- What's the formula for watts to amps?
- It depends on the circuit type. DC: I = P / V (amps = watts ÷ volts). AC single-phase: I = P / (PF × V), where PF is the power factor. AC three-phase line-to-line: I = P / (√3 × PF × V_LL). AC three-phase line-to-neutral: I = P / (3 × PF × V_LN). The calculator picks the right formula from the mode you pick.
- What is power factor and what value should I use?
- Power factor (PF) is the ratio of real power to apparent power on an AC circuit. It's a number between 0 and 1, dimensionless. Purely resistive loads — incandescent bulbs, electric heaters, kettles — have PF = 1. LED lighting is typically 0.9 to 0.95. Mixed industrial loads run around 0.85. Motors, depending on type and loading, sit between 0.75 and 0.9; 0.8 is a common assumption. On DC circuits, power factor doesn't apply — current is just P / V.
- Why does three-phase use √3?
- Because the three phases are 120° out of step with each other, the line-to-line voltage between any two of them is √3 (≈1.732) times the line-to-neutral voltage. When you measure total real power across all three phases and want the line current, you get P = √3 × V_LL × I × PF, which rearranges to I = P / (√3 × V_LL × PF). If you instead measure the per-phase voltage (line-to-neutral), the formula reduces to I = P / (3 × V_LN × PF), since the three phases each carry the same current.
- Should I use line-to-line or line-to-neutral voltage?
- Use the one you actually measured or read off the nameplate. North American 208Y/120V systems: 208V is line-to-line, 120V is line-to-neutral. European 400Y/230V: 400V is line-to-line, 230V is line-to-neutral. Motor nameplates typically list line-to-line. Distribution panels and outlets typically read line-to-neutral. The calculator handles both — pick the one matching your input number.
- Is this the same as the breaker size I need?
- Not directly. This gives you the load current. Breaker sizing in the US follows NEC 210.20 / 240.4: continuous loads must be sized at 125% of the load current, and the breaker is then rounded up to the next standard size (15, 20, 25, 30A, etc.). For a 16A motor load, that's 16 × 1.25 = 20A → 20A breaker. Other jurisdictions have their own rules. The calculator gives you the starting number; the code gives you the safety margin.
- Why is my measured current different from the calculated one?
- Three common reasons. (1) Power factor: if you used PF = 1 but the load is reactive, real current is higher than calculated. (2) Voltage variation: mains voltage can swing ±10% by code; a load drawing 1000W at 120V draws more current if the supply sags to 110V. (3) Inrush: motors and capacitive loads pull 3–10× their steady-state current at startup; a clamp meter catching the inrush moment will read high. The calculated value is the steady-state average.
- Can voltage be zero?
- Mathematically no — you'd be dividing by zero. The calculator blocks that input with a friendly message. Practically: if voltage is zero, there's no current flowing regardless of the wattage rating, because the rating assumes the appliance is energized. Wattage on a nameplate is a design parameter at the rated voltage, not a constant.
- Where does the √3 come from physically?
- Three-phase voltages are sinusoidal waveforms offset by 120°. If V_a = V × sin(ωt), V_b = V × sin(ωt − 120°), V_c = V × sin(ωt − 240°), then the difference (V_a − V_b) — which is what a meter between two phases sees — works out to √3 × V × sin(ωt + 30°). The peak (and RMS) of that difference is √3 times the per-phase value. The factor is exact, not an approximation; it's geometric, not empirical.
- Does this work for kW and MW too?
- Yes — just convert first. 1 kW = 1000 W; 1 MW = 1,000,000 W. Enter the wattage in watts and the calculator handles the rest. For very small or very large numbers it switches to scientific notation rather than show a row of zeros.
