The formula, and the power-factor catch
For DC and simple resistive loads, current is just power divided by voltage. For AC you divide by the voltage and the power factor, so the real current is higher than a plain watts-over-volts guess. Skipping the power factor is the classic mistake — it undersizes wires and breakers.
DC vs single-phase vs three-phase
Same 2000 W load, same 230 V. The power factor pushes the AC current up, while spreading it over three phases pulls the per-line current back down.
A poor power factor raises the current a single circuit must carry; splitting the same power across three phases lowers what each line sees.
Quick reference (single-phase, 230 V, PF 1.0)
| Power | Voltage | Current |
|---|---|---|
| 500 W | 230 V | 2.17 A |
| 1000 W | 230 V | 4.35 A |
| 2000 W | 230 V | 8.70 A |
| 3000 W | 230 V | 13.04 A |
| 5000 W | 230 V | 21.74 A |
On US 120 V circuits the current is nearly double these values for the same wattage. With a power factor below 1, divide again by the PF to get the real draw.
Common questions
How do I convert watts to amps?
Divide the power in watts by the voltage. On a 120-volt DC or resistive circuit, 600 watts is 600 divided by 120, which is 5 amps. For AC you also divide by the power factor, so the current is a bit higher than watts over volts alone.
What is power factor and when does it matter?
Power factor is the ratio of real power (watts) to apparent power (volt-amps), between 0 and 1. Resistive loads like heaters and bulbs are close to 1. Motors, transformers and many electronics run lower, around 0.7 to 0.9, which raises the actual current draw.
Why is the three-phase formula different?
Three-phase power spreads the load across three conductors, so the current in each line is lower. The formula divides by the square root of 3 (about 1.732) times the voltage and power factor, giving roughly 1.7 times less current than the single-phase result.
How do I pick a breaker from the amps?
Work out the current, add about a 25 percent margin, then round up to the next standard breaker size such as 10, 15, 16, 20 or 32 amps. Always follow local wiring codes like the NEC for the exact rule.
