Watts to Amps Calculator: 1φ, 3φ, PF

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The formulas

Power in watts is real power. Current depends on how that power is delivered.

System	Formula for current	Notes
DC	I = P / V	No power factor
Single-phase AC	I = P / (V × PF)	PF = cos φ for sinusoidal loads
Three-phase AC (line-to-line V)	I = P / (√3 × V × PF)	Balanced load, V is line-to-line
Three-phase from line current	P = √3 × V × I × PF	Rearranged
Apparent power	S (VA) = V × I (1φ); √3 × V × I (3φ)	PF = P / S

The constant √3 comes from the geometry of a three-phase system: three windings displaced by 120° produce a line-to-line voltage √3 times the phase voltage. At 400 V line-to-line, each ampere of line current delivers √3 × 400 = 692.8 VA.

Two conversions worth memorising:

1 kW at 230 V, PF 1 = 4.35 A
1 kW at 400 V three-phase, PF 1 = 1.44 A

Conversion table: watts to amps

Rounded to two decimals, resistive load (PF = 1).

Power	120 V, 1φ	230 V, 1φ	400 V, 3φ
100 W	0.83 A	0.43 A	0.14 A
500 W	4.17 A	2.17 A	0.72 A
1,000 W	8.33 A	4.35 A	1.44 A
1,500 W	12.50 A	6.52 A	2.17 A
2,000 W	16.67 A	8.70 A	2.89 A
3,000 W	25.00 A	13.04 A	4.33 A
5,000 W	41.67 A	21.74 A	7.22 A
7,000 W	58.33 A	30.43 A	10.10 A
10,000 W	83.33 A	43.48 A	14.43 A

For a load with PF 0.85, divide the single-phase values by 0.85: 5,000 W at 230 V becomes 25.58 A instead of 21.74 A. That is a 17.7% increase, which is often the difference between a 25 A and a 32 A device.

Worked example 1 — kettle, 230 V

A 1,500 W kettle on a 230 V supply, PF 1.

I = 1,500 / 230 = 6.52 A
A 10 A device and 1.0 mm² flexible cord are adequate for the appliance itself.
On a 16 A socket circuit the kettle uses about 41% of the rating, which is why several appliances can share one ring.

Worked example 2 — space heater, 120 V continuous

A 1,800 W heater on 120 V, operating for more than three hours.

I = 1,800 / 120 = 15.00 A
Continuous load factor: 15.00 × 1.25 = 18.75 A
Device: 20 A, conductor 12 AWG copper (NEC 240.4(D) caps 12 AWG at 20 A)

The naive answer "15 A, so use a 15 A breaker" is wrong here. See the full chain in the breaker size calculator.

Worked example 3 — three-phase motor, 400 V

A 15 kW motor, 400 V, efficiency 0.92, power factor 0.85. Nameplate watts are output watts, so convert to input first.

Input power = 15,000 / 0.92 = 16,304 W
I = 16,304 / (1.732 × 400 × 0.85) = 16,304 / 588.9 = 27.69 A

Keep the chain in one framework. Here it is IEC — the only test is Ib ≤ In ≤ Iz:

Conductor: 6 mm² copper, three loaded conductors, clipped direct (method C). IEC 60364-5-52 gives about 36 A at 30 °C, so it carries 27.7 A with margin. In conduit (method B2) the same cable falls to about 29 A — then the honest answer is 10 mm².
Device: 32 A motor-rated breaker or MCCB, so 27.7 ≤ 32 ≤ 36 holds; a 25 A device would nuisance-trip.
Overload relay: set to the motor's full-load current (25–32 A band), not to the device rating.
Starting current: at 6× FLA the motor briefly pulls about 166 A, so short-circuit protection must ride through the inrush — a motor-rated device with Type 1 or Type 2 coordination to IEC 60947-4-1, not a generic curve B breaker.

Dropping efficiency and power factor from this calculation gives 21.7 A — a 22% error that would undersize the cable.

Worked example 4 — 5 kW load at PF 0.9

A 5,000 W load on 230 V with PF 0.90.

I = 5,000 / (230 × 0.90) = 5,000 / 207 = 24.15 A
Device: 32 A, next preferred IEC rating above Ib; a 25 A device clears Ib by only 3.5%.
Conductor: 6 mm² copper, Iz around 36 A, comfortably above the 32 A device
Apparent power: S = 230 × 24.15 = 5,556 VA, so the cable and source carry 5.56 kVA to deliver 5 kW

That gap is why generators, UPS units and transformers are rated in VA.

