Voltage Drop Calculator
Calculate voltage drop for electrical wiring runs. Enter wire gauge, distance, and amperage to ensure your circuits deliver adequate voltage to loads.
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How to Use This Calculator
Calculate voltage drop to ensure your electrical runs deliver adequate power to loads:
- Enter the wire gauge (AWG) you're using or planning to use.
- Input the one-way distance from power source to load in feet.
- Enter the current (amps) the circuit will carry.
- Select the voltage of your system (120V, 240V, etc.).
- Click 'Calculate' to see voltage drop percentage and actual voltage at the load.
Why This Matters
Voltage drop is the silent killer of electrical efficiency. As current flows through wire, some voltage is lost as heat due to resistance. The National Electrical Code recommends no more than 3% drop for feeders and 5% total for branch circuits. Excessive voltage drop causes lights to dim, motors to run hot and inefficiently, and electronics to malfunction. For long runs to outbuildings, pools, or outdoor lighting, voltage drop often requires larger wire than ampacity alone would suggest.
- Ensure lights operate at full brightness without dimming
- Prevent motors from overheating due to low voltage
- Optimize energy efficiency by minimizing power lost in wiring
- Properly size wire for long runs to outbuildings and remote locations
- Meet NEC recommendations for voltage drop limits
Worked Examples
Long Run to Shed
Running a 20A, 120V circuit 150 feet to a shed using #12 AWG wire.
Landscape Lighting
Low voltage (12V) landscape lights, 5A load, 100 feet of #14 wire.
EV Charger Circuit
50A, 240V circuit, 60 feet from panel to garage, using #6 AWG.
Common Mistakes to Avoid
- Forgetting that voltage drop is a round-trip calculation - current flows out AND back, doubling the wire length.
- Using ampacity tables alone for long runs - you may need larger wire for voltage drop even if ampacity is okay.
- Not accounting for the much higher impact of voltage drop in low-voltage systems (12V, 24V).
- Ignoring NEC guidelines - while not always code requirements, 3%/5% limits are best practice.
- Using aluminum wire ampacity calculations for copper - voltage drop differs between materials.