What size wire do I need?

Common residential: 14 AWG copper for 15A circuits, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40-50A, 6 AWG for 55-65A. Always check local electrical codes and add voltage drop calculations for long runs. NEC code is the legal standard for US installations.

What is voltage drop?

Voltage drop occurs as current flows through wire resistance. The NEC recommends keeping voltage drop below 3% for branch circuits and 5% total. Longer runs need larger wire. For 20A at 100 ft on 12 AWG copper: ~4% drop, just over limit — upgrade to 10 AWG.

Why is smaller AWG number a thicker wire?

Historical: AWG started with the number of drawing operations needed to reduce wire to size. Thinner wire required more passes → higher number. 4/0 AWG (very thick) = "0000", three sizes thicker than "0". Counterintuitive but consistent and standardized worldwide for North America.

What's the difference between copper and aluminum wire?

Copper has ~62% lower resistance than aluminum at same gauge. For same ampacity, aluminum needs 2-3 sizes thicker. Copper: better conductor, more expensive. Aluminum: cheaper, lighter, but requires antioxidant grease, listed terminals, and routine inspection due to creep and corrosion.

How is ampacity determined?

NEC tables based on insulation temperature rating (60°C, 75°C, 90°C). Higher rated insulation allows higher ampacity for same wire. Derated for bundling (more than 3 wires in conduit) and ambient temperature. Modern residential typically uses 75°C-rated wire and terminals.

What is the NEC 80%/125% rule?

For continuous loads (3+ hours): the circuit must be sized at 125% of the load. Or equivalently, only 80% of breaker rating may be used continuously. A 20A breaker may carry 16A continuously. For EV chargers (continuous loads), size for 1.25× actual draw — e.g., 40A draw → 50A circuit → 8 AWG wire minimum.

Can I use stranded wire instead of solid?

Yes, both have same ampacity and resistance for same AWG. Stranded is more flexible and used in cords/cables. Solid is rigid and used in static installations (Romex). Stranded uses 7-19 strands typical; very flexible "welding cable" uses fine stranding (Class K/M).

How do I convert AWG to mm²?

Approximate values: 14 AWG ≈ 2.5 mm², 12 AWG ≈ 4 mm², 10 AWG ≈ 6 mm², 8 AWG ≈ 10 mm², 6 AWG ≈ 16 mm², 4 AWG ≈ 25 mm². Europe uses mm² directly; the US uses AWG. Each three-AWG decrease doubles cross-sectional area approximately.

Wire Gauge (AWG) Calculator

Look up American Wire Gauge (AWG) specifications including maximum ampacity, wire diameter, resistance per foot, and cross-sectional area. Useful for electrical wiring projects.

American Wire Gauge (AWG) is the standard system for sizing electrical wires in North America. Smaller AWG numbers indicate thicker wires (counterintuitive but consistent): 14 AWG (typical 15A circuit) is thinner than 6 AWG (typical 60A feed). The standard runs from 40 AWG (~0.08 mm diameter, electronic instruments) down to 4/0 AWG (~11.7 mm, large service entrance cables).

Each three-AWG decrease approximately doubles the cross-sectional area, halves the resistance, and increases the current-carrying capacity (ampacity). The relationship is geometric: doubling area lets you carry roughly √2 times more current at the same voltage drop and ~40% more current at same temperature rise.

Wire selection involves multiple constraints: NEC-required ampacity (current carrying capacity, based on insulation type and temperature), voltage drop (typically must stay under 3% on branch circuits per NEC recommendation), and physical/mechanical considerations (bend radius, conduit fill, weight). Often the binding constraint for long runs is voltage drop, not ampacity.

Material matters: copper has ~62% lower resistance than aluminum at the same gauge, so aluminum requires 2-3 AWG sizes larger for equivalent performance. Aluminum is cheaper and lighter but needs special connections (antioxidant compound, listed terminals) due to oxidation and creep.

Common applications: residential and commercial wiring, automotive electrical, marine systems, audio wiring (speaker cables, power lines), industrial control panels, solar PV installations, and any electrical work involving conductor selection.

