Electronics

Amps / Watts / Volts Calculator

Convert between amps, watts, and volts for DC, single-phase AC, and three-phase AC circuits. Includes power factor for AC calculations.

Quick Answer

DC: Watts = Volts × Amps. AC single-phase: Watts = Volts × Amps × Power Factor. AC three-phase: Watts = √3 × Volts × Amps × PF.

Solve For

Circuit Type

Amps (A)

Volts (V)

Results

1200.00 W

Power (Watts)

About This Tool

The Amps/Watts/Volts Calculator converts between the three fundamental electrical measurements. It handles DC circuits with the simple P=VI formula, single-phase AC with power factor, and three-phase AC with the √3 factor. Electricians, engineers, and DIY enthusiasts use these conversions daily for sizing wires, breakers, and equipment.

Ohm's Law and the Power Formula

For DC circuits, power in watts equals voltage times current: P = V × I. This can be rearranged to find any unknown: I = P/V or V = P/I. These relationships form the basis of all electrical circuit analysis. Combined with Ohm's law (V = IR), you can calculate any electrical quantity from any two known values.

Power Factor in AC Circuits

AC circuits with motors, transformers, or other inductive loads don't convert all apparent power into useful work. The power factor (PF) represents this efficiency, ranging from 0 to 1. Real power in watts equals apparent power in VA times the power factor. A motor with a PF of 0.85 drawing 10A at 120V delivers 1020W of real power but requires 1200VA from the supply.

Three-Phase Power

Three-phase systems use three conductors carrying AC current offset by 120 degrees. The power formula includes a √3 (1.732) factor: P = √3 × V_line × I_line × PF. Three-phase is more efficient for heavy loads and is standard in commercial and industrial settings. Residential service is typically single-phase.

Practical Applications

Knowing how to convert between amps, watts, and volts helps you size electrical circuits correctly. When installing a new appliance, check the nameplate for watts and your outlet voltage to determine the current draw. This tells you whether your existing circuit breaker can handle the load. Overloaded circuits trip breakers and can cause fires.

Frequently Asked Questions

How do I convert watts to amps?

For DC: Amps = Watts / Volts. For single-phase AC: Amps = Watts / (Volts x Power Factor). For three-phase AC: Amps = Watts / (sqrt(3) x Volts x PF). You always need to know the voltage. A 1500W heater on a 120V circuit draws 12.5 amps.

What is power factor and when does it matter?

Power factor is the ratio of real power (watts) to apparent power (VA) in an AC circuit. It matters for motors, transformers, and fluorescent lighting. Pure resistive loads like heaters have a PF of 1.0. Motors typically range from 0.7 to 0.95. Low power factor means more current for the same real power, requiring larger wires and breakers.

What is the difference between watts and VA?

Watts (W) is real power that does actual work. Volt-amps (VA) is apparent power, the product of voltage and current. In DC and purely resistive AC circuits, they are equal. In AC circuits with reactive loads, VA is always higher than watts. UPS systems and generators are rated in VA because they must supply the full apparent power.

Why does three-phase use the sqrt(3) factor?

The sqrt(3) factor (1.732) comes from the geometry of three-phase power. Three voltage waveforms offset by 120 degrees combine to deliver sqrt(3) times more power than a single phase at the same voltage and current per conductor. This makes three-phase more efficient for transmitting large amounts of power.

How many watts can a 15-amp circuit handle?

A 15-amp circuit at 120V can theoretically handle 1800W. However, NEC code requires that continuous loads use no more than 80% of the breaker rating, limiting you to 1440W for loads running more than 3 hours. Always leave headroom for startup surges and other devices on the same circuit.

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