Calculate Current (Amps) with Ohm's Law and the Power Formula

I = Current (amps)
E = Electromotive Force or Voltage (volts)
R = Resistance (ohms)
P = Power (watts)

How to Calculate Current When Voltage and Resistance are Known:    I = E / R

This formula is known as Ohm's law which describes the mathematical relationship between current, voltage and resistance in an electrical circuit. German physicist Georg Simon Ohm determined that voltage and current are directly proportional and published Ohm's Law in 1827. According to Ohm's Law, when voltage increases across two points at opposite ends of a conductor, current through the conductor increases proportionally.

Example: Find the current of a 120 volt circuit with a resistance of 60 ohms.

120 volts / 60 ohms = 2 amps

How to Calculate Current When Power and Voltage are Known:    I = P / E

This formula is the power formula which describes the mathematical relationship between current, voltage and power in an electrical circuit. Power and current are directly proportional.

Example: Find the current of a 120 volt circuit with a 1440 watt load.

1440 watts / 120 volts = 12 amps

How to Calculate Current When Resistance and Power are Known:    I = √ (P / R)

Using a combination of Ohm's law and the power formula, we can determine the mathmatical relationship between current, power and resistance.

Example: Find the current of circuit that consumes 625 watts through a 12.75 ohm resistor.

√ (625 / 12.75) = 7 amps

* Electric motors are rated in horsepower (hp). 1 hp = 746 watts.
** Generators are rated in kilowatts (kW). 1 kW = 1000 watts.

Warning: When using this information to perform electrical work, call a licensed electrician and consult the NEC® for safety. All licensed electricians have passed examinations covering the National Electric Code®, know state and local building codes, and may carry insurance to cover damages.