Ohm's Law Calculator
Ohm's Law — The Foundation of Electrical Work
Ohm's Law is the most fundamental relationship in practical electronics and electrical work. It describes how voltage, current, resistance, and power relate to each other in any DC circuit — and mastering it means you can size wires, diagnose faults, and calculate loads with confidence. Whether you're troubleshooting a house circuit or designing electronics, every calculation starts here.
How to Use This Calculator
Select which two values you know from the dropdown, enter them in the fields, and hit Calculate. The calculator solves for the remaining two values and shows the load percentage against standard 15A and 20A residential circuits.
The Four Ohm's Law Formulas
Example: a 120 V circuit with a 12 Ω load. V=120, R=12 → I = 120 ÷ 12 = 10 A → P = 120 × 10 = 1,200 W. That's 80% of a 15A circuit's 1,500 W capacity — a high but legal load for a dedicated circuit.
North American Circuit Standards
- 15A circuit (14 AWG): 1,800 W max — NEC 80% rule limits continuous load to 1,440 W. Used for lighting and general outlets.
- 20A circuit (12 AWG): 2,400 W max — 1,920 W continuous. Required for kitchen outlets, bathrooms, and laundry rooms.
- 30A circuit (10 AWG): 3,600 W max — used for clothes dryers, water heaters, EV charger level 1.
- 50A circuit (6 AWG): 6,000 W max — electric ranges, large hot tubs, EV charger level 2.
The NEC 80% Rule
Continuous loads — those running for 3 hours or more — must not exceed 80% of a circuit's rated ampacity. A 15A circuit should carry no more than 12A continuously; a 20A circuit, no more than 16A. This headroom prevents heat buildup in wiring and protects the breaker's overload mechanism from continuous high-temperature operation. HVAC systems, water heaters, and EV chargers are all considered continuous loads.
Frequently Asked Questions
What's the difference between AC and DC Ohm's Law?
In a DC circuit, Ohm's Law applies directly: V = I × R. In AC circuits, the effective resistance is called impedance (Z), which includes resistance plus reactive components from capacitors and inductors. For purely resistive AC loads like heating elements, R and Z are the same. For motors, transformers, and reactive circuits, use Z instead of R. This calculator assumes resistive (DC-equivalent) loads.
Why does my breaker trip when I run several appliances at once?
Each appliance draws current. When total current exceeds the breaker's rating, it trips to prevent wire overheating. Add up the watt ratings of everything on the circuit and divide by 120V to get total amps. Hair dryer (1,875 W) + space heater (1,500 W) = 3,375 W ÷ 120 V = 28.1 A — well over a 20A circuit's limit. The fix is dedicated circuits for high-draw appliances.
How do I size a circuit for a specific appliance?
Find the wattage on the nameplate. Divide by 120 (or 240 for 240V appliances) to get amps. Add 25% safety margin for motor inrush current. The result determines minimum wire gauge and breaker size. A 2,400W electric dryer: 2,400 ÷ 240V = 10A × 1.25 = 12.5A → use a 15A or 20A 240V circuit with 12 AWG wire minimum.
What happens if resistance increases in a circuit?
Higher resistance reduces current flow (I = V ÷ R). In practice: a loose connection, corroded wire, or undersized wire increases resistance. This causes voltage drop (less voltage at the load), heat at the high-resistance point, and potential fire risk. This is why maintaining low-resistance connections and properly-sized wiring is a fundamental electrical safety requirement — not just a code issue.
Can I use this calculator for 12V automotive or solar circuits?
Yes — Ohm's Law applies at any voltage. A 12V 120W car amplifier: 120 ÷ 12 = 10A draw, 12 ÷ 10 = 1.2Ω impedance. A 12V 100W solar panel under full sun outputs 8.3A. Size your wires for the current draw, not the wattage, and apply the 80% continuous-use rule. Use our Wire Gauge Calculator to verify voltage drop for long DC runs.