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

Why Voltage Drop Matters

Every foot of wire has resistance. Current flowing through that resistance creates a voltage drop — the longer the run and the smaller the wire, the more voltage you lose before it reaches the load. On a short 15-foot run from the panel to a bedroom outlet, it's negligible. On a 200-foot run to a detached garage, it can be the difference between equipment working correctly and a callback.

Low voltage at the load means motors run hotter and less efficiently, LED lights can flicker, and sensitive electronics may malfunction. It also means wasted energy — the voltage lost in the wire is converted to heat, and the customer pays for every watt of it.

NEC Requirements

The National Electrical Code doesn't set a hard limit on voltage drop, but NEC 210.19 Informational Note No. 4 recommends no more than 3% for branch circuits and 5% total (feeder + branch combined). These aren't code violations if you exceed them, but they're the standard that inspectors and engineers expect.

For most residential and commercial work, hitting the 3% target on each branch circuit means you'll stay within 5% total without worrying about the feeder calculation separately.

The Voltage Drop Formula

For single-phase circuits:

VD = (2 × L × I × R) ÷ 1000

For three-phase circuits:

VD = (√3 × L × I × R) ÷ 1000

Where L is one-way length in feet, I is current in amps, and R is the conductor resistance per 1000 feet (which depends on wire gauge and material).

Wire Sizing: Two Checks, Not One

This is where apprentices often get tripped up. Sizing a wire requires passing two independent checks:

1. Ampacity. The wire must safely carry the load current without overheating. NEC Table 310.16 gives the maximum ampacity for each gauge at different temperature ratings. For 75°C THWN-2 copper: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 50A, 6 AWG = 65A.
2. Voltage drop. The wire must keep voltage drop within acceptable limits for the given distance and load.

The final wire size is whichever check requires the larger conductor. On short runs, ampacity usually governs. On long runs, voltage drop almost always requires upsizing beyond the ampacity minimum.

Copper vs. Aluminum

Aluminum wire has about 61% of copper's conductivity, so it needs to be roughly two sizes larger to carry the same current. For a 100A feeder, you'd use 1 AWG copper or 2/0 aluminum.

Aluminum is significantly cheaper for large feeders (like service entrances and sub-panel feeds) and is standard practice for those applications. For branch circuits, copper is almost universally used because the smaller wire sizes are easier to work with.

Real-World Examples

Detached garage sub-panel

Say you're running a 60A sub-panel feed to a detached garage 150 feet from the main panel at 240V single-phase. NEC ampacity says 6 AWG copper handles 65A — that passes. But check the voltage drop: at 60A over 150 feet with 6 AWG (0.491 ohms/1000ft), you get about 8.8V drop, which is 3.7%. That exceeds the 3% recommendation. Upsize to 4 AWG and the drop drops to about 5.5V or 2.3% — well within range.

LED lighting circuit

A 20A lighting circuit on 12 AWG at 120V with a 75-foot run to the farthest fixture. Even at only 10A actual load, the voltage drop is about 3.0%. Right at the limit. If the circuit might carry more load later, or if the customer has dimming LEDs (which are more sensitive to voltage variations), upsizing to 10 AWG is cheap insurance.

When to Upsize Beyond Code

NEC minimums are exactly that — minimums. Consider upsizing when:

• The load includes motors (they draw more current on startup and are sensitive to low voltage)
• LED lighting is on the circuit (flickering from voltage fluctuation is visible and annoying)
• The circuit might carry additional loads in the future
• Energy cost savings from reduced I²R losses justify the wire cost difference

Use our Wire Size Calculator to find the right gauge for your specific distance and load, and the Voltage Drop Calculator to verify an existing installation.