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Alternating current from the mains or a generator, or direct current from a battery, a solar array or a DC bus.
Mains supplies are AC. DC has no power factor and no phases, so choosing it hides both fields and works volts times amps directly.
Single phase for houses and most appliances; three phase for larger commercial and industrial loads.
A North American 240 V circuit for a dryer or an EV charger is single phase: two hot conductors from a split-phase service, 240 V between them. Three phase means three line conductors with the load shared across them; enter the line-to-line voltage below, and the current in one line.
The supply's nominal voltage — for three phase, the voltage between two lines.
Pick the nominal voltage the circuit is supplied at, or choose another voltage to type one in. For three phase it is the line-to-line figure — 208, 240, 400, 415 or 480 V — never the line-to-neutral one: 120 or 277 V in North America, 230 or 240 V elsewhere. A North American 240 V delta supply is 240 V between lines. The nominal figure is the one equipment is rated against; a meter reading a few percent above or below it is normal.
The figure on the nameplate, the datasheet or the meter: watts, amps, or kVA.
Watts are real power, the rate work is done. Amps are the current, which is what heats a cable and trips a breaker. kVA is apparent power, volts times amps, which is what generators, transformers and supply agreements are rated in. Each converts into the other two at the voltage and power factor below.
The real power in watts, from the rating plate or a data sheet.
Use the running rating. For kilowatts, multiply by 1,000 — 7.2 kW is 7,200 W. Where a plate gives both a heating and a standby figure, the heating figure is the load. A motor is the exception: its hp or kW is its shaft output, not what it draws, and the single-phase and three-phase motor pages divide by its efficiency as well as its power factor.
1 for heaters, kettles and incandescent lamps; lower for motors and inductive loads.
The ratio of real power to apparent power. A resistive load — an element, a kettle, a dryer's heater — is 1. A loaded three-phase induction motor is roughly 0.8 to 0.9, and less when lightly loaded; a small single-phase motor can be far lower — WEG's four-pole US motors up to 1 hp list 0.59 to 0.68 — though one built with start and run capacitors, such as WEG's European W22, lists 0.75 to 0.99. Electronics with power-factor correction sit close to 1. Take it from the nameplate where one is printed. At 1, watts and volt-amperes are the same number.
Current or power
30 A
Single-phase AC: apparent power is volts times amps, and real power is that times the power factor. At a power factor of 1 the two are the same; below 1 the supply carries more current than the watts alone suggest.
- Apparent power
- 7.2 kVA
- Reactive power
- 0 kvar
- Voltage used
- 240 V
- Power factor used
- 1
They open the calculator with your figures already in it
Watts, Amps and kVA Calculator (Any Voltage, AC or DC): 30 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Definitions of real, apparent and reactive power: DC P = V × I; single-phase S = V × I and P = V × I × PF; three-phase S = √3 × V(line-to-line) × I and P = √3 × V × I × PF for a balanced load; Q = S × √(1 − PF²)
- Balanced three-phase systems: the line-to-line voltage is √3 times the line-to-neutral voltage (phases 120° apart), so three loads at V/√3 each draw √3 × V × I in total
- WEG Rolled Steel General Purpose 1-phase Motors, performance data (AutomationDirect catalogue page tMTR-54): the four-pole motors from 1/4 to 1 hp at full-load power factors of 0.59 to 0.68; the 1 hp motors listed at 6.8 A (1,745 rpm, 71.0%, 0.68) and 5.06 A (3,500 rpm, 70.0%, 0.92) on 230 V
- WEG W22 Single-Phase Electric Motor, Commercial Catalogue, European Market (Cod. 50069268, Rev. 00, 01/2017), page 5: motors with start and run capacitors, 0.18 to 9.2 kW, at full-load power factors of 0.75 to 0.99
Inputs used
- Supply
- AC (mains)
- Phases
- Single phase
- Voltage
- 240 V (North American dryer, range, EV charger or heat pump circuit)
- Voltage you are working at (V)
- 220
- What you know
- The power in watts — find the amps
- Power (W)
- 7200
- Current (A)
- 32
- Apparent power (kVA)
- 10
- Power factor
- 1
Intermediate steps
- Apparent power
- 7.2 kVA
- Reactive power
- 0 kvar
- Voltage used
- 240 V
- Power factor used
- 1
Confidence note: Single-phase AC: apparent power is volts times amps, and real power is that times the power factor. At a power factor of 1 the two are the same; below 1 the supply carries more current than the watts alone suggest.
What this calculation does not cover
- These are the defining relationships between volts, amps, watts and volt-amperes, and nothing else. The page does not size a conductor, a breaker, a fuse or a plug, and it applies no continuous-load allowance, diversity or derating; those are decided against the wiring rules where the work is done, by the person who signs for it.
- The voltage is the nominal one you chose. A real supply sits somewhere inside a tolerance band and sags along a loaded circuit; a constant-power load such as a motor drive or a charger draws proportionally more current as the voltage falls, so the current at a weak point of a long run is higher than the nominal answer.
- Three phase assumes a balanced load: the same current in each line. An unbalanced load puts current in the neutral and the line currents stop being interchangeable, so one figure no longer describes the circuit; work each phase as a single-phase load at the line-to-neutral voltage instead.
- The power factor entered is taken as the true power factor. Variable-speed drives, LED drivers and switch-mode supplies draw distorted current, and a meter that reports only the displacement factor flatters the answer; on those loads the nameplate's current or VA figure is the safer starting point.
- Running current only. Motors, compressors and transformers draw several times their running current for the first moments after switching on, and that inrush, not this figure, is what a protective device's type and a generator's size have to allow for.
- A heat pump's or an air conditioner's circuit is built to the minimum circuit ampacity and maximum overcurrent protection printed on its nameplate, not to a current worked back from its wattage; the same holds for any appliance whose installation instructions name a circuit.
- A motor's rating is the power at its shaft, not the power it draws, so a rating entered here as watts understates the current: a 1 hp single-phase motor entered at its rating with a power factor of 0.85 reads 3.8 A on 230 V, where WEG lists its own 1 hp motors at 5.06 and 6.8 A. The single-phase and three-phase motor pages divide by the motor's efficiency as well as its power factor.
Add the equipment this sizes
This result is a specification — 30 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations4
- Definitions of real, apparent and reactive power: DC P = V × I; single-phase S = V × I and P = V × I × PF; three-phase S = √3 × V(line-to-line) × I and P = √3 × V × I × PF for a balanced load; Q = S × √(1 − PF²)
- Balanced three-phase systems: the line-to-line voltage is √3 times the line-to-neutral voltage (phases 120° apart), so three loads at V/√3 each draw √3 × V × I in total
- WEG Rolled Steel General Purpose 1-phase Motors, performance data (AutomationDirect catalogue page tMTR-54): the four-pole motors from 1/4 to 1 hp at full-load power factors of 0.59 to 0.68; the 1 hp motors listed at 6.8 A (1,745 rpm, 71.0%, 0.68) and 5.06 A (3,500 rpm, 70.0%, 0.92) on 230 V
- WEG W22 Single-Phase Electric Motor, Commercial Catalogue, European Market (Cod. 50069268, Rev. 00, 01/2017), page 5: motors with start and run capacitors, 0.18 to 9.2 kW, at full-load power factors of 0.75 to 0.99
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