Watts to Amps Calculator

Convert a load in watts to the current it draws, for single-phase or three-phase power at the voltages datacenters and offices use.

Inputs

Result

Enter the power in watts to see the current.

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How the calculation works

Current depends on the power drawn, the voltage, whether the circuit is single-phase or three-phase, and the power factor (PF):

Single-phase: amps = watts ÷ (volts × PF)
Three-phase: amps = watts ÷ (√3 × volts × PF), where volts is the line-to-line voltage and √3 ≈ 1.732

Watts here means real power, the figure on most power meters and data sheets. The power factor is the ratio of real power (watts) to apparent power (volt-amperes, VA). A PF of 1.0 means the two are equal, which is true of resistive loads such as heaters. Equipment with a PF below 1.0 draws more current for the same watts, so the calculator also shows the apparent power in kVA (watts ÷ PF ÷ 1,000), the number UPS and generator ratings are often quoted in.

Line-to-line or line-to-neutral? A three-phase system has two voltages. Line-to-line is measured between two phase conductors; line-to-neutral is measured between one phase and neutral, and it is the line-to-line voltage divided by √3. A 208Y/120 V system is 208 V line-to-line and 120 V line-to-neutral. The three-phase formula above uses the line-to-line voltage. A single-phase circuit taken from a three-phase panel uses the voltage of that circuit: 120 V between a phase and neutral, or 208 V between two phases.

The last line of the result is context, not electrical advice. A circuit serving a continuous load is commonly limited to 80% of its breaker rating in North America, so the calculator divides the current by 0.8 to show the smallest circuit rating that keeps this load at or below 80%.

Worked example

A colocation cabinet will hold equipment drawing 5,000 W, with a power factor of 0.95, fed at 208 V.

On a single-phase 208 V circuit:

amps = 5,000 ÷ (208 × 0.95) = 5,000 ÷ 197.6 = 25.30 A
at 80% loading, the circuit needs a rating of at least 25.30 ÷ 0.8 = 31.63 A, so a 30 A circuit is too small and a 40 A circuit fits

On a three-phase 208 V circuit:

amps = 5,000 ÷ (1.732 × 208 × 0.95) = 5,000 ÷ 342.3 = 14.61 A per phase
at 80% loading that needs at least 14.61 ÷ 0.8 = 18.26 A, so a 20 A three-phase circuit fits

In both cases the apparent power is 5,000 ÷ 0.95 ÷ 1,000 = 5.26 kVA. With redundant A and B feeds, each feed has to carry the full load on its own if the other fails, so size each one for the whole 5,000 W.

Frequently Asked Questions

How many amps is 1,000 watts?

It depends on the voltage. At a power factor of 1.0 on a single-phase circuit, 1,000 W draws about 8.33 A at 120 V, 4.81 A at 208 V, 4.35 A at 230 V and 4.17 A at 240 V. The higher the voltage, the lower the current for the same power.

Which voltage do I enter for three-phase power?

Enter the line-to-line voltage, the voltage measured between two of the three phase conductors. On a 208Y/120 V system that is 208 V; on 480Y/277 V it is 480 V; on 415Y/240 V it is 415 V. The three-phase formula already includes the √3 factor, so do not enter the line-to-neutral voltage (120 V, 277 V or 240 V in those examples).

What power factor should I use?

Use 1.0 for purely resistive loads such as heaters. IT equipment with power-factor-corrected power supplies usually runs close to 1.0 under load, and 0.9 or 0.95 are common planning assumptions. If the equipment's nameplate, data sheet or a PDU reading gives a power factor, use that value.

Why does three-phase power draw fewer amps for the same load?

A three-phase circuit spreads the load across three conductors. For the same watts and line-to-line voltage, the current on each phase is the single-phase figure divided by √3 (about 1.732). That is one reason high-density racks are often fed with three-phase power.

Can I load a circuit up to its breaker rating?

Usually not for loads that run for hours, like servers. In North America the National Electrical Code commonly limits a continuous load to 80% of the breaker rating, and colocation providers typically apply the same 80% limit to the circuits they sell. Rules differ by country and installation, so treat the calculator's 80% figure as a planning guide and confirm with your electrician or provider.