Wire Gauge Guide: AWG Sizes, Ampacity and Voltage Drop Explained

What Is AWG (American Wire Gauge)?

American Wire Gauge is the standard system for measuring wire diameter in the United States and Canada. Established in 1857, the AWG system uses a counterintuitive numbering scheme: smaller numbers indicate thicker wire. This dates back to the manufacturing process — wire was drawn through successively smaller dies, and the gauge number represented how many draws it took.

The system follows a precise geometric progression. Each step down in gauge number increases the diameter by a factor of approximately 1.123, and every decrease of 6 AWG doubles the diameter. Every decrease of 3 AWG doubles the cross-sectional area (and therefore the current-carrying capacity).

AWG Size Chart: Diameter, Area and Resistance

Ampacity: How Much Current Each Wire Can Carry

Ampacity is the maximum current a wire can safely carry without exceeding its temperature rating. The National Electrical Code (NEC) Table 310.16 provides standard ampacity values. According to the 2023 NEC (the most recent edition as of 2026), these are the copper wire ratings:

The NEC restricts 14 AWG and 12 AWG to their 60°C ampacity regardless of insulation type for branch circuit wiring. According to the National Fire Protection Association (NFPA), electrical distribution and lighting equipment were involved in an average of 34,600 home fires per year between 2017 and 2021, making proper wire sizing critical.

Voltage Drop: Why Wire Length Matters

Every wire has resistance, and that resistance causes voltage to drop as current flows through it. The NEC recommends:

• 3% maximum voltage drop on branch circuits (NEC 210.19(A) Informational Note)
• 5% maximum total voltage drop from service entrance to the farthest outlet

The voltage drop formula for single-phase circuits is:

V_drop = 2 × I × R × L / 1000

Where I is current in amps, R is resistance in ohms per 1,000 feet, and L is one-way length in feet. The factor of 2 accounts for the round trip (hot and neutral wires).

Voltage Drop Example

A 20-amp circuit using 12 AWG copper wire running 75 feet to a workshop outlet:

V_drop = 2 × 20 × 1.588 × 75 / 1000 = 4.76 volts (4.0% on a 120V circuit)

This exceeds the 3% recommendation. Upgrading to 10 AWG (0.999 Ω/1000ft) reduces the drop to 3.0 volts (2.5%), which is within limits.

Copper vs Aluminum Wire

According to the Copper Development Association, copper remains the standard for branch circuit wiring in residential construction. Aluminum is widely used for service entrance cables and large feeders where the cost savings on long runs of thick wire is significant. The 2023 NEC requires AL-rated connectors and anti-oxidant compound on all aluminum wire terminations.

Common Residential Circuits

EV Charger Circuits

Most Level 2 EV chargers draw 32–48 amps. Per the NEC continuous load rule (Article 625), the circuit must be rated at 125% of the charger's draw. A 48-amp charger needs a 60-amp breaker with 6 AWG copper. According to the Department of Energy, over 4.2 million EVs were registered in the U.S. in 2025, driving rapid growth in home charging installations.

Kitchen and Bathroom Circuits

The NEC requires at least two 20-amp small-appliance branch circuits in the kitchen (12 AWG minimum) and at least one 20-amp circuit for bathroom receptacles. GFCI protection is mandatory for both.

240V Appliance Circuits

Electric ranges typically need a 50-amp circuit with 6 AWG wire. Electric dryers need a 30-amp circuit with 10 AWG wire. Central air conditioning units vary by size but commonly require 30–60 amp circuits.

Frequently Asked Questions

What wire gauge do I need for a 20-amp circuit?

A 20-amp circuit requires 12 AWG copper wire minimum per the National Electrical Code (NEC). For runs longer than 50 feet, consider upsizing to 10 AWG to minimize voltage drop. 12 AWG copper has an ampacity of 20 amps at 60°C and 25 amps at 75°C.

What does AWG stand for and how does the numbering work?

AWG stands for American Wire Gauge. The numbering is counterintuitive: smaller numbers mean larger wire. Each decrease of 6 AWG roughly doubles the wire diameter, and each decrease of 3 AWG doubles the cross-sectional area. The system originated from the number of drawing dies a wire was pulled through during manufacturing.

What is voltage drop and when does it matter?

Voltage drop is the reduction in voltage along a wire due to its resistance. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total from service entrance to the farthest outlet. For a 120V circuit, that means no more than 3.6V drop. Long runs and undersized wire increase voltage drop.

Can I use aluminum wire instead of copper?

Yes, but aluminum wire must be larger than copper for the same ampacity. Aluminum has about 61% the conductivity of copper, so you typically need to go up 2 AWG sizes (e.g., 2 AWG aluminum replaces 4 AWG copper for a 100-amp circuit). Aluminum is common for service entrance cables and large feeders.

What size wire do I need for an EV charger?

Most Level 2 EV chargers draw 32–48 amps. A 48-amp charger needs a 60-amp breaker (continuous load rule: 48 × 1.25 = 60) with 6 AWG copper wire. For runs over 50 feet, 4 AWG may be needed to stay within voltage drop limits.