Turning a wattage rating into real line current
Equipment is often rated in watts, but to size a breaker or conductor you need amps. The conversion depends on the voltage and, for AC loads, the power factor. Watts is real power — only the part doing useful work — so for inductive loads such as motors the actual line current is higher than the wattage alone suggests. This calculator covers DC, single-phase, and three-phase loads and includes the power factor term so the result reflects the true current the circuit must carry.
How it works
For DC and AC single-phase loads the current is:
I = W / (V × PF)For AC three-phase loads the line current is:
I = W / (√3 × V × PF)with V the line-to-line voltage for three-phase, PF the power factor (1.0 for resistive loads, lower for motors), and √3 ≈ 1.732. DC has no power factor, so the tool fixes it at 1.0 in that mode. Dividing by the power factor is what corrects for the reactive current an inductive load draws on top of its real-power current.
Example
A 3600 W resistive water-heater element at 240 V single-phase draws 3600 ÷ (240 × 1.0) = 15 A. A 3600 W motor at 240 V with a 0.85 power factor draws 3600 ÷ (240 × 0.85) ≈ 17.6 A — the same watts but more current because of the reactive component.
Understanding power factor in practice
Power factor is the ratio of real power (watts) to apparent power (volt-amps). A purely resistive load — incandescent lamps, strip heaters, electric water heaters — converts nearly all the supplied voltage-times-current into useful heat or light. The power factor is 1.0, and the watts-to-amps formula is straightforward.
Inductive loads — motors, transformers, ballasts, variable-frequency drives — draw a lagging current that is larger than the real-power component alone would indicate. A motor rated at 1,000 W with a power factor of 0.80 draws 1,000 W of real power but pulls 1,000 ÷ 0.80 = 1,250 volt-amps of apparent power from the supply. The current is sized for the apparent power, so the wire and breaker must be rated for the higher current.
Common power factor ranges:
- Resistive heaters, incandescent lamps: ~1.0
- Induction motors (fully loaded): 0.80–0.92
- Induction motors (lightly loaded): 0.40–0.70
- Fluorescent lighting with magnetic ballast: ~0.50
- LED drivers and modern switch-mode power supplies: 0.90–0.99 (high-PF units)
- Older electronic equipment and small motors: 0.60–0.80
When the nameplate does not show a power factor, use a typical figure for the equipment type rather than assuming 1.0.
Three-phase specifics
The √3 factor arises from the geometry of three-phase power: three conductors 120 degrees apart. For a balanced three-phase load, you always use the line-to-line voltage in the formula — using the line-to-neutral voltage would give a current that is √3 times too high and lead to significant undersizing of the conductors and breaker.
Common US commercial/industrial line-to-line voltages: 208 V (120/208 Y wye system), 240 V (delta or 120/240 high-leg delta), 480 V (277/480 Y), 600 V (Canadian). Always confirm which voltage applies to your panel before sizing conductors.
From amps to breaker sizing
The calculated current is the operating current under the stated load. For circuit breaker and conductor sizing, the NEC (in the US) requires a 125% factor for continuous loads — loads expected to run for three hours or more. If the calculated current is 17.6 A for the motor example, multiply by 1.25 to get 22 A, then select the next standard breaker size above 22 A. Conductor ampacity must also account for installation temperature, conduit fill, and any bundling derating per NEC Table 310.15. This calculator gives the starting point; always confirm the final sizing against the applicable electrical code for your jurisdiction.