WiremanWireman · Ohm's law and power calculator (single and three phase) · 2023 NEC ·
Formulas
Ohm's law & power calculator
Fill in any two of volts, amps, ohms and watts and get the other two. Switch to AC for single- or three-phase power with power factor.
How this is calculated
- DC and purely resistive loads: V = I × R and P = V × I. The other ten formulas are these two rearranged.
- Single-phase AC: VA = V × I, W = VA × PF. Three-phase AC: VA = V × I × 1.732 (line-to-line voltage), W = VA × PF.
- VAR (reactive power) = √(VA² − W²). One horsepower = 746 W of output; input watts are higher by the motor’s efficiency.
- For motors, size conductors and protection from the NEC full-load current tables, not the nameplate or these formulas. Use the motor calculator.