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Ohm's Law & Power Wheel

Ohm's Law formulas — V=IR, P=VI, P=I²R, P=V²/R — with the complete power wheel for voltage, current, resistance, and power calculations.

Formulas

V = I × R

Voltage equals current times resistance

I = V / R

Current equals voltage divided by resistance

R = V / I

Resistance equals voltage divided by current

P = V × I

Power equals voltage times current

P = I² × R

Power equals current squared times resistance

P = V² / R

Power equals voltage squared divided by resistance

Variable Definitions

V=Voltage(volts (V))
I=Current(amperes (A))
R=Resistance(ohms (Ω))
P=Power(watts (W))

Worked Examples

Example 1

A 120V circuit has a resistance of 10Ω. Find the current and power.

  1. I = V / R = 120 / 10 = 12A
  2. P = V × I = 120 × 12 = 1,440W

Answer: 12A current, 1,440W power

Example 2

A 240V heater draws 20A. Find its resistance and power.

  1. R = V / I = 240 / 20 = 12Ω
  2. P = V × I = 240 × 20 = 4,800W

Answer: 12Ω resistance, 4,800W (4.8 kW) power

When to Use

  • •Calculating current draw for any resistive load
  • •Determining voltage drop across a known resistance
  • •Finding power consumption of any electrical device
  • •Sizing conductors and overcurrent devices

Standards & References

  • Fundamental electrical law — applies to all NEC calculations

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