Voltage Drop Calculator
Calculate voltage drop across AC and DC electrical circuits over distance. Ensure subpanels, outbuilding feeds, submersible pumps, and long solar array runs do not exceed the National Electrical Code (NEC) recommended 3% branch circuit drop limit.
Gauge of installed copper wire
Nominal source voltage at breaker panel (e.g. 120V, 240V, or 12V DC)
Operating current load in amperes
Distance from the electrical source to the load terminal
Single phase uses multiplier 2.0; three phase uses √3 ≈ 1.732
Actual voltage drop dissipated across the wire run
How to Use This Calculator
- Select the wire gauge size (AWG) of your circuit conductor.
- Enter your source supply voltage (120V, 240V, or 12V DC).
- Enter the actual operating load current in amps.
- Enter the one-way distance in feet.
- Review the voltage loss and check if it satisfies the NEC 3% recommendation.
Formula & Calculation Methodology
Voltage drop is governed by Ohm’s Law (V = I × R). For copper conductors at 75°C, the resistivity constant K is 12.9 ohms-cmil/ft. Dividing by the wire circular mils yields total loop drop.
Worked Example
16 Amps on 12 AWG copper, 120V single-phase run 75 feet
30 Amps on 10 AWG copper, 240V run 50 feet (electric water heater)
Frequently Asked Questions
What are the consequences of excessive voltage drop?↓
Excessive voltage drop causes electric motors (in refrigerators, air conditioners, and well pumps) to draw higher current and overheat, drastically shortening their lifespan. It also causes flickering lights and electronic component resets.
What is the National Electrical Code (NEC) rule on voltage drop?↓
NEC Informational Note 210.19(A) recommends that voltage drop should not exceed 3% on branch circuits, and total combined drop across feeder and branch circuits should not exceed 5% from the main service panel to the farthest outlet.
Why does 12V DC experience much worse percentage drop than 120V AC?↓
Because percentage drop is relative to supply voltage. Losing 2.4 volts on a 120V circuit is only a 2% drop, but losing the exact same 2.4 volts on a 12V solar battery system is a crippling 20% loss.
What is a limitation when measuring distance for voltage drop?↓
Remember to measure the actual winding path of the wire through joists, studs, conduits, and vertical drops rather than straight-line Euclidean distance.
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Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.