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Ohm's Law / Power Wheel Calculator

Solve voltage, current, resistance, and power for DC, single-phase AC, and three-phase AC circuits

Free Ohm's Law and Watt's Law calculator for electricians, maintenance technicians, and electrical engineers who need to solve any two-known, two-unknown relationship across all 12 standard formulas. Select DC, single-phase AC, or three-phase AC mode, enter any two known values (voltage, current, resistance, or power), and the calculator returns the remaining two. AC modes include power factor input and display the power triangle: real power (W), reactive power (VAR), and apparent power (VA). Three-phase mode handles both line and phase quantities. Every electrical troubleshooting call starts with these relationships. Whether you are checking voltage drop across a motor starter, verifying current draw against a breaker rating, or sizing a resistive heater element, these four quantities and their interrelationships are the foundation. The calculator eliminates mental math errors on formulas that most electricians know by heart but occasionally transpose under pressure, especially when converting between three-phase line and phase values.

Pro Tip: When troubleshooting a three-phase motor that is drawing higher than expected current on one leg, measure all three line voltages and all three line currents before reaching for this calculator. A voltage imbalance of just 3 percent can cause a current imbalance of 18 to 25 percent due to the negative-sequence impedance of the motor. If the voltages are balanced but the currents are not, the problem is in the motor or its connections, not the supply.

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Ohm's Law / Power Wheel Calculator
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How It Works

  1. Select Circuit Mode

    Choose DC, single-phase AC, or three-phase AC. For AC modes, the power factor field becomes active and defaults to 1.0 (unity). Enter the actual power factor if known, typically 0.80 to 0.95 for motor loads.

  2. Enter Any Two Known Values

    Enter two of the four quantities: voltage (V), current (A), resistance (ohms), or power (W). The calculator solves for the remaining two using the appropriate Ohm's Law and Watt's Law formulas for the selected mode.

  3. Review Results and Power Triangle

    The output displays all four quantities plus, in AC modes, the power triangle showing real power (W), reactive power (VAR), and apparent power (VA). Three-phase mode shows both line and phase values for voltage and current.

Assumptions

  • Circuits are assumed to be balanced (equal voltage and current on all phases for three-phase mode).
  • Resistance is assumed constant (no temperature coefficient correction).
  • AC calculations assume sinusoidal waveforms with no harmonic content.
  • Power factor is applied as a simple cosine term (displacement power factor only, not total power factor).

Limitations

  • Does not model unbalanced three-phase loads or single-phasing conditions.
  • Does not include conductor impedance, voltage drop, or cable reactance.
  • Does not calculate harmonic distortion effects on power factor or current.
  • Does not account for motor starting transients or locked-rotor conditions.

References

  • Ugly's Electrical References (Jones & Bartlett Learning)
  • IEEE Std 141 (Red Book) - IEEE Recommended Practice for Electric Power Distribution for Industrial Plants
  • NEMA MG 1 - Motors and Generators

Frequently Asked Questions

The factor 1.732 (the square root of 3) appears because three-phase power is calculated from line quantities, and the line voltage in a balanced three-phase system is the square root of 3 times the phase voltage. The formula P = 1.732 x V_line x I_line x PF accounts for this geometric relationship. If you used phase voltage and phase current instead, you would multiply by 3 (three individual phases), and the result would be the same total power. The 1.732 factor is not an approximation. It is exact for balanced three-phase systems.
Use apparent power (VA or kVA) when sizing transformers, generators, UPS systems, conductors, and switchgear, because these components must carry the full current regardless of power factor. Use real power (W or kW) when calculating energy consumption, heat dissipation, or mechanical output. A 100 kVA transformer feeding a load at 0.85 power factor delivers only 85 kW of real power, but the transformer, cables, and breakers must be rated for the full 100 kVA current.
Ohm's Law applies to AC circuits when you replace simple resistance (R) with impedance (Z), which includes both resistance and reactance. For purely resistive loads such as heaters and incandescent lamps, R and Z are the same and the basic V = I x R formula works directly. For motors, transformers, and capacitor banks, the impedance includes inductive or capacitive reactance, and you need to account for phase angle (power factor). This calculator handles that by including the power factor input for AC modes.
You generally should not use DC resistance to analyze a running motor. The relevant quantity for a running motor is impedance, not resistance. The DC winding resistance (measured with a milliohm meter) is only useful for winding temperature tests and continuity checks. During operation, the motor's impedance includes inductive reactance from the stator windings and back-EMF from the rotor. If you need running current, use the NEC table FLA values from the Motor FLA Lookup tool, not an Ohm's Law calculation based on winding resistance.
Disclaimer: This calculator solves standard Ohm's Law and Watt's Law relationships for ideal circuit conditions. It does not account for temperature-dependent resistance changes, harmonic distortion, or transient conditions. Use field measurements with calibrated instruments to verify calculated values.

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