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# Formula for Ohm’s Law for AC Circuits:

where:

 I = current in amperes V = voltage in volts Z = impedance in ohms

# Formula for Power in AC Circuits:

 W = VI(pf)

where:

 W = power in watts pf = power factor

The power factor is the ratio of actual power in a circuit measured in watts to the apparent power in volt-amperes. In a circuit with only resistive loads, such as incandescent lights or electric heating elements, impedance equals resistance, and the power factor is 1.0. Therefore, W = VI.

## Example

Find the current in a 120 volt circuit serving nine 150 watt downlights. From the formula just given,

Since the power factor is 1, then

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