How to calculate amps from kw

Written by johnmcgee
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How to calculate amps from kw
Delivery of three-phase electricity (power image by zusyl from Fotolia.com)

Kilowatt or kW means a 1,000 watts. A watt measures the rate at which energy in an electrical current flow dissipates. For a simple circuit, wattage relates to amperage according to the formula P=IV. For circuits with coils and alternating current, some of the energy stored in the coil as magnetic energy returns to the circuit when the current changes direction. Therefore less energy dissipates than indicated by the formula P=IV and a multiplier less than one called a "power factor" is needed.

Skill level:
Moderate

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Instructions

  1. 1

    Calculate current in amps from power in kilowatts in a simple circuit by dividing the power by the voltage. For example, if the direct-current power source is a battery with nine volts and the power is 0.001kW, then the current is 9V / 0.001kW = 9V / 1W = 9 amps.

  2. 2

    Determine the current if you know the circuit's resistance instead of its voltage by using the formula P=I^2*R, where "^2" means "squared." For example, if a battery of unknown voltage connects to a single-loop circuit with 5 ohms of resistance, producing 0.05kW of power, then the current is the square root of 50W / 5 ohms, or 3.16 amps after rounding.

  3. 3

    Determine the current for a single-phase alternating circuit with the formula 1,000 * (power in kW) / [voltage * power factor]. The power factor accounts for inductance in the circuit that stores energy and returns it back into the circuit. A good estimate for the power factor of a residential home is around 0.85.

  4. 4

    Determine the current for a three-phase alternating circuit with the formula 1,000 * (power in kW) / [voltage * power factor * √3]. (ref. 3) √3 equals 1.73, after rounding. Three-phase AC is how most electricity is delivered throughout the world.

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