Watts to Amps Calculator

The formula#

DC circuits — the simple case, no power factor:

I = P / V

Single-phase AC:

I = P / (V × PF)

Three-phase AC:

I = P / (√3 × V × PF)

Where P is real power in watts, V is volts, PF is power factor, and I is current in amps.

Worked examples#

A 1500 W heater on a 230 V single-phase supply. A heater is a resistive load, so the power factor is 1:

I = 1500 / (230 × 1) = 6.5 A

Comfortable on a 16 A socket circuit, and the reason a 6 A socket is not enough for one.

The same heater on a 120 V supply:

I = 1500 / (120 × 1) = 12.5 A

Halve the voltage and the current doubles for the same power. On a North American 15 A branch circuit, that single heater occupies 83 % of the circuit before anything else is plugged in — which is why the 12 A / 1440 W limit on portable heaters exists.

A 1500 W motor at 0.85 power factor, 230 V single-phase:

I = 1500 / (230 × 0.85) = 7.7 A

Same watts, 18 % more current than the heater, because the power factor is below 1.

Appliance chart at 230 V#

Current drawn by common single-phase loads, with the power factor each type actually runs at:

Two things this chart does not show. Motor-driven appliances — fridges, air conditioners, washing machines — draw a starting current several times these figures for a second or so. And an inverter air conditioner varies its consumption continuously rather than cycling, so its running figure is whatever the compressor is doing at that moment, not a fixed number.

Watts, VA, and the UPS trap#

This is the conversion that costs people money.

A UPS is rated in VA, apparent power. Your equipment is rated in watts, real power. They are not the same number, and the ratio between them is the power factor:

VA = W / PF

A 600 W load at 0.6 power factor needs 1000 VA of UPS. Buy a "1000 VA" UPS for a 1000 W load and it will overload immediately — that unit is designed for about 600 W.

Older UPS units state a power factor of 0.6, newer ones 0.8 to 0.9. The nameplate always shows both figures; use the watts one. And leave headroom: a UPS running at its rating runs hot, and hot is what kills the battery.

The watts to VA calculator does this conversion directly.

Three-phase, and the size of the difference#

A 15,000 W load:

• Single-phase, 230 V, PF 0.9: 15000 / (230 × 0.9) = 72.5 A
• Three-phase, 415 V, PF 0.9: 15000 / (1.732 × 415 × 0.9) = 23.2 A

A third of the current for the same work, spread across three conductors instead of one. This is why anything much above 5 kW is supplied three-phase — the cable, the switchgear and the losses all scale with current, not with power.

Sizing the circuit around the answer#

The current is where the design starts:

Continuous loads get 125 %. A load running more than three hours continuously — heating, lighting, a compressor on a duty cycle — is conventionally sized at 125 % of its current. Our 6.5 A heater wants a circuit rated at least 8.1 A, so a 10 A device.

The cable must survive the device, not the load. If the protective device is 16 A, the cable has to carry 16 A after derating for its ambient temperature and how many other cables share its route. Cable sizing handles the derating; MCB and MCCB sizing handles the device.

Volt drop over the run. On anything longer than about 20 m, volt drop rather than heating usually decides the size. Limits are 3 % for lighting and 5 % for power. The voltage drop calculator covers it.

Common mistakes#

Assuming PF 1 for everything. True for heaters, kettles, filament lamps and geysers. Wrong for motors, air conditioners and anything with a switch-mode supply, and it understates the current by 10 to 40 %.

Using the wrong voltage. 230 V line-to-neutral for single-phase, 415 V line-to-line with √3 for three-phase. Mixing them up is a factor of nearly two either way.

Reading the surge rating from a nameplate. Some appliances state peak input rather than continuous. If the figure looks unusually high for the appliance, check whether it is a surge or a sustained rating.

Adding up nameplate watts for a whole installation. Nothing runs everything at once. Connected load times a diversity factor gives maximum demand — see connected load and diversity for the factors that apply.