Ohm’s law and power (DC, 1- and 3-phase)
In DC mode enter any two values and the tool works out the rest. The AC modes give active, reactive and apparent power and the load current.
Result
- Voltage U
- 24 V
- Current I
- 500 mA
R = U / I
P = U · I
Ohm’s law and power formulas
Ohm’s law links voltage, current and resistance: U = I · R. Power is P = U · I. Together they mean that any two of the four quantities give you the other two.
Three-phase power
In a balanced three-phase system, active power is P = √3 · U · I · cos φ, where U is the line-to-line voltage (400 V in Europe) and I is the current in one phase. Apparent power is S = √3 · U · I and reactive power is Q = √3 · U · I · sin φ.
Example: a 7.5 kW motor at 400 V with cos φ = 0.85 and 90% efficiency draws about 14.2 A.
Common questions
Where does √3 come from?
Line-to-line voltage is √3 times the phase voltage (400 V ≈ √3 · 230 V). The formula with √3 and line voltage equals three times the power of one phase.
What cos φ should I use if I don’t know it?
Usually 0.8–0.9 for induction motors and 1 for heaters. The exact value is on the nameplate.