Calculating kVA Load

The calculation for single phase loads on a three phase generator is a bit complex due to the fact that a three phase generator has to be de-rated by 1/3 when used on single phase.  This is because the current for an equivalent kVA, single phase is much higher than for three phase.

Formula:

V = Voltage generated

I = Amps Available

kVA = Kilo Volt Amps

.66 = De-rating factor of 1/3

Calculation Example 1

The current which can be supplied by a three-phase 50 kVA generator when connected 240 volt single phase is determined as follows:

            First de-rate by 1/3 e.g. 50kVA x .66 = 33 3VA

            for single phase   I = kVA x 1000 ÷ Volts   = 33 x 1000 ÷ 240 = 132.5 Amps

Calculation Example 2

You require a 30 Amp single phase 240 volt capable unit to be powered  .  What size generator do you require?

            for single phase generator   kVA     V x I ÷ 1000    
                      =    240 x 30 ÷ 1000    
                      = 7.2 kVA

            for a three phase generator  kVA = 7.2 ÷ .66 = 10.9 kVA

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Last modified: April 17, 2010