What is voltage drop and why does it matter?

Voltage drop is the reduction in voltage as electricity travels through a wire due to the wire's resistance. Excessive voltage drop (typically over 3% for branch circuits or 5% total for the entire system) can cause: dim lights, motors running hot or inefficiently, appliances not functioning properly, and wasted energy. Proper wire sizing minimizes voltage drop and ensures safe, efficient electrical operation.

How do I calculate wire length for electrical circuits?

For wire length calculations, you must account for both the supply wire AND return wire - electricity completes a circuit. If your device is 50 feet from the power source, the total wire length is 100 feet (50 feet there + 50 feet back). Always double the distance when calculating or selecting wire. This is crucial for accurate voltage drop calculations and proper wire gauge selection.

What wire gauge (AWG) should I use for my circuit?

Wire gauge depends on: amperage (current) the circuit carries, wire length (longer runs need thicker wire), acceptable voltage drop (typically 3% max), and wire material (copper or aluminum). Common residential uses: 14 AWG for 15-amp circuits, 12 AWG for 20-amp circuits, 10 AWG for 30-amp circuits. For long runs (over 50-100 feet), you may need to upsize (use thicker wire) to prevent excessive voltage drop.

What's the difference between copper and aluminum wire?

Copper is more conductive than aluminum, meaning copper wire can carry more current for the same gauge, or you can use smaller copper wire for the same current. Aluminum wire must be about two gauges larger than copper for equivalent capacity (e.g., 10 AWG copper ≈ 8 AWG aluminum). Copper is more expensive but easier to work with and generally preferred for residential wiring. Aluminum is lighter and less expensive, common in service entrance cables.

How much voltage drop is acceptable?

The National Electrical Code (NEC) recommends: maximum 3% voltage drop on branch circuits (from panel to outlet/fixture) and maximum 5% voltage drop for the entire system (service to final point of use). For sensitive electronics or motors, even lower voltage drop (1-2%) is preferable. Exceeding these recommendations results in inefficiency and performance issues, though it's not necessarily a code violation in all cases.

Home & Construction

Wire Length & Voltage Drop Calculator

Calculate electrical wire length needed and estimate voltage drop.

Distance (One Way) in Feet

Voltage (V)

Load (Amps)

Wire Gauge (AWG)

Enter circuit details.

How to Use This Calculator

Enter Circuit Details

Input the one-way distance from power source to load (the calculator will automatically double this for total wire length), the circuit voltage (120V or 240V for residential), the amperage (current) the circuit will carry, and the wire material (copper or aluminum).

Select or Calculate Wire Gauge

Either enter the wire gauge (AWG) you plan to use, or let the calculator recommend the minimum gauge needed to keep voltage drop within acceptable limits (typically 3%). Remember that local electrical codes may have specific requirements that supersede calculations.

Calculate Voltage Drop and Wire Length

The calculator will show: total wire length needed (one-way distance × 2), the voltage drop as both volts and percentage, and whether the voltage drop is acceptable (under 3%). If voltage drop is excessive, the calculator will recommend using a larger (lower AWG number) wire gauge.

Verify and Purchase Materials

Review the recommendations and verify they meet local electrical codes. Purchase the recommended wire gauge with some extra length for routing and connections. **Important: This calculator is for planning purposes. Always consult local electrical codes and consider hiring a licensed electrician for actual electrical work to ensure safety and code compliance.**