Inputs

Wire Gauge (AWG)

Wire Length (feet)

System Voltage (V)

Current Load (A)

Actual current draw for voltage drop calculation

Results

Max Ampacity

20 A

Voltage Drop

4.76 V (4.0%)

Diameter

2.05 mm

Wire Gauge Specifications

Parameter	Value
Wire Gauge	12 AWG
Diameter	80.8 mils (2.053 mm)
Cross-Section Area	3.3089 mm²
Max Ampacity (60°C)	20 A
Resistance per ft	0.001588 Ω/ft
Total Resistance (round trip)	0.3176 Ω
Voltage Drop	4.764 V (3.97%)
Voltage Drop Status	Caution (3-5%)
Voltage at Load	115.24 V
Power Lost in Wire	71.46 W

Last updated: May 29, 2026

Formula

**AWG to diameter conversion:** d (in inches) = 0.005 × 92^((36 - AWG) / 39) d (in mm) = 0.127 × 92^((36 - AWG) / 39) **AWG to cross-sectional area:** A (in kcmil) = (d/0.001)² (where d is in inches) A (in mm²) = π/4 × d² **Resistance per length (copper, 20°C):** R (Ω/kft) = 10.371 / A (in kcmil) Or: R (Ω/m) = 0.017 × 10⁻⁶ / A (in m²) Where 0.017 × 10⁻⁶ Ω·m is copper's resistivity. **Cross-sectional area scaling:** A doubles with every 3 AWG decrease: - 14 AWG: 2.08 mm² - 11 AWG: 4.17 mm² - 8 AWG: 8.37 mm² - 5 AWG: 16.8 mm² **Worked example: 12 AWG copper** Diameter: 0.127 × 92^((36-12)/39) = 0.127 × 92^0.615 = 0.127 × 16.27 ≈ 2.05 mm. Actual: 2.05 mm = 0.0808 inches. Area: π/4 × (2.05)² = 3.31 mm² = 6,530 cmil. Resistance: 1.588 Ω/kft = 5.21 mΩ/m. NEC ampacity (75°C): 20 A. **AWG reference table (copper):** | AWG | Diameter (mm) | Area (mm²) | Ω/kft | Ω/km | Ampacity (75°C) | |---|---|---|---|---|---| | 4/0 | 11.68 | 107.2 | 0.049 | 0.161 | 230 A | | 3/0 | 10.40 | 85.0 | 0.0618 | 0.203 | 200 A | | 2/0 | 9.27 | 67.4 | 0.0779 | 0.255 | 175 A | | 1/0 | 8.25 | 53.5 | 0.0983 | 0.323 | 150 A | | 1 | 7.35 | 42.4 | 0.124 | 0.406 | 130 A | | 2 | 6.54 | 33.6 | 0.156 | 0.513 | 115 A | | 4 | 5.19 | 21.2 | 0.249 | 0.815 | 85 A | | 6 | 4.11 | 13.3 | 0.395 | 1.296 | 65 A | | 8 | 3.26 | 8.37 | 0.628 | 2.061 | 50 A | | 10 | 2.59 | 5.26 | 0.999 | 3.277 | 35 A | | 12 | 2.05 | 3.31 | 1.588 | 5.211 | 25 A | | 14 | 1.63 | 2.08 | 2.525 | 8.286 | 20 A | | 16 | 1.29 | 1.31 | 4.016 | 13.18 | (not for power) | | 18 | 1.02 | 0.82 | 6.385 | 20.95 | (low-V signal) | | 20 | 0.812 | 0.518 | 10.15 | 33.31 | (signal) | | 22 | 0.644 | 0.326 | 16.14 | 52.96 | (signal) | | 24 | 0.511 | 0.205 | 25.67 | 84.21 | (electronics) | | 26 | 0.405 | 0.129 | 40.81 | 133.9 | (very small) | | 28 | 0.321 | 0.081 | 64.90 | 212.9 | (tiny electronics) | | 30 | 0.254 | 0.0509 | 103.2 | 338.6 | (PCB wire wrap) | **NEC ampacity (60°C / 75°C / 90°C insulation):** | AWG | 60°C | 75°C | 90°C | |---|---|---|---| | 14 | 15 | 20 | 25 | | 12 | 20 | 25 | 30 | | 10 | 30 | 35 | 40 | | 8 | 40 | 50 | 55 | | 6 | 55 | 65 | 75 | | 4 | 70 | 85 | 95 | | 2 | 95 | 115 | 130 | | 1/0 | 125 | 150 | 170 | | 4/0 | 195 | 230 | 260 | Most modern residential wiring uses 75°C-rated terminals (NM-B Romex). **Common residential circuits:** | Circuit | AWG | Breaker | Use | |---|---|---|---| | 14 AWG | 15 A | Lighting, small outlet | | 12 AWG | 20 A | Kitchen outlets, bathroom | | 10 AWG | 30 A | Dryer, electric water heater | | 8 AWG | 40-50 A | Range, electric heater | | 6 AWG | 50-65 A | EV charger, subpanel | | 4 AWG | 70-85 A | Larger subpanel | | 2 AWG | 100-115 A | Whole house in some cases | | 1/0 AWG | 125-150 A | 100A main feeder | | 4/0 AWG | 195-230 A | 200A main feeder | **Aluminum wire:** Aluminum requires bigger gauge for same ampacity: - 12 AWG Cu = 10 AWG Al (for 20A) - 6 AWG Cu = 4 AWG Al (for 60A) - 1/0 AWG Cu = 2/0 AWG Al (for 150A) Modern aluminum (AA-8000 series) used commonly for service entrance and subpanel feeders. **Special considerations:** - **Bundling derating**: 3+ wires in conduit reduces ampacity (NEC table). - **Ambient temperature derating**: hot environments reduce ampacity. - **Continuous loads**: NEC requires sizing 125% of continuous load. - **Voltage drop**: long runs may require upsizing beyond ampacity. **Stranded vs solid:** Both have same gauge designation (same total copper area). - **Solid**: rigid, easier to terminate, used in residential. - **Stranded**: flexible, used in cords, panels, automotive. Same AWG = same ampacity and resistance. **Specialty wire types:** | Type | Use | Notes | |---|---|---| | THHN/THWN | Building wire | Heat/water resistant | | Romex (NM-B) | Residential branch | Cable assembly | | MC | Metal-clad | Commercial/industrial | | UF | Underground | Direct burial | | TFFN | Fixture/appliance | Flexible | | SE / SER | Service entrance | Aluminum common | **Common applications:** - **Residential**: standard branch circuits, lighting, outlets. - **Commercial**: larger feeders, machinery. - **Industrial**: motor starters, control panels. - **Marine**: tin-coated for corrosion resistance. - **Automotive**: stranded for vibration. - **Audio**: speaker wire (typically 14-16 AWG for moderate runs). - **Solar PV**: PV wire and combiner cables. - **Communications**: data cabling (CAT5-8 use ~24 AWG inside). **Conduit fill:** NEC limits number of wires per conduit (typically 40% fill). - 1/2" EMT: 9 × 12 AWG, 6 × 10 AWG, ... - 3/4" EMT: 16 × 12, 11 × 10, ... Each insulation type has different effective diameter. **Wire gauge conversion:** AWG to mm² approximate: | AWG | mm² | (rounded) | |---|---|---| | 18 | 0.82 | 1.0 | | 16 | 1.31 | 1.5 | | 14 | 2.08 | 2.5 | | 12 | 3.31 | 4.0 | | 10 | 5.26 | 6.0 | | 8 | 8.37 | 10 | | 6 | 13.3 | 16 | | 4 | 21.2 | 25 | | 2 | 33.6 | 35 | Europe uses mm² directly; US uses AWG. International standards (IEC) tend toward mm². **Resistance temperature coefficient:** R changes with temperature: ~0.4%/°C for copper. At 75°C (typical operating): R is ~22% higher than 20°C value. NEC tables typically use 75°C resistance. **Voltage drop calculation:** For a run of length L (one-way, ft) at current I (A): V_drop = 2 × L × R_per_kft × I / 1000 Round-trip factor of 2 because current flows out AND back.

