AWG Wire Gauge Guide: Sizes, Ampacity & Selection

AWG (American Wire Gauge) is the standard sizing system for electrical wire and cable in the United States. The gauge number defines the conductor's diameter, cross-sectional area, and current-carrying capacity. Smaller AWG numbers indicate thicker wire—14 AWG is thicker than 22 AWG, and 4/0 AWG is the thickest standard size at 11.684 mm diameter. Understanding AWG sizes is essential for selecting the correct wire for any electrical circuit.

This guide covers the complete AWG size chart from 4/0 to 40 AWG, ampacity ratings per NEC Table 310.16, conductor resistance values, and practical selection guidance for building wire, industrial cable, and low-voltage applications.

What Size Wire Do I Need? Quick Lookup by Breaker Size

This table shows the minimum copper and aluminum wire gauge for standard residential and commercial circuit breaker sizes, based on NEC Table 310.16 at 75°C with standard installation conditions. For dwelling unit services, NEC 310.12 permits smaller conductors using the 83% rule.

*NEC 310.12 allows dwelling unit service and feeder conductors to be sized at 83% of the service rating. Always verify conductor sizing with NEC ampacity tables, voltage drop calculations, and your local authority having jurisdiction (AHJ).

AWG Wire Gauge Size Chart

The AWG system uses a geometric progression where each step down in gauge number increases the conductor diameter by a fixed ratio. The diameter in inches for any solid round conductor can be calculated as: D = 0.005 × 92^{(36−AWG)/39}. Every decrease of 6 AWG roughly doubles the wire diameter, and every decrease of 3 AWG roughly doubles the cross-sectional area.

Values shown are for solid bare copper conductor at 20°C. Stranded conductors have a slightly larger overall diameter than solid conductors of the same gauge due to the air gaps between strands. Resistance values are nominal DC resistance per ASTM B258.

AWG Ampacity Ratings (NEC Table 310.16)

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.16 (formerly Table 310.15(B)(16) in earlier code editions) provides allowable ampacities for insulated conductors rated 0–2000V in raceways, cables, or direct burial, based on an ambient temperature of 30°C with no more than three current-carrying conductors.

These values assume not more than three current-carrying conductors in a raceway, cable, or earth at an ambient temperature of 30°C (86°F). For higher ambient temperatures or bundled conductors, apply the correction and adjustment factors in NEC 310.15(B) and 310.15(C).

NEC Overcurrent Protection Limits

Even when a conductor's ampacity exceeds the circuit breaker size, NEC 240.4(D) limits the overcurrent protection for small conductors: 14 AWG copper is limited to 15A, 12 AWG to 20A, and 10 AWG to 30A. These limits apply regardless of the insulation temperature rating. This means you cannot use 14 AWG copper on a 20A breaker even though the 90°C ampacity column shows 25A.

How Does the AWG Numbering System Work?

The AWG system originated from the number of drawing dies a copper rod had to pass through to reach a given diameter. More drawing passes produced thinner wire and a higher gauge number. This is why AWG numbering runs "backwards"—a larger number means a smaller wire.

The system is based on two fixed reference points: 4/0 AWG (0.4600 inches) and 36 AWG (0.0050 inches), with 39 geometric steps between them. Each step changes the diameter by a factor of 1.1229 (the 39th root of 92), which means:

• Every decrease of 3 AWG sizes roughly doubles the cross-sectional area (and weight per foot)
• Every decrease of 6 AWG sizes roughly doubles the diameter
• Every decrease of 10 AWG sizes multiplies the area by approximately 10

For conductors larger than 4/0 AWG, the industry switches from gauge numbers to kcmil (thousand circular mils), where the size describes the conductor's cross-sectional area directly. Common large sizes include 250 kcmil, 350 kcmil, 500 kcmil, and 750 kcmil.

What Is the Difference Between Solid and Stranded Wire?

Solid wire consists of a single conductor, while stranded wire bundles multiple smaller wires to achieve the same total cross-sectional area. Both carry the same AWG rating and have the same current-carrying capacity, but they differ in flexibility, installation characteristics, and typical applications.

NEC 310.106(C) requires conductors 8 AWG and larger to be stranded unless specifically permitted to be solid. For most building wire applications above 8 AWG, you will use stranded building wire such as THHN or XHHW.

