How to Calculate Voltage Drop: Step-by-Step Engineering Guide

Learn how to calculate cable voltage drop for long wire runs, apply NEC 3% and 5% rules, and select correct conductor sizes.

Step-by-Step Procedure

Determine Circuit Current and Nominal Voltage

Identify the continuous or maximum operating current ( $I$ ) in Amps and the supply voltage ( $V$ ).

Measure the One-Way Distance

Measure the physical one-way cable routing length ( $L$ ) in feet or meters from the power source / breaker panel to the final load point.

Find Conductor Resistance (R)

Look up the conductor resistance per unit length (Ohms/1,000 ft or Ohms/km) from standard wire tables based on conductor gauge (AWG or mm²) and material (Copper/Aluminum) at operating temperature.

Calculate Total Loop Voltage Drop

For DC / 1-Phase: $\Delta V = 2 \times I \times (R_{\text{unit}} \times L)$ . For 3-Phase: $\Delta V = \sqrt{3} \times I \times (R_{\text{unit}} \times L)$ .

Evaluate Percentage Drop vs. Standards

Compute $%\Delta V = (\Delta V / V_{\text{supply}}) \times 100\%$ . Verify that the drop is $\le 3\%$ for branch circuits and $\le 5\%$ total.

Worked Practical Example

150-Foot Detached Garage Subpanel Feeder

Supplying a 240V 40A feeder to a subpanel located 150 feet away using 8 AWG Copper wire at 20°C.

•One-way length: 150 ft (0.150 kft)
•Resistance of 8 AWG Copper: 0.6282 Ω / 1,000 ft
•One-way resistance: 0.150 × 0.6282 = 0.09423 Ω
•Two-wire loop resistance: 2 × 0.09423 = 0.18846 Ω
•Voltage Drop: ΔV = 40 A × 0.18846 Ω = 7.54 V
•Drop Percentage: (7.54 V / 240 V) × 100 = 3.14%
•Conclusion: 8 AWG gives 3.14% drop. Upsizing to 6 AWG reduces drop to 1.98% (4.74 V), well within the 3% target.

✓ Recommend upsizing to 6 AWG Copper for optimal efficiency.

Common Mistakes to Avoid

✕Forgetting to double the one-way distance for single-phase and DC circuits (current must travel out and return).
✕Ignoring temperature rise: copper resistance increases by ~0.393% per °C above 20°C.
✕Using the 90°C ampacity table without checking voltage drop over long runs.

Frequently Asked Questions

At what distance does voltage drop become a concern?

For standard 120V branch circuits, voltage drop typically becomes significant on cable runs exceeding 50 to 75 feet. For 240V circuits, runs over 100 to 150 feet warrant careful calculation.

Related Calculators & Engineering Tools

Voltage Drop Calculator

Wire Size Calculator