Watts to Amps
With watts or amps 1500, voltage 120 V, power factor 1, watts to amps comes to 12.50 A — amps. It is reached in 4 steps, the last of which is 1 * (1500 / 120) + 0 * 1500, and each one is printed on the page with its numbers filled in. The formula is the one published by IEEE Std 141, not an approximation fitted to it.
Watts to amps and back, with the power factor and the phase that AC needs. A DC answer applied to a 3-phase motor is out by 73%.
Formula and sources checked · How we check
I know the Watts — find amps, Watts or amps 1500, Voltage 120 V, Supply AC single phase — a household circuit
12.50 A
Amps for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Current
1 * (1500 / 120) + 0 * 150012.5 A- Real power
1 * 1500 + 0 * (1500 * 120)1,500 W- Apparent power
12.5 * 120 * 11,500 VA- Continuous load at 80% of a breaker
12.5 / 0.815.625 A
Ask about this in the chatWatts to Amps Chart
Worked example
A 1,500 W space heater on a 120 V circuit draws 12.5 A. It is a continuous load, so the circuit has to be rated for 15.6 A — which is why two of them on one 20 A circuit trips it and one does not.
How to work it out yourself
- 1.Pick the supply first. It is the input that moves the answer most: 1,500 W is 12.5 A on single phase and 7.2 A on three phase at the same voltage, because of the √3.
- 2.Enter the voltage the equipment actually sees. A US household outlet is 120 V, a dryer or range circuit 240 V, and a three-phase nameplate gives the line-to-line figure.
- 3.Set the power factor for anything with a motor or a transformer in it. At 0.8 the same watts draw 25% more current, and that current is what the wire and the breaker have to carry.
- 4.Read the breaker line for anything running more than three hours at a stretch. The NEC sizes those circuits at 125% of the load, so a 12.5 A heater needs a circuit good for 15.6 A.
Watts to amps at 120 V, single phase, unity power factor
| Watts or amps | Amps | Continuous load at 80% of a breaker |
|---|---|---|
| 100 | 0.83 A | 1.04 A |
| 250 | 2.08 A | 2.6 A |
| 500 | 4.17 A | 5.21 A |
| 750 | 6.25 A | 7.81 A |
| 1000 | 8.33 A | 10.42 A |
| 1200 | 10.00 A | 12.5 A |
| 1500 | 12.50 A | 15.63 A |
| 1800 | 15.00 A | 18.75 A |
| 2000 | 16.67 A | 20.83 A |
| 2400 | 20.00 A | 25 A |
| 3000 | 25.00 A | 31.25 A |
The second column is what a continuously running load needs from the breaker. NEC 210.19(A)(1) sizes a circuit at 125% of a continuous load, which is the same thing as loading a breaker to no more than 80%.
The formula
- Current
1 * (1500 / 120) + 0 * 1500 - Real power
1 * 1500 + 0 * (1500 * 120) - Apparent power
12.5 * 120 * 1 - Continuous load at 80% of a breaker
12.5 / 0.8
Source: IEEE Std 141 — the √3 factor in three-phase power, NFPA 70 (NEC) 210.19(A)(1) — branch-circuit conductors sized at 125% of continuous load, NIST SP 811 — units for electrical quantities
Questions people actually ask
- How many amps is 1000 watts?
- 8.33 A at 120 V, 4.17 A at 240 V, and 2.78 A on 208 V three phase — all at unity power factor. The question has no single answer because watts are volts times amps: without the voltage there is nothing to divide by.
- What is the formula for watts to amps?
- Amps = watts ÷ volts for DC. For AC single phase, amps = watts ÷ (volts × power factor). For AC three phase, amps = watts ÷ (volts × power factor × √3), where the voltage is line-to-line. The √3 is 1.732, and forgetting it overstates a three-phase current by 73%.
- What power factor should I use?
- 1.0 for anything purely resistive — heaters, incandescent bulbs, kettles, electric ovens. 0.8 to 0.9 for induction motors under load, lower when lightly loaded. Above 0.9 for modern LED drivers and computer supplies, which are required to correct it. If a nameplate gives a figure, use that one.
- Why do watts and VA differ?
- Because current and voltage can be out of step. Watts are the power actually converted to work or heat; volt-amperes are what the wiring carries regardless. The ratio is the power factor, so a 0.8 PF load drawing 800 W occupies 1,000 VA of a supply — which is why generators and UPS units are rated in kVA rather than kW.
- How many amps can a 20 amp breaker really carry?
- 16 A continuously. NEC 210.19(A)(1) requires a branch circuit to be rated at 125% of a continuous load — one running three hours or more — and 20 ÷ 1.25 is 16. The full 20 A is available only for intermittent loads.
- Does this work for a generator?
- Yes, but generators are rated in kVA rather than kW, so enter the current and read the apparent power line. A 10 kVA generator supplies 10,000 VA whatever the power factor; at 0.8 PF that is only 8,000 W of real power available to the load.
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