Voltage Drop and Wire Sizing: The Complete Electrician's Guide

Every electrical wire has resistance. When current flows through that resistance, it loses voltage along the way — a phenomenon called voltage drop. For short runs this is negligible, but for long circuits feeding workshops, outbuildings, EV chargers, or industrial motors, undersized wire causes serious problems: equipment runs hotter than spec, motors fail early, and you can violate NEC code.

This guide explains the physics behind voltage drop, shows you the exact calculation method, and gives you practical wire sizing guidance.

---

Why Voltage Drop Matters

Appliances and motors are designed to operate within a specific voltage window. A motor rated at 120V that receives only 108V (10% drop) works harder for the same output, draws more current, overheats, and has a significantly shorter service life.

The National Electrical Code (NEC) recommends:

• Maximum 3% voltage drop on any individual branch circuit
• Maximum 5% total from the service panel to the final outlet (feeder + branch combined)

These are recommendations, not hard rules — but exceeding them voids warranties, causes equipment failures, and can fail electrical inspections.

---

The Voltage Drop Formula

For single-phase AC and DC circuits, the standard formula is:

Vd = (2 × K × I × L) / Area

Where:

• Vd = Voltage Drop (Volts)
• K = Resistivity constant (Copper = 12.9, Aluminum = 21.2)
• I = Load current in Amperes
• L = One-way wire length in feet
• Area = Cross-sectional area of the wire in circular mils (cmil)

The factor of 2 accounts for both the hot wire and the neutral return wire.

Percentage Loss = (Vd / Source Voltage) × 100

---

AWG Wire Size Chart (Circular Mils)

---

Worked Example: 100-Foot Run to a Workshop

A 120V, 15A circuit runs 100 feet (one-way) to a detached garage using 12 AWG copper wire.

• K = 12.9 (copper)
• I = 15 A
• L = 100 ft
• Area = 6,530 cmil (12 AWG)

Vd = (2 × 12.9 × 15 × 100) / 6,530 = 38,700 / 6,530 = 5.93 V

Percentage = 5.93 / 120 × 100 = 4.94%

This exceeds the 3% NEC recommendation. Upgrading to 10 AWG (10,380 cmil):

Vd = (2 × 12.9 × 15 × 100) / 10,380 = 3.73 V = 3.11% ✓

---

Copper vs. Aluminum: Which to Choose?

Aluminum wire carries 64% more resistance per circular mil than copper. To match copper's voltage drop, aluminum wire must be two AWG sizes larger. For example, use 1/0 AWG aluminum where you would use 2 AWG copper.

Safety Note: Aluminum wiring in branch circuits requires special connectors marked AL/CU and anti-oxidant compound at all terminations. Never mix aluminum conductors directly with standard copper-rated receptacles or switches.

---

EV Charger Sizing Example (Level 2, 240V, 32A)

A Level 2 EV charger is installed with a 50-foot one-way run using 8 AWG copper at 240V.

• K = 12.9, I = 32A, L = 50 ft, Area = 16,510 cmil

Vd = (2 × 12.9 × 32 × 50) / 16,510 = 41,280 / 16,510 = 2.50 V

Percentage = 2.50 / 240 = 1.04% ✓ Well within NEC limits.

Use our Voltage Drop Calculator to size any circuit instantly.

---

Common Mistakes

1. Using one-way length only — always double it in the formula to account for the return wire.
2. Ignoring temperature — wire resistance increases with heat. Cables in conduit in hot attics carry less current safely.
3. Forgetting future loads — size for 125% of the continuous load, not just the nameplate rating.
4. Using aluminum in branch circuits without proper fittings — this has caused house fires. Always use AL/CU-rated terminations.