A 3-phase power circuit consists of three conductors of alternating current combined into a single power line. The current in each conductor is 1/3 cycle out of phase with the other two. This arrangement produces a smoother power flow and permits lower line voltages. Three-phase power circuits are commonly used in heavy-duty systems such as power transmission lines and large electric motors. However, the procedure to calculate 3-phase power differs from that used for conventional circuits because the relationship between current (amperes), voltage and power consumption (watts) is different.
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Determine the line voltage. Most of the large systems that use 3-phase circuits are equipped with instruments that provide this information. Alternatively, check the manufacturer’s operating manual for the system.
Measure the current flow in amperes. Again, you can usually get this from instrument readings. However, you may need to independently verify the current flow in some situations. The easiest way is to use a clamp-on ammeter to determine the current in the line. Ammeter designs vary, so consult the manufacturer’s instructions for use. Ammeters are available at electronics supply stores.
Calculate 3-phase power by multiplying the voltage times the amperes times a power factor. The power factor adjusts the equation to allow for the effects of the 3-phase circuit. For 3-phase circuits, the power factor is the square root of 3 (about 1.73 if your calculator doesn’t have a square root function). For example, with a line voltage of 250 and a current flow of 75 amps, you have 250 x 75 x 1.73. The power consumption is 32,437.5 watts.
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