ToolwickHow to Use This Watts to Amps Calculator
Type the load's power in watts (or switch the unit to kW), enter the supply voltage, and choose DC, single-phase AC or three-phase AC. For AC loads, set the power factor with the slider; leave it at 1.00 for resistive loads such as heaters. The result shows the current in amps, the design current, a minimum standard breaker size, a minimum copper wire gauge, and the same load at other common voltages.
Why This Matters
Nameplates list watts, but breakers, wire, extension cords, generators and inverters are limited by amps. A 1,800 W space heater looks modest until you see that it pulls 15 amps at 120 V. That is the entire rating of a standard 15 A circuit, with nothing left for a lamp or a phone charger. Electricians run this conversion when adding a circuit for a hot tub (often 50 A at 240 V), or when checking whether a 2,000 W generator can carry a 1,500 W load. Camper and solar builders use it constantly: a 1,000 W inverter on a 12 V battery draws over 83 A, which is why the cables are so thick and the fuses so large.
Homeowners use the same math to stop tripping breakers, and landlords use it to check a tenant's appliance load. DIYers use it to buy the right wire before they start. An error in one direction wastes money on oversized cable. An error in the other direction can overheat conductors and start a fire. A quick, consistent calculation that always rounds toward the safe side is worth keeping bookmarked.
How It's Calculated
Power, voltage and current are linked by P = V × I, so current is power divided by voltage. AC circuits add the power factor (PF), which accounts for the part of the current that does no useful work. Three-phase circuits add the square root of 3 (about 1.732).
For three-phase, P is the total power of all three phases and V is the line-to-line voltage, such as 208 V or 480 V.
The sizing steps follow common NEC (NFPA 70) practice. A continuous load multiplies the current by 1.25, per NEC 210.20(A) and 215.2(A)(1). The tool then rounds up to the next standard breaker rating from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and so on. Wire size is the smallest copper conductor whose NEC Table 310.16 ampacity is at least the breaker rating. It uses the 60 °C column up to 100 A and the 75 °C column above that (NEC 110.14(C)). Results always round up, never down. The tool assumes no more than three current-carrying conductors, a 30 °C ambient temperature and a short run. Local codes vary, and the output is not engineering, permit or inspection approval.
Worked Example
A 1,800 W space heater plugs into a 120 V single-phase circuit and will run for hours. Power factor is 1.00 because it is a resistive load. These are the default values in the calculator above.
- Current: 1,800 ÷ (120 × 1.00) = 15.00 A.
- Continuous load: 15.00 × 1.25 = 18.75 A design current.
- Next standard breaker at or above 18.75 A: 20 A.
- Smallest copper wire with at least 20 A in the 60 °C column: 12 AWG (20 A).
So the heater needs a dedicated 20 A circuit on 12 AWG wire. On a 15 A breaker it would run at 100% of the rating, well above the 80% guideline for continuous loads.
Three-phase variation: a 10,000 W load at 208 V, PF 0.85, draws 10,000 ÷ (1.732 × 208 × 0.85) = 32.66 A. With the 125% rule that is 40.82 A, which rounds up to a 45 A breaker and 6 AWG copper (55 A).
Tips & Common Mistakes
- Use the real voltage. A constant-power load such as a motor drive or inverter draws more amps at 208 V than at 240 V. Check the nameplate rather than assuming 120 or 240.
- Don't ignore power factor on motors. Compressors, pumps and transformers often sit at 0.80–0.85, which raises the current by 15–25% compared with PF 1.00. Motor nameplate watts are often output power, so divide by efficiency to get input watts.
- Remember startup surge. Motors can draw 3–7 times their running amps for a moment. Size generators and inverters for the surge, not just the running figure.
- Never "fix" a tripping breaker by upsizing it. The breaker protects the wire. A larger breaker on the same wire removes that protection.
- Long runs need extra checking. The wire size here ignores voltage drop and temperature derating. Use the Voltage Drop Calculator for runs over about 50 feet.
Frequently Asked Questions
How many amps is 1,000 watts?
At 120 V and a power factor of 1.00, 1,000 W is 8.33 A. At 240 V it is 4.17 A, and at 12 V DC it is 83.33 A. The same watts mean far more amps at low voltage.
Can I convert watts to amps without knowing the voltage?
No. Amps depend on both watts and volts, so you need the voltage. Check the appliance nameplate, or use the nominal supply voltage such as 120 V or 240 V in North America.
What is the formula for watts to amps?
For DC and purely resistive loads, amps = watts ÷ volts. For AC single-phase, divide by volts × power factor. For three-phase, divide by 1.732 × volts × power factor.
How many amps is 3,000 watts at 240 volts?
At a power factor of 1.00, 3,000 W at 240 V is 12.50 A. At a power factor of 0.90 it is 13.89 A. With the continuous-load 125% rule, the design current is 15.63 A (PF 1.00), so the minimum breaker is 20 A.
What breaker do I need for a 2,000 watt load at 120 volts?
The load draws 16.67 A. If it runs 3+ hours, the 125% rule gives 20.83 A, so the next standard breaker is 25 A with 10 AWG copper. If it is not continuous, a 20 A breaker with 12 AWG wire is the minimum.
Is the calculation different for three-phase power?
Yes. Three-phase current is lower for the same total watts because the load is shared across three conductors. Use the total watts and the line-to-line voltage, and divide by 1.732 × volts × power factor.