Formula
DC: I = P ÷ V
AC single phase: I = P ÷ (V × PF)
AC three phase: I = P ÷ (√3 × VL-L × PF)
Worked example
A 1,500 W heater on a 230 V single-phase supply (PF = 1 for resistive loads): I = 1500 ÷ 230 = 6.52 A.
Reference table
Quick reference (PF = 1)
| Watts | Amps @ 120 V | Amps @ 230 V |
|---|---|---|
| 100 | 0.83 | 0.43 |
| 500 | 4.17 | 2.17 |
| 1,000 | 8.33 | 4.35 |
| 1,500 | 12.5 | 6.52 |
| 2,000 | 16.67 | 8.7 |
| 3,000 | 25 | 13.04 |
Where this shows up in the real world
This is the homeowner's conversion: will a 1,500 W space heater trip a 15 A bedroom circuit (at 120 V it draws 12.5 A — legal, but at 83% of the breaker, nothing else can share that circuit)? Can a 13 A UK plug run a 2.8 kW kettle plus a toaster? Wattage is printed on every appliance; amps are what the wiring cares about.
Common mistakes to avoid
Two traps: first, continuous loads in the US are limited to 80% of breaker rating under NEC 210.20 — a 15 A circuit is really 12 A for anything running hours at a time. Second, power factor: the formula's PF term matters for motors and electronics; assuming PF = 1 for a shop full of machines understates current by 15–25%.
Frequently asked questions
What power factor should I use?
Resistive loads (heaters, incandescent lamps) are close to 1.0. Motors typically run 0.8–0.9. If unsure for a mixed AC load, 0.8 is a conservative estimate that yields a higher (safer) current figure.
Does this work for choosing breaker size?
It gives the running current, which is the starting point. Breakers and cables must also account for inrush current, duty cycle and local code rules — confirm with your electrical code.
Why is three-phase current lower for the same watts?
The load is spread across three conductors, so each line carries less current for the same total power.