Watts to Amps: Complete Conversion Guide
Watts to Amps Formulas
- DC: I (A) = P (W) / V (V)
- Single-phase AC: I (A) = P (W) / (V (V) × PF)
- Three-phase AC: I (A) = P (W) / (VLL (V) × PF × √3)
Quick Conversion at Common Voltages
| Watts | 120V (DC/PF=1) | 120V (PF=0.8) | 240V (DC/PF=1) | 240V (PF=0.8) |
|---|---|---|---|---|
| 100W | 0.83A | 1.04A | 0.42A | 0.52A |
| 500W | 4.17A | 5.21A | 2.08A | 2.60A |
| 1,000W | 8.33A | 10.42A | 4.17A | 5.21A |
| 1,500W | 12.50A | 15.63A | 6.25A | 7.81A |
| 2,000W | 16.67A | 20.83A | 8.33A | 10.42A |
| 3,000W | 25.00A | 31.25A | 12.50A | 15.63A |
| 5,000W | 41.67A | 52.08A | 20.83A | 26.04A |
| 10,000W | 83.33A | 104.17A | 41.67A | 52.08A |
Sizing Breakers from Watts
Step 1: Calculate amps: I = P / V (or P / (V × PF) for AC motors)
Step 2: For continuous loads (>3 hours): multiply by 1.25
Step 3: Select breaker ≥ calculated amps
Step 4: Verify wire ampacity ≥ breaker rating
Examples
1,500W space heater at 120V: I = 1,500/120 = 12.5A. Continuous: 12.5 × 1.25 = 15.6A → 20A breaker, 12 AWG wire.
7,200W electric dryer at 240V: I = 7,200/240 = 30A. Continuous: 30 × 1.25 = 37.5A → 40A breaker, 8 AWG wire.
3,600W AC motor at 240V, PF 0.85: I = 3,600/(240 × 0.85) = 17.6A. Breaker per NEC 430: 17.6 × 1.5 = 26.4A → 30A breaker, 10 AWG wire.
Common Mistakes
- Ignoring power factor for motors: A 1,000W motor at 120V, PF 0.7 draws 11.9A, not 8.3A. The wire must carry the full current regardless of PF.
- Not applying the 80% rule: Continuous loads (heaters, EV chargers) require 125% breaker sizing.
- Using wrong voltage: 240V equipment on 120V draws double the amps. Always check the nameplate.
Standards Reference
- NEC Article 220 — Branch Circuit Calculations
- NEC 210.20 — Continuous Loads
- NEC Article 430 — Motors
Frequently Asked Questions
How many amps is 1500 watts?
At 120V: 1,500W / 120V = 12.5A. At 240V: 1,500W / 240V = 6.25A. For AC motors with PF 0.8 at 120V: 1,500 / (120 × 0.8) = 15.6A. Most 1,500W space heaters are designed for 120V, 12.5A — fitting on a 15A or 20A circuit.
How do I convert watts to amps for a motor?
Use I = P / (V × PF). Motor power factor is typically 0.7–0.85. A 2,000W motor at 240V, PF 0.8: I = 2,000 / (240 × 0.8) = 10.4A. Don't forget to apply NEC 430 rules for motor wire and breaker sizing.
How many amps does a refrigerator use?
A typical refrigerator uses 150–400W running but draws 1,200W+ during compressor startup. Running amps: 150/120 = 1.25A to 400/120 = 3.3A. Startup surge can be 8–12A for a few seconds. A dedicated 20A circuit is recommended.
How many amps is 5000 watts?
At 120V: 5,000 / 120 = 41.7A (requires 50A circuit, 6 AWG wire). At 240V: 5,000 / 240 = 20.8A (requires 25A circuit, 10 AWG wire). High-wattage loads like ranges and water heaters typically use 240V to keep current manageable.
What size breaker for 3000 watts?
At 120V: 3,000 / 120 = 25A → 30A breaker, 10 AWG wire. At 240V: 3,000 / 240 = 12.5A → 15A breaker, 14 AWG wire (but 20A/12 AWG is more common). For continuous loads: multiply amps by 1.25 first.