Voltage Drop Calculator

Quick summary. Voltage drop is the reduction in delivered voltage caused by conductor resistance across the full circuit path. In a wire harness or cable assembly, long runs, high current, and smaller conductors raise that loss, so this calculator helps engineers screen branch designs before prototype validation and thermal testing.

Understanding Voltage Drop

What is Voltage Drop?

Voltage drop is the reduction in voltage as electrical current flows through a conductor's resistance. Every wire has inherent resistance that causes some voltage to be lost as heat when current passes through it. Excessive voltage drop can cause equipment malfunction, reduced performance, or safety hazards.

Voltage Drop Formula

Voltage Drop (V) = 2 × I × R × L / 1000

Where:

• I = Current in Amperes (A)
• R = Resistance per kilometer (Ω/km)
• L = One-way cable length in meters (m)
• 2 = Factor for round trip (positive and negative conductors)

Acceptable Voltage Drop Limits

• NEC Recommendation: ≤3% — The National Electrical Code recommends maximum 3% voltage drop for branch circuits and feeders.
• Combined System: ≤5% — Total combined voltage drop (feeder + branch) should not exceed 5%.
• Critical Loads: ≤2% — Sensitive equipment and critical applications often require <2% voltage drop.

Factors Affecting Voltage Drop

• Wire Size: Larger wire (lower AWG) has less resistance and lower voltage drop
• Current: Higher current increases voltage drop proportionally
• Length: Longer cables have more resistance and higher voltage drop
• Temperature: Higher temperatures increase resistance (~0.4% per °C)
• Material: Copper has lower resistance than aluminum

Solutions for High Voltage Drop

• Increase wire size (use lower AWG number)
• Reduce cable length if possible
• Use multiple parallel conductors
• Increase system voltage (e.g., 24V instead of 12V)
• Install transformer or power supply closer to load

Voltage Drop Design Targets