Watts, VA and VAR are not interchangeable

W (watts) — real power, the part that does work and generates heat.
VA (volt-amperes) — apparent power, the product of RMS voltage and RMS current. Cable, breaker and transformer are limited by this.
VAR — reactive power, the part that oscillates between source and magnetic fields.

For a sinusoidal load, S² = P² + Q² and PF = P / S. For non-sinusoidal electronic loads the cosine is not enough: use true RMS current or the manufacturer's PF / crest factor data. An LED driver measuring PF 0.55 can draw nearly twice the current you would calculate from its wattage.

From amps to a protective device

Once you have Ib, the rest of the chain is standard:

Multiply by 1.25 if the load is continuous under NEC rules.
Select the next standard rating (NEC 240.6 or IEC preferred values).
Verify the cable's corrected ampacity is at least the device rating.
Check voltage drop over the real run length — a 40 m circuit at 16 A on 1.5 mm² loses far more than the tables suggest at full load.

Voltage drop is an amps problem too: ΔV = 2 × I × R × L single-phase, with R per metre at operating temperature. Halving current halves the drop; more area cuts R roughly in proportion.

Crossing AWG and metric cable? Convert the size first with the AWG to mm² chart, then run the ampacity check.

Calculate a whole dwelling, not one appliance: start the GorkyCAD demo and let the load schedule sum the circuits.

Always verify against the current edition of NFPA 70 or IEC 60364, plus local amendments. Nameplate data overrides generic assumptions about power factor and efficiency. This is technical guidance, not a compliance statement.

Common mistakes

Using watts as VA. For anything with a motor or electronics, watts divided by volts underestimates current.
Forgetting efficiency. Motor nameplate kW is shaft output; input current needs P/η.
Mixing line-to-line and line-to-neutral voltage. 230 V to neutral inside a 400 V system is not the same calculation as a 230 V single-phase supply.
Applying the 125% factor twice. It applies to the load, once — not again to the selected breaker.
Mixing frameworks in one chain. The 125% of full-load current is an NEC Article 430 rule; IEC uses Ib ≤ In ≤ Iz with an overload relay. Pick one and say which on the schedule.
Ignoring starting current. A 6× inrush trips a magnetic-only setting that looked fine on running current.
Assuming PF 1 for LED and IT loads. Check the driver or PSU datasheet; 0.5–0.6 is common for cheap supplies.

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Frequently asked questions

How many amps is 1,000 W at 230 V?

4.35 A at PF 1. At PF 0.9 it is 4.83 A. At 120 V the same 1,000 W draws 8.33 A.

What is the formula for watts to amps in three-phase?

I = P / (√3 × V × PF), where V is line-to-line voltage. For 400 V and PF 1, that is P / 692.8.

Do I divide or multiply by power factor?

Divide. Current is real power divided by (voltage × PF), because apparent power is larger than real power. PF below 1 always increases current for the same wattage.

Why does my 2,000 W appliance draw more than 8.7 A at 230 V?

Either the power factor is below 1, the appliance's rated watts are output rather than input, or the supply voltage is below nominal. Measure voltage and current together before concluding the appliance is faulty.

Is watts to amps the same for AC and DC?

For DC, I = P / V. For single-phase AC, divide by power factor as well. Three-phase adds the √3 factor. The unit conversion is the same; the phase arrangement changes the divisor.

How do I choose a breaker after converting watts to amps?

Multiply by 1.25 for continuous loads, pick the next standard rating at or above the result, then confirm the cable's corrected ampacity is at least that rating. The breaker sizing guide shows the full sequence.

Does a higher voltage always mean lower current?

Yes for the same power, in proportion: doubling voltage halves current. That is why 400 V three-phase carries about three times the power of a 230 V single-phase circuit at the same current.

Sources

NFPA 70, National Electrical Code — Articles 210, 220, 430; Table 310.16.
IEC 60364-5-52, Selection and erection of wiring systems — current-carrying capacity.
IEC 60038, Standard voltages — nominal 120/230/400 V system values.
IEEE 1459, Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions.
BIPM, The International System of Units (SI Brochure), 9th edition — unit definitions for W, VA and A.

Next: see plans, read the docs, or browse the electrical drawing checklist before you issue the package.

Watts to Amps Calculator: Single-Phase, Three-Phase and Power Factor