Ohm's Law Basics for Electricians: V = IR and Beyond

The Three Formulas That Drive Everything

Ohm’s Law and the power equation form the foundation of every electrical calculation. If you know any two values, you can find the rest.

Ohm’s Law: V = I × R (Voltage equals Current times Resistance) Power: P = V × I (Power in watts equals Voltage times Current) Combined: P = I² × R and P = V² / R

Where V is voltage in volts, I is current in amperes, R is resistance in ohms, and P is power in watts.

These aren’t academic formulas — they’re troubleshooting tools. When a circuit isn’t behaving correctly, Ohm’s Law tells you what’s wrong.

Practical Applications on the Job

Calculating current draw from wattage. A 1,500W space heater on a 120V circuit draws: I = P/V = 1500/120 = 12.5 amps. On a 15A circuit, that leaves only 2.5A of headroom for anything else on that circuit.

Verifying a circuit has expected resistance. If you measure 120V at the panel and 10A of current flowing, the circuit resistance should be: R = V/I = 120/10 = 12 ohms. If your meter shows significantly different resistance, something is wrong — a bad connection, damaged conductor, or unintended load.

Estimating voltage from known load. If you know a motor draws 8A and has a rated resistance of 14.4 ohms: V = I × R = 8 × 14.4 = 115.2V. If the measured voltage is significantly lower, voltage drop on the feeder may be the problem.

Understanding Power Factor

In AC circuits, the simple P = V × I relationship gives you apparent power (VA or kVA), not true power (watts). The difference is power factor (PF).

True Power (W) = V × I × PF

Resistive loads (heaters, incandescent lights) have a power factor near 1.0 — apparent power equals true power. Inductive loads (motors, fluorescent ballasts, transformers) have power factors between 0.7 and 0.95, meaning they draw more current than the watt rating alone would suggest.

This matters for wire sizing. A motor rated at 1,500W with a 0.85 power factor actually draws: I = P / (V × PF) = 1500 / (120 × 0.85) = 14.7A — not the 12.5A you’d calculate ignoring power factor.

Single-Phase vs Three-Phase

For three-phase circuits, the formulas add a √3 factor:

Three-phase power: P = V × I × √3 × PF Three-phase current: I = P / (V × √3 × PF)

For a 10 kW three-phase load at 480V with 0.9 PF: I = 10,000 / (480 × 1.732 × 0.9) = 13.4A per phase.

This is where a calculator saves time and prevents errors. The √3 multiplication combined with power factor makes mental math unreliable.

Try the free Ohm’s law calculator → | FieldLab Electrician NEC Calculator →

Troubleshooting With Ohm’s Law

When a circuit behaves unexpectedly, Ohm’s Law narrows the diagnosis:

Higher than expected current: Either voltage is higher than expected (check the source) or resistance is lower than expected (look for a short circuit, ground fault, or additional unintended loads).

Lower than expected current: Either voltage is low (check for voltage drop on the feeder) or resistance is high (look for a loose connection, corroded terminal, or damaged conductor).

Equipment running hot: Excessive current through resistance generates heat (P = I²R). Check for loose connections — a slightly loose wire nut creates resistance that generates heat proportional to the square of the current flowing through it.

The formulas don’t change. The application is knowing which two values to measure and which third value to calculate.