Copper vs. Aluminum Wire: AWG and Ampacity Differences

Copper and aluminum are the two primary conductor materials. Copper has higher conductivity, which means a copper conductor carries more current at the same AWG size than its aluminum equivalent. The general rule is to upsize aluminum by one to two AWG sizes to match the ampacity of copper.

Aluminum wire is lighter and less expensive per foot, making it the standard choice for large feeders, service entrance cable, and utility distribution. For residential branch circuits 10 AWG and smaller, copper is the dominant material due to its smaller diameter, easier termination, and well-established installation practices. Browse our selection of copper building wire for your project.

AWG to Metric (mm²) Conversion

Most of the world uses metric wire sizes measured in square millimeters (mm²) of cross-sectional area, while the U.S. and Canada use AWG. The two systems do not align exactly, so conversions are approximate. This table shows the closest metric equivalent for common AWG sizes.

When ordering cable for international projects or matching imported equipment specifications, use the exact mm² value rather than the "nearest" metric size to ensure the conductor area meets the circuit requirements. IEC 60228 defines the standard metric conductor sizes used in most countries outside North America.

Common AWG Wire Sizes by Application

Selecting the right wire gauge depends on the circuit's current draw, voltage, run length, and installation environment. Here are the most common applications for each standard AWG size.

How Does Wire Gauge Affect Voltage Drop?

Voltage drop is the reduction in voltage along a conductor caused by the wire's resistance. Longer runs and smaller gauges produce higher voltage drop, which can cause equipment malfunction, dimming lights, and motor overheating. The NEC recommends (but does not require) limiting voltage drop to 3% for branch circuits and 5% total for feeders plus branch circuits combined (NEC 210.19(A) Informational Note No. 4).

The voltage drop formula for single-phase circuits is: V_drop = 2 × I × R × L / 1000, where I is the current in amps, R is the conductor resistance in ohms per 1000 feet, and L is the one-way circuit length in feet.

Voltage Drop Example

A 20A circuit on 12 AWG copper running 150 feet one-way: V_drop = 2 × 20 × 1.588 × 150 / 1000 = 9.53V. On a 120V circuit, that is 7.9% voltage drop—well above the 3% recommendation. Upsizing to 10 AWG (0.9989 Ω/1000ft) reduces the drop to 5.99V (5.0%), and 8 AWG (0.6282 Ω/1000ft) brings it to 3.77V (3.1%). For this run length, 8 AWG would be the practical choice to stay within the 3% guideline.

For long runs on commercial and industrial projects, voltage drop calculations often drive wire gauge selection more than ampacity alone. Voltage drop becomes more significant on longer runs—typically over 100 feet—and should always be evaluated to ensure the installation remains within the NEC recommended limits of 3% for branch circuits and 5% total.

When Do You Need to Derate Wire Ampacity?

The ampacity values in NEC Table 310.16 assume ideal conditions: 30°C ambient temperature and no more than three current-carrying conductors in a raceway. When real-world conditions differ, you must derate (reduce) the allowable ampacity.

Temperature Correction (NEC 310.15(B))

When ambient temperature exceeds 30°C, the ampacity must be reduced using correction factors from NEC Table 310.15(B)(1). For example, at 40°C ambient, 75°C-rated conductors are multiplied by 0.88, and 90°C-rated conductors by 0.91. In hot environments like rooftops, attics, or industrial plants, this correction can significantly affect wire sizing.

Conductor Bundling (NEC 310.15(C))

When more than three current-carrying conductors share a raceway or cable, heat dissipation is reduced and ampacity must be adjusted:

When both temperature correction and bundling adjustment apply, multiply the base ampacity by both factors. The 90°C column in Table 310.16 is primarily used as a starting point for derating calculations—even when you must terminate at 75°C equipment, you can use the higher 90°C ampacity as the basis before applying correction and adjustment factors, as long as the final derated value does not exceed the 75°C ampacity (per NEC 110.14(C)).

Why Does Wire Insulation Temperature Rating Matter at Terminations?

NEC 110.14(C) requires that the ampacity used for conductor sizing must not exceed the lowest temperature rating in the circuit—including the termination points (breakers, lugs, receptacles). This is the most commonly misunderstood derating rule.

For circuits rated 100A or less (14 AWG through 1 AWG): terminations are assumed to be rated 60°C unless marked otherwise. For circuits over 100A (1/0 AWG and larger): terminations are assumed to be rated 75°C. This means a 12 AWG THHN conductor (rated 30A at 90°C) connected to a standard 60°C receptacle is limited to the 60°C ampacity of 20A.

