Amp calculator
Convert watts to amps or amps to watts for a 120 V or 240 V circuit, three-phase or DC, and see how much a breaker can carry.
That load draws
12.50 amps
That current carries
1,500 watts
- Amps
- 12.50 A
- Watts
- 1,500 W
- Kilowatts
- 1.500 kW
- Volt-amps
- 1,500 VA
- Breaker if it runs 3+ hours
- at least 15.6 A
- Load running 3+ hours
- up to 1,200 W
A load that runs 3 hours or more needs a breaker rated at least 125% of its current (NEC 210.20(A)), so it can use about 80% of the breaker. Follow local code and have a licensed electrician size circuits.
How to convert watts to amps
Divide the watts by the volts. On AC circuits, also divide by the power factor, and on three-phase circuits divide by 1.732 (the square root of 3) as well. Turn it around to get watts from amps.
- DC: amps = watts ÷ volts
- AC single-phase: amps = watts ÷ (volts × power factor)
- AC three-phase: amps = watts ÷ (1.732 × volts × power factor), with line-to-line volts
- kVA: amps = kVA × 1,000 ÷ volts (÷ 1.732 more on three-phase); power factor doesn't apply
Worked example
A 1,500 W space heater on a 120 V circuit: 1,500 ÷ 120 = 12.5 amps. A heater is a resistive load, so its power factor is 1. It can run for hours, so the breaker must be at least 15.6 A. That's more than a 15 A breaker allows for a continuous load (1,440 W), so check the circuit with an electrician before running it for hours.
Watts to amps chart
Single-phase, power factor 1 (heaters, incandescent bulbs and other resistive loads).
| Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|
| 100 W | 0.83 | 0.42 |
| 400 W | 3.33 | 1.67 |
| 500 W | 4.17 | 2.08 |
| 1,000 W | 8.33 | 4.17 |
| 1,500 W | 12.50 | 6.25 |
| 1,800 W | 15.00 | 7.50 |
| 2,000 W | 16.67 | 8.33 |
| 3,000 W | 25.00 | 12.50 |
| 5,000 W | 41.67 | 20.83 |
How many watts a breaker can handle
Full rating, with the continuous-load limit (80%, for loads running 3 hours or more) in brackets. Single-phase, power factor 1.
| Breaker | At 120 V | At 240 V |
|---|---|---|
| 15 A | 1,800 W (1,440) | 3,600 W (2,880) |
| 20 A | 2,400 W (1,920) | 4,800 W (3,840) |
| 30 A | 3,600 W (2,880) | 7,200 W (5,760) |
| 40 A | 4,800 W (3,840) | 9,600 W (7,680) |
| 50 A | 6,000 W (4,800) | 12,000 W (9,600) |
Continuous-load rule from NFPA 70, National Electrical Code, 2023 edition, Article 100 and 210.20(A). Breakers and equipment listed for 100% operation are an exception. The wire must also be rated for the breaker; see the wire size calculator.
Amps and watts questions
How many amps is 1,500 watts?
12.5 amps at 120 volts, or 6.25 amps at 240 volts, for a heater or other load with a power factor of 1. Amps = watts ÷ volts, so the higher the voltage, the less current the same wattage draws.
How many watts can a 20 amp breaker handle?
2,400 watts at 120 volts (20 × 120). For a continuous load, one that runs 3 hours or more like a space heater, keep it to 1,920 watts, because the National Electrical Code requires the breaker to be at least 125% of a continuous load. The table above covers 15, 30, 40 and 50 amp breakers and 240 volts.
How many watts are in an amp?
It depends on the voltage: watts = amps × volts. One amp is 120 watts on a 120 V circuit and 240 watts at 240 V. On AC circuits with motors or electronics, multiply by the power factor too.
What is power factor?
The share of the current that does real work. Heaters, toasters and incandescent bulbs have a power factor of 1, so watts = volts × amps. Motors, compressors and some electronics draw extra current that isn't turned into work, so their power factor is below 1 and they draw more amps per watt. Use the figure on the equipment's nameplate or spec sheet; if you don't know it, 1 gives the lowest current, so treat the result as a minimum.
How many amps does my refrigerator or microwave use?
Look at the nameplate or label, usually inside the door, on the back or on the bottom. It lists the volts and either the amps or the watts. Enter the watts above to get the amps. Refrigerators, air conditioners and other motor loads draw a short surge when they start, so leave headroom on the circuit.
How do three-phase amps work?
Three-phase power is split across three wires, so the current on each is lower: amps = watts ÷ (1.732 × volts × power factor), using the line-to-line voltage such as 208 or 480 V. Choose "AC three-phase" above. For long runs, also check the voltage drop, and have a licensed electrician size the circuit.