How to use this calculator

Select AWG (smaller number = thicker wire).
Enter wire length in feet (one-way).
Enter system voltage and current load.
Calculator returns diameter, area, resistance, ampacity, and voltage drop.
For aluminum, expect 2-3 AWG sizes larger needed for equivalent performance.
Always check NEC and local codes for legal compliance.

Worked examples

Standard 20A circuit

**Scenario:** 20A kitchen outlet circuit, 50 ft from panel using 12 AWG copper. **Calculation:** R_total = 2 × 50 × 0.00159 = 0.159 Ω. V_drop = 20 × 0.159 = 3.18 V (2.65%). Diameter: 2.05 mm. NEC ampacity: 20A (75°C) — exactly at limit. **Result:** Wire is at NEC ampacity limit but voltage drop is acceptable (<3%). For longer runs, upgrade to 10 AWG. For dedicated 20A continuous (5+ hours), NEC requires 25A wire = 10 AWG. Code rules become controlling factor often.

EV charger installation

**Scenario:** 50A EV charger circuit, 80 ft run, 240V split-phase. Wire size? **Calculation:** Need ≥ 50A ampacity. 8 AWG copper rated 50A (75°C) — exactly meets. R_total = 2 × 80 × 0.000628 = 0.100 Ω. V_drop = 50 × 0.100 = 5.0 V (2.08%). Within NEC. **Result:** 8 AWG copper minimum. For continuous EV charging, NEC requires 125% sizing = 62.5A wire = 6 AWG. Use 6 AWG for safety margin. Aluminum equivalent: 4 AWG. Always derate for continuous loads exceeding 3 hours.

