Table of Contents
Watts to Amps Conversion
Converting wattage to amperage is essential for electrical safety and circuit design. The current draw of an appliance determines what size wire and circuit breaker are needed. Undersized wiring or breakers can cause overheating and fire hazards, while oversized components waste money.
For DC circuits, the conversion is straightforward: divide watts by volts. For AC circuits, the power factor must be considered because reactive loads (motors, transformers) draw more current than their wattage alone would suggest.
Conversion Formulas
Quick Reference Table (120V, PF=1)
| Watts | Amps @120V | Amps @240V |
|---|---|---|
| 100 | 0.83 | 0.42 |
| 500 | 4.17 | 2.08 |
| 1000 | 8.33 | 4.17 |
| 1500 | 12.50 | 6.25 |
| 2000 | 16.67 | 8.33 |
| 3000 | 25.00 | 12.50 |
Breaker Sizing Guide
The NEC requires that continuous loads not exceed 80% of the breaker rating. This means a 15A breaker can safely handle 12A continuous, and a 20A breaker can handle 16A continuous. Always size up to the next standard breaker size.
- 15A breaker: up to 1,440W at 120V (12A continuous)
- 20A breaker: up to 1,920W at 120V (16A continuous)
- 30A breaker: up to 5,760W at 240V (24A continuous)
- 50A breaker: up to 9,600W at 240V (40A continuous)
Frequently Asked Questions
Why does my 1500W space heater trip a 15A breaker?
A 1500W heater at 120V draws 12.5A. A 15A breaker is rated for 12A continuous (80% rule). The heater exceeds this, causing the breaker to trip. Use a 20A circuit for 1500W heaters.
Does voltage affect amperage?
Yes. Higher voltage means lower amperage for the same wattage. A 2400W appliance draws 20A at 120V but only 10A at 240V, which is why large appliances use 240V circuits for thinner, cheaper wiring.
What is apparent power vs real power?
Real power (watts) does actual work. Apparent power (VA) includes reactive power that oscillates in the circuit. For resistive loads (heaters, incandescent bulbs), they are equal. For inductive loads (motors), apparent power exceeds real power by the power factor ratio.