How to Calculate Three Phase Amps From Megawatts
Megawatt 3 phase power applies primarily to large power distribution systems. In fact, the unit of watts represent the actual power used by system after a percentage of the power is lost due to inefficiencies of the load. Therefore, the total power supplied by the power supply is higher than the actual power and is in the form of voltamperes, or in this case, Megavoltamperes or MVA. You need to know the MVA to figure 3phase amps. To figure MVA from megawatts, you need to know the power factor associated with the load, which measures the level of inefficiency of the load.
Instructions

Find the phase voltage, or "Vphase," associated with the 3phase system. Refer to system specifications. For example, assume 4,000 volts, which is typical for power in the megawatt range

Find the power factor, "pf," of the load powered by the megawatt power distribution system. Refer to specifications of the load. A typical power factor for 3phase loads is 0.8.

Find the total power delivered by the power distribution system in Megavoltsamperes or "MVA." Use the formula MVA = MW/pf where MW is the megawatt value of the system. For example, if MW is 20MW and pf is 0.8:
MVA = 20/0.8 = 25 MVA

Calculate 3 phase amps, or "I", using the formula: I = (MVA x 1,000, 000)/(Vphase x 1.732). The 1,000,000 represents "mega" where 1 megavolt is 1,000,000 volts. Continuing with the example:
I = (25 x 1,000,000)/(4,000 x 1.732) = 25,000,000/6,928 = 3608.5 amps.
References
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