline/sqrt(3)
daestrom
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I should have said root(3)* line voltage*line current or 3*phase voltage* phase current Both are valid for star or for delta. Very often you only have access to 3 terminals so that only the line values can be measured.
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Don Kelly snipped-for-privacy@shawcross.ca