How watts convert to amps
Power is voltage times current. On a DC circuit, or an AC circuit feeding a purely resistive load such as a heater, that is the whole story. AC motor and transformer loads add a power factor, and three-phase circuits add √3 because the three legs share the load.
DC: watts = volts × amps
Single-phase AC: watts = volts × amps × power factor
Three-phase AC: watts = √3 × line-to-line volts × amps × power factor
Rearrange the same equation for whichever value you are after:
amps = watts ÷ (√3 × volts × power factor) (drop the √3 for single-phase and DC)
volts = watts ÷ (√3 × amps × power factor)
kilowatts = watts ÷ 1,000
DC resistance = volts ÷ amps
For the breaker, a continuous load is multiplied by 125% and rounded up to the next standard rating:
breaker ≥ amps × 1.25, then the next size up from NEC 240.6(A)
Standard sizes start at 15, 20, 25, 30, 35, 40, 45, 50, 60 A and continue up to 600 A in this calculator.
Use the nominal system voltage: 120 or 240 V for a house, 208 or 480 V line-to-line for most commercial three-phase. Take the watts from the appliance nameplate or the manual. If the nameplate lists amps instead, switch "Solve for" to watts. For a DC system such as a 12 V battery bank, choose DC; power factor does not apply, and the calculator also gives the load's resistance.
Worked example: 1,500 W at 120 V
The calculator opens on a 1,500 W space heater on a 120 V single-phase circuit at a power factor of 1.
- Amps: 1,500 ÷ (120 × 1) = 12.500 A.
- Kilowatts: 1,500 ÷ 1,000 = 1.500 kW.
- Continuous load: 12.5 × 1.25 = 15.625 A.
- Breaker: the next standard rating at or above 15.625 A is 20 A.
A 15 A breaker is too small for this heater even though 12.5 A is less than 15, because the 125% rule pushes the requirement past 15 A.
Watts to amps conversion table
Amps for common household loads at 120 V and 240 V, for resistive loads (power factor 1). Check the nameplate on your own appliance; ratings vary by model.
| Load | Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|---|
| Incandescent bulb | 100 | 0.8 | 0.4 |
| Coffee maker | 1,000 | 8.3 | 4.2 |
| Portable space heater | 1,500 | 12.5 | 6.3 |
| Hair dryer | 1,875 | 15.6 | 7.8 |
| Water heater element | 4,500 | 37.5 | 18.8 |
| Clothes dryer element | 5,000 | 41.7 | 20.8 |
| Level 2 EV charger (30 A) | 7,200 | 60.0 | 30.0 |
| Electric furnace heat strip | 10,000 | 83.3 | 41.7 |
Large loads such as water heaters, dryers and EV chargers are wired for 240 V, which halves the current compared with 120 V and keeps the wire size down.
Sizing the circuit for a continuous load
- A continuous load runs at full current for 3 hours or more. NEC 210.20(A) requires the breaker to be at least 125% of it, so a 15 A circuit carries at most 1,440 W of continuous load at 120 V and a 20 A circuit 1,920 W.
- Non-continuous loads, like a microwave or a power tool, are counted at 100%. Add them to 125% of the continuous loads on the same circuit.
- The breaker protects the wire. Pick a conductor whose ampacity is at least the breaker rating before you install it.
- Motors are different: their breakers and wire are sized from the full-load current tables in NEC Article 430, not from the nameplate watts.
- For three-phase, enter line-to-line volts (208 or 480). The answer is the current in each leg of a balanced load.
When to hire an electrician
Use these numbers to plan and to check a quote. Adding a circuit, changing a breaker or wiring a 240 V appliance needs a permit in most places, and an electrician will also check the panel's capacity, the wire type and the length of the run. Service upgrades, three-phase equipment and motor circuits should be designed by a licensed electrician or engineer.
Sources
- Ohm’s law and the power equation: P = V × I × PF (DC and single-phase); P = √3 × V(line-to-line) × I × PF (three-phase)
- NFPA 70, National Electrical Code (2023), Article 100 (definition of continuous load), 210.20(A) and 215.3 (continuous loads at 125%)
- NFPA 70, National Electrical Code (2023), 240.6(A) (standard ampere ratings for fuses and inverse-time circuit breakers)