Frequently Asked Questions

What AWG wire do I need for a 200 amp service?

For a 200A residential service entrance, use 2/0 AWG copper or 4/0 AWG aluminum. Per NEC Table 310.16 at 75°C, 3/0 copper is rated 200A and 4/0 aluminum is rated 205A. For dwelling unit services, NEC 310.12 allows conductor sizing at 83% of the service rating, which permits 2/0 copper (175A at 75°C) and 4/0 aluminum for 200A services. Verify with your local authority having jurisdiction (AHJ).

What size wire do I need for a 100 amp subpanel?

For a 100A subpanel feeder, use 2 AWG copper (rated 115A at 75°C) or 1/0 AWG aluminum (rated 120A at 75°C). Add a separate equipment grounding conductor sized per NEC Table 250.122.

Can I use 14 AWG wire on a 20 amp breaker?

No. NEC 240.4(D) limits 14 AWG copper to 15A overcurrent protection. Even though 14 AWG THHN is rated for 25A at 90°C, the NEC restricts the maximum breaker size to 15A for 14 AWG conductors. Use 12 AWG for 20A circuits.

What is the difference between AWG and kcmil?

AWG (American Wire Gauge) is a sizing system for conductors up to 4/0. For larger conductors, the industry uses kcmil (thousand circular mils), which directly describes the cross-sectional area. The transition happens at 4/0 AWG (211.6 kcmil), with the next standard size being 250 kcmil.

How do I convert AWG to mm²?

Use the formula: Area (mm²) = 0.012668 × 92^{(36−AWG)/19.5}. For quick reference: 14 AWG = 2.08 mm², 12 AWG = 3.31 mm², 10 AWG = 5.26 mm². The metric sizes do not align exactly with AWG, so always use the precise mm² value when specifying for international standards.

Is thicker wire always better?

Thicker wire reduces voltage drop and runs cooler, but it costs more, is harder to install, and takes up more space in conduit. The goal is to select the smallest wire gauge that safely handles the circuit's current, meets voltage drop requirements, fits the conduit fill limits per NEC Chapter 9, and satisfies the termination temperature rating per NEC 110.14(C).

What wire gauge do I need for an EV charger?

Most Level 2 home EV chargers draw 32A–48A continuously, requiring 40A–60A circuit protection. For a 48A charger (requiring a 60A breaker per NEC 625.41 continuous load rule), most installations use 6 AWG copper THHN or equivalent. However, conductor sizing should always be verified against NEC ampacity tables, termination temperature ratings, and voltage drop for the specific run length. Always size the circuit for 125% of the continuous load per NEC 210.20(A).

Does stranded wire have the same ampacity as solid wire?

Yes. Solid and stranded conductors of the same AWG have the same cross-sectional area and the same ampacity rating. Stranded wire has a slightly larger overall diameter due to air gaps between strands, which may affect conduit fill calculations, but the current-carrying capacity is identical.

Related Resources

• THHN vs THWN Building Wire: Ratings & Applications
• How to Choose the Right Cable for Your Project
• Plenum vs. Riser Cable: CMP, CMR, CL2P & CL3P Ratings Explained
• MV-105 Medium Voltage Cable Overview (5kV–35kV)
• Direct Burial Cable: Types, Depth Requirements & Selection
• How to Read a Cable Print Legend

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Disclaimer: This guide is provided for informational purposes only and is not installation advice. It does not constitute professional electrical, engineering, or code-compliance advice. Installing wire & cable can be dangerous and pose a risk of possible electric shock or other hazards. Building codes, NEC editions, and local amendments change periodically. Always consult a licensed electrician and your local authority having jurisdiction (AHJ) before specifying or installing cable. Ampacity values shown are from NEC Table 310.16 and are subject to correction and adjustment factors. Images are for illustration purposes and may not reflect actual installed products.

The information on this page is provided for general reference only and may contain errors or omissions. NEC® is a registered trademark of the National Fire Protection Association (NFPA®). UL® is a registered trademark of Underwriters Laboratories. All other trademarks, product names, and brand names referenced on this page are the property of their respective owners. Ramcorp Wire & Cable is not affiliated with or endorsed by these organizations unless explicitly stated.