Cable & Wire Length Calculator

Voltage Drop Calculation

Voltage drop occurs when electrical current flows through a wire due to the wire’s resistance. The formula used for calculating voltage drop is:

• Vd = Voltage drop in volts
• I = Current in amperes
• L = One-way wire length in feet
• R = Wire resistance in ohms per 1000 feet

Wire Size Selection

Wire size selection is based on two main criteria:

1. Ampacity (current-carrying capacity) – The wire must be thick enough to carry the current without overheating. This is determined by the NEC (National Electrical Code) tables.
2. Voltage drop – The wire must be sized to keep voltage drop within acceptable limits (usually 3% for general circuits and 5% for the total branch circuit).

The final wire size selected is always the larger of these two requirements.

Safety First

• Always turn off power before working with electrical wiring
• Use proper PPE including gloves and safety glasses
• Never exceed the ampacity rating of a wire
• Follow local electrical codes and obtain permits when required
• When in doubt, consult a licensed electrician

Voltage Drop Considerations

• Keep voltage drop under 3% for branch circuits, 5% maximum total
• For long runs, consider upgrading wire size
• Sensitive electronics may require stricter limits (1-2%)
• Motors are particularly affected by voltage drop during startup

Wire Selection Tips

• Choose the correct insulation type for your application (THHN, XHHW, etc.)
• Consider future load increases when sizing wire
• For outdoor use, ensure wire is rated for wet locations
• Color-code wires according to standards (black/red for hot, white for neutral, green for ground)

Installation Best Practices

• Leave extra wire at junction boxes (6-8 inches minimum)
• Use proper connectors and terminations for the wire size
• Avoid sharp bends that could damage insulation
• Label wires for future identification
• Keep wires neat and organized in panels and junction boxes