How to Use This Calculator
Ohm’s Law is the foundation of electrical work. It shows the relationship between voltage (pressure), current (flow), and resistance (opposition). If you know any two of these values, you can solve for the third. Power (in watts) can also be calculated from any pair of these values, which is why we include Watt’s Law here.
Enter the two values you know, and the calculator will solve for voltage, current, resistance, and power. For example, if you know a circuit is 120 volts and the breaker is 15 amps, you can find the minimum resistance that breaker will tolerate: 120V ÷ 15A = 8 ohms. If a device’s resistance is lower than that, it will trip the breaker.
This calculator is indispensable for troubleshooting. A dryer not heating? Measure the voltage at the element. Get less than 240V? You have a bad connection or undersized feeder—that lower voltage means less power. A motor running slow? Could be voltage drop, which you can verify using voltage and nameplate current.
Formula
Ohm’s Law: V = I × R
Where:
- V = voltage in volts
- I = current in amps
- R = resistance in ohms
Rearranged:
- I = V / R (current equals voltage divided by resistance)
- R = V / I (resistance equals voltage divided by current)
Watt’s Law: P = V × I
Where:
- P = power in watts
- V = voltage in volts
- I = current in amps
Alternative Power Formulas:
- P = I² × R (power equals current squared times resistance)
- P = V² / R (power equals voltage squared divided by resistance)
When to Use This
Every day. You’re checking a loose breaker connection and wondering if the wire is getting hot? Calculate the resistance based on voltage and current and see if it’s drawing more power than it should. A 120V outlet is only giving you 100V—that’s a 14% voltage drop. If devices are plugged in that need 120V, they won’t run properly or at all.
On service calls, this is your diagnostic tool. Measure two parameters with a meter, plug them into Ohm’s Law, and you’ll know if the problem is the supply voltage, the circuit resistance, or the load. I’ve used this to determine that a burned-up contactor was drawing too much current before failing (high current = low resistance = heat buildup).
Code References
- NEC Article 110.3(B): Equipment must be used according to instructions; understanding rated voltage and amperage (based on Ohm’s Law) is fundamental
- NFPA 70E: Work on electrical circuits requires understanding voltage, current, and resistance to assess shock and arc flash hazards
Frequently Asked Questions
If I increase voltage on a circuit, does current automatically go up?
Only if the resistance stays the same. V = I × R. If you double voltage and resistance is unchanged, current doubles. But if resistance increases (due to a corroded connection, for instance), the current increase is less dramatic. Voltage and resistance both affect current.
What’s the difference between power in watts and power in volt-amps?
For DC or purely resistive loads, they’re identical. For AC circuits with inductive or capacitive loads (motors, fluorescent ballasts), power factor comes into play and watts differ from volt-amps. For this calculator, assume resistive loads unless you have a power factor correction.
My circuit breaker is 20 amps at 120V. What’s the maximum safe resistance?
R = V / I = 120V / 20A = 6 ohms. Any load with a resistance lower than 6 ohms will trip the breaker. Keep in mind this is a theoretical floor; the breaker will actually trip slightly above or below the nominal 20A depending on its design and temperature.
How do I know if a wire is overheating?
If current flows through a wire with resistance, it generates heat: P = I² × R. High current in a small wire (high resistance per foot) produces heat. That’s why we upsize wire for long runs and high currents. If you touch a wire and it’s hot, you have either high current, high resistance, or both—and you need to find out which before something fails.
Can I use Ohm’s Law to size a circuit breaker?
No. Ohm’s Law tells you what’s happening in a circuit, but the NEC determines breaker sizing based on the ampacity of the conductor, not Ohm’s Law. A 10 AWG copper wire is rated for 30 amps in free air (cooler conditions). You install a 30-amp breaker to protect it. If you load that circuit to exactly 30 amps, you’re at 100% of its rating, which the NEC discourages for continuous loads. Use NEC tables, not Ohm’s Law, for breaker selection.