# 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: 2026-04-08