Understanding Voltage Drop and Why It Matters in Your Wiring

Voltage drop is the loss of electrical potential as current flows through a conductor. Every wire has resistance, and that resistance converts some of your electrical energy into heat. While a small voltage drop is normal, excessive drop causes motors to overheat, lights to dim, electronics to malfunction, and energy bills to climb. The NEC recommends keeping branch-circuit voltage drop under 3% and total system drop under 5%.

The Voltage Drop Formula

V_d = (2 × K × I × L) ÷ CM

Where K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), I is the current in amps, L is the one-way wire length in feet, and CM is the circular mil area of the conductor. For three-phase circuits, multiply by 1.732 instead of 2. Our calculator uses these exact constants so your results match hand calculations.

Why 3% Is the Magic Number

A 3% voltage drop on a 120V circuit means only 116.4V reaches your load. On a 240V circuit, that is a loss of 7.2V. While small motors and resistive loads tolerate this, sensitive electronics, LED drivers, and variable-speed equipment may malfunction. For branch circuits serving receptacles and lighting, aim for under 2% if possible. The 3% threshold is a maximum, not a target.

When Distance Becomes the Problem

Short runs (under 50 feet) rarely have voltage drop issues with properly sized wire. The problem appears with long runs: outbuildings, detached garages, well pumps, distant HVAC condensers, and EV charger installs in the driveway. At 200+ feet, you may need to upsize your wire one or two gauges beyond what ampacity alone requires.

Example: Wire Size for 100 Amp Service 150 Feet

A homeowner asking “what size wire for 100 amp service 150 feet” needs to consider both ampacity and voltage drop. While #2 AWG copper handles 100A at 75°C, a 150-foot run at 240V may show a 4–5% voltage drop — exceeding the 3% recommendation. Upsizing to #1/0 AWG or #2/0 AWG copper brings the drop back under 3%. Our calculator flags this automatically.

Aluminum Conductors and Long Runs

Aluminum has higher resistance per ampacity than copper, meaning voltage drop is worse for the same load at the same distance. However, aluminum is significantly cheaper per ampere at large sizes. For long service entrance runs and feeders, aluminum is often the practical choice — you just need to account for the larger gauge requirement. Check both materials in our wire size calculator to compare cost vs. performance.

Single-Phase vs. Three-Phase

Three-phase systems experience lower voltage drop than single-phase at the same current because the 1.732 multiplier (versus 2 for single-phase) reflects the phase relationship. Commercial and industrial facilities with three-phase power have a built-in advantage for long runs. Our calculator supports both configurations.

Practical Tips to Reduce Voltage Drop

Upsize the wire — The most direct solution. One AWG size smaller reduces resistance by roughly 20%.
Shorten the run — If possible, route the cable the most direct path. Every 50 feet matters.
Use parallel conductors — For very large feeders (4/0 and above), running two parallel conductors per phase halves the resistance.
Balance three-phase loads — Uneven loading increases neutral current and effective drop.
Increase voltage — Running at 240V instead of 120V halves the current for the same power, which quarters the voltage drop.

After Voltage Drop Is Under Control

Once your wire size handles both ampacity and voltage drop, verify that your conduit can physically accommodate the wire. Upsizing wire for voltage drop sometimes means moving to a larger conduit. Use our conduit fill calculator to check.

This calculator provides estimates based on NEC standard formulas. For critical installations, consult a licensed electrician and verify against your local electrical code amendments.