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Watts to Amps Calculator

Convert watts to amps for DC, single-phase, and three-phase circuits. Enter power, voltage, and power factor to calculate current draw.

How to Convert Watts to Amps

Watts, Amps, and Voltage

The relationship between watts (power), amps (current), and volts (voltage) is one of the most fundamental concepts in electrical work. Power is the rate at which electrical energy is transferred, current is the flow of electrons through a conductor, and voltage is the electrical pressure that pushes that current. These three quantities are directly related — if you know any two, you can calculate the third. This calculator solves for current (amps) when you know power (watts) and voltage (volts).

The Watts to Amps Formula

DC circuits: I = P / V

Single-phase AC: I = P / (V × PF)

Three-phase AC: I = P / (V × PF × √3)

Where I = current (A), P = power (W), V = voltage (V), PF = power factor, √3 ≈ 1.732

The DC formula is the simplest because there is no phase angle between voltage and current. In AC circuits, the power factor accounts for the phase difference between voltage and current waveforms caused by inductive or capacitive loads. For purely resistive loads (like heaters and incandescent lights), the power factor is 1.0 and can be ignored. For motors, transformers, and other inductive loads, the power factor is typically between 0.8 and 0.9.

The three-phase formula includes the √3 factor because three-phase power is delivered across three conductors with voltages 120° apart. The line-to-neutral voltage in a three-phase system relates to the line-to-line voltage by a factor of √3.

Worked Examples

Example 1 — Single-phase resistive load: A 1500W electric heater running on 120V with a power factor of 1.0.

  1. I = P / (V × PF) = 1500 / (120 × 1.0)
  2. I = 1500 / 120 = 12.5A
  3. A standard 15A breaker can handle this load. For continuous duty (3+ hours), apply the 125% rule: 12.5 × 1.25 = 15.63A — a 20A breaker is required.

Example 2 — Three-phase motor: A 3000W motor running on 240V three-phase with a power factor of 0.85.

  1. I = P / (V × PF × √3) = 3000 / (240 × 0.85 × 1.732)
  2. I = 3000 / 353.33 ≈ 8.49A
  3. For continuous motor duty per NEC 430.22, size the breaker at 125%: 8.49 × 1.25 = 10.61A — a 15A breaker is appropriate.

Practical Tips

  • Resistive loads (heaters, toasters, incandescent bulbs) have a power factor of 1.0 — you can ignore the PF input for these.
  • Motor loads typically have a power factor between 0.8 and 0.9. Check the motor nameplate for the exact rating.
  • Continuous loads (running for 3 hours or more) must be sized at only 80% of the breaker rating per NEC 210.20. For example, a 15A breaker can only carry 12A continuously.
  • Always round up when selecting wire sizes and breakers — never round down.

Code References

NEC 220.5

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Frequently Asked Questions

How many amps is 1500 watts at 120 volts?
At 120V with a power factor of 1.0 (single-phase), 1500 watts draws 12.5 amps (1500 ÷ 120 = 12.5A). This is a common scenario for space heaters, microwave ovens, and other 120V household appliances. Since this exceeds 80% of a 15A breaker (12A), a 20A circuit is recommended for continuous use.
What is power factor and when does it matter?
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amps). For purely resistive loads like heaters and incandescent lights, PF = 1.0 and can be ignored. For inductive loads like motors, transformers, and fluorescent ballasts, PF is typically 0.8–0.9. A lower power factor means more current is drawn for the same amount of real power, which affects wire sizing and breaker selection.
Why is the three-phase formula different from single-phase?
The three-phase formula includes a √3 (approximately 1.732) multiplier because three-phase power is distributed across three conductors with voltages 120° out of phase. This geometric relationship means that three-phase systems can deliver the same total power with less current per conductor compared to single-phase, which is why industrial facilities use three-phase power — it is more efficient.
How do I account for continuous loads when sizing breakers?
Per NEC 210.20, continuous loads (those expected to run for 3 hours or more) must be sized at no more than 80% of the circuit breaker rating. This is the 125% rule: multiply the calculated current by 1.25 to find the minimum breaker size. For example, a 12A continuous load requires a minimum 15A breaker (12 × 1.25 = 15A). Always round up to the next standard breaker size.