Watts to amps formulas
Current equals power divided by voltage. For AC loads, divide by the power factor (PF) too, because a load with PF below 1 draws more current than its watts alone suggest:
DC: A = W ÷ V
AC single phase: A = W ÷ (V × PF)
AC three phase: A = W ÷ (√3 × VL-L × PF) ≈ W ÷ (1.732 × V × PF)
For three phase, the answer is the current in each line, and V is the line-to-line voltage (208, 480 or 600 V). For a 240 V single-phase appliance such as a dryer or water heater, divide by 240, not 120. Each hot leg carries the full current.
Worked examples
1. A 1500 W space heater at 120 V
A = 1500 ÷ 120 = 12.5 A
A heater runs for hours, so treat it as a continuous load. On a 15 A breaker, the continuous limit is 80%, or 12 A, so a 1500 W heater exceeds it; a dedicated 20 A circuit (16 A continuous) has room for it. That is also why a heater on a shared 15 A circuit trips the breaker.
2. A 4500 W water heater at 240 V
A = 4500 ÷ 240 = 18.75 A
NEC 422.13 treats a storage water heater of 120 gallons or less as a continuous load, so the branch circuit must be rated at least 125% of 18.75 A, which is 23.4 A. The next standard breaker is 25 A, and 30 A with 10 AWG copper is also a common choice. Check the run length with the voltage drop calculator.
3. A 10 kW three-phase heater at 208 V
A = 10,000 ÷ (1.732 × 208) = 27.8 A per line
The same heater at 480 V draws 10,000 ÷ (1.732 × 480) = 12.0 A, and at 600 V it draws 9.6 A. Higher voltage means less current and smaller wire for the same power.
4. A 300 W load on a 12 V DC system
A = 300 ÷ 12 = 25 A
Low-voltage DC needs a lot of current for modest power. Voltage drop on long 12 V runs is often the deciding factor, not ampacity.
5. A 2000 W motor load with PF 0.85
A = 2000 ÷ (240 × 0.85) = 9.8 A
At unity power factor the answer would be 8.33 A. Ignoring PF understates the current by about 18% here.
Watts to amps chart (PF = 1)
Single-phase columns use W ÷ V. Three-phase columns use W ÷ (1.732 × V).
| Watts | 12 V DC | 120 V | 240 V | 277 V | 347 V | 208 V 3φ | 480 V 3φ | 600 V 3φ |
|---|---|---|---|---|---|---|---|---|
| 100 W | 8.33 A | 0.83 A | 0.42 A | 0.36 A | 0.29 A | 0.28 A | 0.12 A | 0.10 A |
| 500 W | 41.7 A | 4.17 A | 2.08 A | 1.81 A | 1.44 A | 1.39 A | 0.60 A | 0.48 A |
| 1,000 W | 83.3 A | 8.33 A | 4.17 A | 3.61 A | 2.88 A | 2.78 A | 1.20 A | 0.96 A |
| 1,200 W | 100.0 A | 10.0 A | 5.00 A | 4.33 A | 3.46 A | 3.33 A | 1.44 A | 1.15 A |
| 1,500 W | 125.0 A | 12.5 A | 6.25 A | 5.42 A | 4.32 A | 4.16 A | 1.80 A | 1.44 A |
| 1,800 W | 150.0 A | 15.0 A | 7.50 A | 6.50 A | 5.19 A | 5.00 A | 2.17 A | 1.73 A |
| 2,000 W | 166.7 A | 16.7 A | 8.33 A | 7.22 A | 5.76 A | 5.55 A | 2.41 A | 1.92 A |
| 2,400 W | 200.0 A | 20.0 A | 10.0 A | 8.66 A | 6.92 A | 6.66 A | 2.89 A | 2.31 A |
| 3,000 W | 250.0 A | 25.0 A | 12.5 A | 10.8 A | 8.65 A | 8.33 A | 3.61 A | 2.89 A |
| 3,600 W | 300.0 A | 30.0 A | 15.0 A | 13.0 A | 10.4 A | 9.99 A | 4.33 A | 3.46 A |
| 4,000 W | 333.3 A | 33.3 A | 16.7 A | 14.4 A | 11.5 A | 11.1 A | 4.81 A | 3.85 A |
| 4,500 W | 375.0 A | 37.5 A | 18.8 A | 16.2 A | 13.0 A | 12.5 A | 5.41 A | 4.33 A |
| 5,000 W | 416.7 A | 41.7 A | 20.8 A | 18.1 A | 14.4 A | 13.9 A | 6.01 A | 4.81 A |
| 7,200 W | 600.0 A | 60.0 A | 30.0 A | 26.0 A | 20.7 A | 20.0 A | 8.66 A | 6.93 A |
| 9,600 W | 800.0 A | 80.0 A | 40.0 A | 34.7 A | 27.7 A | 26.6 A | 11.5 A | 9.24 A |
| 10,000 W | 833.3 A | 83.3 A | 41.7 A | 36.1 A | 28.8 A | 27.8 A | 12.0 A | 9.62 A |
277 V and 480 V are common for lighting and motors in US commercial buildings. 347 V and 600 V are the Canadian equivalents.
Circuit capacity in watts
Going the other way is just as useful: how much can a circuit carry? Multiply the breaker rating by the voltage, then take 80% for continuous loads.
| Breaker | Volts | Maximum watts | Continuous (80%) |
|---|---|---|---|
| 15 A | 120 V | 1,800 W | 1,440 W |
| 20 A | 120 V | 2,400 W | 1,920 W |
| 15 A | 240 V | 3,600 W | 2,880 W |
| 20 A | 240 V | 4,800 W | 3,840 W |
| 30 A | 240 V | 7,200 W | 5,760 W |
| 40 A | 240 V | 9,600 W | 7,680 W |
| 50 A | 240 V | 12,000 W | 9,600 W |
The continuous-load rule in the US and Canada
The two codes say the same thing from opposite directions. In the US, NEC 210.19(A) and 210.20(A) require the conductor and breaker to be rated at least 125% of a continuous load, meaning one that runs for three hours or more. In Canada, CEC Rule 8-104 limits a continuous load to 80% of the rating of a standard breaker, or 100% for an assembly marked for continuous operation at 100%. Since 1 ÷ 1.25 = 0.8, both rules land on the same breaker for the same load.
After you have the current, pick the conductor with the wire size calculator (NEC) or the wire size calculator for Canada (CEC), and check the run with the CEC voltage drop calculator if you work in Canada.
Watts, VA and amps
Nameplates on motors, transformers, UPS units and LED drivers often list VA or kVA instead of watts. For VA, skip the power factor: A = VA ÷ V, or VA ÷ (1.732 × V) for three phase. Use the kVA calculator for transformer full-load current, and the amps to watts calculator to convert back.
Common mistakes
- Dividing by 120 V for a 240 V appliance, which doubles the current.
- Dividing by 1.732 on a single-phase load, or forgetting it on a three-phase load.
- Using PF = 1 for motors and compressors, which understates the current.
- Sizing a breaker at exactly the calculated current for a heater, EV charger or water heater instead of applying the 125% continuous-load rule.