Three phase power circuits are often used in power transmission lines and large electric motors because they allow lower line voltages and provide a smoother flow of electricity. A three phase circuit consists of three alternating current conductors combined into a single power line. Each conductor is 1/3 cycle out of phase with the other two. Calculating three phase amperage or other electrical values is a little more complicated than for conventional circuits because a “power factor” must be incorporated into the computation.

Consult the operating manual or manufacturer’s specifications for the line voltage.

Look at the power consumption indicator for the system. Everyday appliances and motors don’t usually have a power consumption indicator. However, the large systems that rely on three phase power circuits routinely come with readouts. Make a note of the scale. Due to the size of the systems, the reading may be in kilowatts rather than watts. If so, multiply by 1000 to convert kilowatts to watts.

Divide the power consumption in watts by the line voltage multiplied by the power factor to find the amperage. For three phase circuits the power factor is the square root of 3. If your calculator doesn’t have a square root function, use 1.73 as an approximation of the square root of 3. For example, a three phase circuit using 25,000 watts of power and a line voltage of 250 will have a current flow of 25,000/(250 x 1.73), which is equal to 57.80 amperes.