Speaker wire

**Scenario:** 8 ohm speakers at 30 ft from amp. Wire gauge? **Calculation:** For < 5% loss, R_wire / R_speaker < 0.05. Need R_wire < 0.4 Ω total (200 round-trip ft). 16 AWG = 4 × 0.004 = 0.24 Ω (acceptable). 14 AWG = 0.15 Ω (better). **Result:** 16 AWG OK for typical home audio. 14 AWG for slightly better signal integrity. 12 AWG for very long runs (50+ ft) or premium installs. Below 16 AWG, signal degradation becomes audible. Solid or stranded both work; insulation matters less than gauge.

When to use this calculator

**Use AWG sizing for:**

- **Electrical wiring**: residential, commercial, industrial. - **Automotive wiring**: 12V/24V systems. - **Marine electrical**: corrosion-rated cables. - **Audio systems**: speaker wire, signal wire. - **Electronics projects**: hookup wire, PCB connections. - **Solar PV**: combiner box to inverter runs. - **Cordsets**: power cords for appliances.

**Selection process:**

1. **Determine current load**: appliance ratings or breaker size. 2. **Find NEC ampacity** for that wire size and insulation type. 3. **Calculate voltage drop** at planned length. 4. **Choose larger** of ampacity-required or VD-required size. 5. **Verify code compliance** (NEC, local).

**Common pitfalls:**

- **Smaller AWG number = thicker wire** (counterintuitive). - **Aluminum needs 2-3 sizes larger** than copper. - **Continuous loads** (>3 hrs): size at 125% of load per NEC. - **Don't oversize** unnecessarily — cost + bend radius issues. - **Temperature derating**: hot conduit or high ambient reduces capacity.

**Mechanical limits:**

Smaller wire = more flexible but lower current. Larger wire = stiffer, requires bigger bend radius: - 1/4" diameter wire: ~1.5" bend radius minimum. - 1" diameter cable: 8-10" bend radius.

**Common applications:**

- **Lighting**: 14 AWG copper, 15A breaker. - **Outlets**: 12 AWG copper, 20A breaker. - **Dryer**: 10 AWG copper, 30A breaker. - **Range/Oven**: 6-8 AWG copper, 40-50A. - **EV Charger**: 6-8 AWG, 40-60A. - **Subpanel (60A)**: 6 AWG copper. - **Main service**: 2/0-4/0 AWG copper or aluminum.

**Stranded vs solid:**

- **Solid**: rigid, easier termination, used in static installations. - **Stranded**: flexible, used where movement occurs.

| Stranding | Use | |---|---| | Solid | Residential building wire | | 7-strand | Most automotive, audio | | 19-strand | Welding cables, audio | | Class K/M (very fine) | Very flexible cords |

Same AWG = same ampacity regardless of strand count.

**Color codes (US convention):**

| Color | Use | |---|---| | Black | Hot (phase) | | Red | Hot (second phase) | | White | Neutral | | Green | Ground (safety) | | Bare copper | Ground (NM cable) | | Blue | Hot (3-phase, optional) |

International conventions differ. Always verify with local code.

**Software:**

- **Voltage drop calculators**: online or app. - **NEC chapter 9 tables**: official ampacity. - **Electrical CAD**: AutoCAD Electrical, SmartDraw. - **PowerCAD**: commercial design.

**Pitfalls:**

- **Smaller AWG = thicker wire** (confusing). - **Aluminum requires bigger gauge** than copper for same ampacity. - **Continuous loads need oversizing** (NEC 80% rule). - **Forgetting voltage drop on long runs** (often dictates wire size, not ampacity). - **Mixing copper/aluminum** without proper connectors (corrosion). - **Underestimating motor starting current** (5-7× running). - **Temperature derating** for hot conduits or ambient.

Common mistakes to avoid

Confusing AWG numbering (smaller number = thicker wire).
Using aluminum gauge tables for copper installations (or vice versa).
Sizing wire for ampacity only, ignoring voltage drop on long runs.
Forgetting continuous load derating (NEC 125% rule).
Connecting copper to aluminum without listed connectors.
Ignoring conduit fill limits.
Using wrong insulation temperature rating.
Forgetting wire de-rating for high ambient temperatures.

Frequently Asked Questions

Sources & further reading

SponsoredShop Top Deals on AmazonSupport CalcMountain — browse top-rated products at no extra cost to you.