Ohm’s law and the power formula
Ohm’s law links voltage, current and resistance. The power formula adds watts. Between them, any two known values give you the other two:
Rearranging those two equations gives all twelve combinations:
| To find | Formulas |
|---|---|
| Voltage V | I × R, P ÷ I, √(P × R) |
| Current I | V ÷ R, P ÷ V, √(P ÷ R) |
| Resistance R | V ÷ I, V² ÷ P, P ÷ I² |
| Power P | V × I, I² × R, V² ÷ R |
Worked examples
A 1500 W space heater on 120 V
At 12.5 A, the heater uses most of a 15 A circuit on its own. It's also a continuous load, so it shouldn't share that circuit with much else. That's why heaters are a classic cause of nuisance trips.
A resistor for an LED on 12 V
A red LED drops about 2 V and wants 20 mA. The resistor has to drop the remaining 10 V:
Enter 10 V and 0.02 A above to check it. The voltage you enter is the voltage across the resistor, not the supply.
Using Ohm’s law on real circuits
- Breaker sizing: divide a load’s watts by the voltage to get its current, then compare it with the circuit rating. Continuous loads (three hours or more) shouldn't exceed 80% of the breaker rating.
- Troubleshooting: a measured current far above V ÷ R points to a partial short. Zero current with full voltage points to an open circuit.
- Voltage drop: the drop along a wire is just I × R of the wire itself. Our voltage drop calculator does this using code resistance values.
- Heat: P = I² × R explains why loose connections get hot. A small extra resistance at a high current becomes significant heat at the terminal.
AC loads and power factor
On AC circuits, these formulas are exact for purely resistive loads like heaters, toasters, incandescent lamps and water heater elements. Motors, transformers, fluorescent ballasts and electronic power supplies draw current that is out of step with the voltage. For those loads:
For conductor and breaker sizing, the current on the nameplate is what matters. Use it directly rather than calculating it from watts.
Frequently asked questions
How many amps is 1500 watts at 120 volts?
12.5 amps (1500 ÷ 120). At 240 volts, the same 1500 watts draws 6.25 amps.
How do I convert watts to amps?
Divide watts by volts: I = P ÷ V. For three-phase loads, divide by (volts × 1.732). For motor loads, also divide by the power factor, or use the nameplate current.
What’s the difference between Ohm’s law and the power law?
Ohm’s law (V = I × R) relates voltage, current and resistance. The power law (P = V × I) relates those to watts. Combined, they give all twelve formulas in the table above.
Why does my calculation not match my meter?
Real resistances change with temperature. An incandescent filament’s cold resistance is about a tenth of its hot resistance. Supply voltage also varies, and many loads aren’t purely resistive. Use measured values under operating conditions where possible.
Last reviewed October 11, 2026. Results are estimates for planning. Always verify against the edition of the NEC adopted in your area, local amendments and the authority having jurisdiction. Found a mistake? Tell us.