# Wattage to Amperage Calculator

Convert watts to amps using I = P / V. Supports DC, single-phase, and three-phase AC circuits with power factor. Includes breaker size recommendation.

## What this calculates

Need to know how many amps a device draws? Divide watts by volts. A 1,500-watt space heater on a 120V circuit draws 12.5 amps. This calculator handles DC, single-phase AC, and three-phase AC, and it suggests the right breaker size based on the NEC 80% continuous load rule.

## Inputs

- **Power** (W) — min 0 — Power consumption or rating in watts.
- **Voltage** (V) — min 0 — Supply voltage in volts.
- **Circuit Type** — options: DC / Single-Phase (PF=1), Single-Phase AC, Three-Phase AC — DC uses I = P/V. AC includes power factor.
- **Power Factor** — min 0.1, max 1 — AC power factor (0 to 1). Only used for AC circuits.

## Outputs

- **Current** (A) — Current draw in amperes.
- **Current** (mA) — Current in milliamperes.
- **Suggested Breaker** — formatted as text — Recommended breaker size based on 80% rule.

## Details

The formulas

- DC: I = P / V

- Single-phase AC: I = P / (V × PF)

- Three-phase AC: I = P / (√3 × V × PF)

Where I is current in amps, P is power in watts, V is voltage, and PF is the power factor.

Why power factor matters for AC

AC motors and other inductive loads do not use all the current they draw to do useful work. The power factor (typically 0.8 to 0.95) accounts for this. A motor rated at 1,000W with a power factor of 0.85 on 120V actually draws about 9.8 amps, not the 8.3 amps you would expect from simple division.

The 80% breaker rule

The National Electrical Code says that for continuous loads (running 3+ hours), a circuit breaker should only be loaded to 80% of its rating. So a 20A breaker should carry no more than 16A continuously. This calculator factors that in when suggesting a breaker size.

Quick references

- 1,500W on 120V = 12.5A (needs 20A breaker)

- 3,600W on 240V = 15A (needs 20A breaker)

- 5,000W on 240V = 20.8A (needs 30A breaker)

## Frequently Asked Questions

**Q: How do you convert watts to amps?**

A: Divide watts by volts. For a 1,200W microwave on a 120V circuit, divide 1,200 by 120 to get 10 amps. For AC circuits with inductive loads, also divide by the power factor.

**Q: Why do I need to know the power factor?**

A: Power factor matters for AC circuits with motors, compressors, or transformers. These devices draw more current than the simple watts/volts formula predicts. A power factor of 0.85 means the device draws about 18% more current than the wattage alone would suggest.

**Q: What breaker size do I need for a given wattage?**

A: Divide the wattage by the voltage to get amps, then select a breaker rated at least 125% of that value (the NEC 80% rule in reverse). For example, 1,800W on 120V is 15A, so you need at least a 20A breaker for continuous use.

**Q: What is the difference between single-phase and three-phase?**

A: Single-phase uses one pair of power conductors and is standard in homes. Three-phase uses three conductors 120 degrees apart and delivers more power with less current per conductor. Industrial equipment, large motors, and commercial buildings typically use three-phase power.

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Source: https://vastcalc.com/calculators/physics/wattage-to-amperage
Category: Physics
Last updated: 2026-04-21