Watts to Amps Calculator

The Watts to Amps Calculator converts electrical power (in watts) into current (in amperes) for DC, AC single-phase, and AC three-phase circuits. Enter your power, voltage, and — for AC — the power factor, and the tool returns the exact current, the formula it used, a step-by-step breakdown, an animated current-flow diagram, and a suggested copper wire gauge and breaker size so you can turn the number into a real-world wiring decision.

How to Convert Watts to Amps

The relationship between power, voltage, and current comes from the power equation P = V × I. Rearranging it to solve for current gives you three formulas, depending on the type of circuit:

Where I is current in amps, P is real power in watts, V is voltage in volts, and PF is the power factor (a number between 0 and 1). The factor √3 ≈ 1.732 appears in three-phase systems because the line-to-line voltage is √3 times the phase voltage.

Worked Example: 1500 W Heater at 120 V

A 1500-watt space heater is a purely resistive load, so its power factor is 1.0. On a 120-volt AC single-phase outlet:

The heater draws 12.5 amps, which is why it needs a dedicated 15 A or 20 A circuit and should not share an outlet with other high-draw appliances.

Watts to Amps Conversion Chart (Single-Phase, PF = 1.0)

What is Power Factor?

Power factor (PF) is the ratio of real power (watts, the power that does useful work) to apparent power (volt-amps, the power the circuit actually carries). It ranges from 0 to 1:

• PF = 1.0 — purely resistive loads such as heaters, incandescent bulbs, and toasters. Watts and volt-amps are equal.
• PF ≈ 0.8–0.95 — typical mixed residential and commercial loads.
• PF ≈ 0.7–0.85 — motors and inductive loads. They draw more current than the wattage alone suggests, which is why motor circuits are sized generously.

Because current rises as power factor falls, a low power factor means more amps for the same watts. For DC circuits there is no power factor, so the tool fixes it at 1.0.

Why Three-Phase Uses √3

In a balanced three-phase system, total real power is P = √3 × V_LL × I × PF, where V_LL is the line-to-line voltage. Solving for current gives I = P / (√3 × V_LL × PF). Three-phase distributes the same power across three conductors, so for a given wattage each line carries less current than an equivalent single-phase circuit — one reason industrial and commercial equipment favours three-phase power.

What Affects the Current Draw?

Wire Gauge & Breaker Sizing

Once you know the amperage, this calculator suggests the smallest common copper conductor (75°C ampacity) that covers 125% of the load — the continuous-load rule used in the US National Electrical Code — along with the next standard breaker size. Small conductors (14, 12, and 10 AWG) also have fixed overcurrent ceilings (15 A, 20 A, and 30 A), which the tool respects. These figures are a planning estimate only: real installations depend on conductor insulation rating, ambient temperature, how many wires share a conduit, cable length and voltage drop, and your local electrical code. Always confirm with a licensed electrician before wiring.

How to Use This Calculator

1. Choose the current type: DC, AC single-phase, or AC three-phase, to match your circuit.
2. Enter power and voltage: Type the power in watts, kilowatts, or horsepower, and enter the supply voltage. For three-phase, use the line-to-line voltage.
3. Enter the power factor: For AC, enter the power factor (around 0.9 for mixed loads, 1.0 for heaters and resistive loads).
4. Click Calculate: See the current in amps, the formula used, the animated flow diagram, and the suggested wire gauge and breaker size.

Frequently Asked Questions

How do you convert watts to amps?

For a DC circuit, amps equal watts divided by volts (I = P / V). For AC single-phase, divide by voltage times power factor (I = P / (V × PF)). For AC three-phase, divide by the square root of 3 times the line-to-line voltage times the power factor (I = P / (1.732 × V × PF)).

What is the formula for watts to amps?

The base relationship is P = V × I, so I = P / V. Alternating-current circuits add a power factor, and three-phase circuits add a factor of the square root of 3 (about 1.732) for line-to-line voltage.

How many amps is 1500 watts at 120 volts?

On a 120-volt AC single-phase circuit with a power factor of 1.0, 1500 watts draws 1500 / 120 = 12.5 amps. A common 1500 W space heater therefore needs a 15 or 20 amp circuit.

What is power factor and why does it matter?

Power factor is the ratio of real power (watts) to apparent power (volt-amps) in an AC circuit, between 0 and 1. Motors and inductive loads have a power factor below 1, so they draw more current than the watts alone suggest. Purely resistive loads such as heaters and incandescent bulbs have a power factor of 1.

Why does three-phase use the square root of 3?

In a balanced three-phase system the line-to-line voltage is the square root of 3 (about 1.732) times the phase voltage. Total power is P = √3 × line voltage × line current × power factor, so solving for current divides the power by 1.732 × V × PF.

What wire size do I need for the calculated amps?

This tool suggests a copper wire gauge whose ampacity covers 125% of the load, following the continuous-load rule, plus the next standard breaker size. It is a planning estimate only. Actual wire sizing depends on conductor temperature rating, ambient conditions, bundling, distance and voltage drop, so always confirm with a licensed electrician and local code.

Additional Resources

• Electric Power - Wikipedia
• Power Factor - Wikipedia
• Three-Phase Electric Power - Wikipedia