Wire Gauge Calculator Guide: Wire Sizing Explained (2026)
Safety Disclaimer: Electrical wiring must comply with local codes and the National Electrical Code (NEC). Improper wire sizing is a fire hazard. Always consult a licensed electrician for electrical work. This guide is for educational purposes only.
Quick Answer
- *AWG is counterintuitive: smaller gauge number = thicker wire.
- *Common residential: 14 AWG (15A), 12 AWG (20A), 10 AWG (30A).
- *Keep voltage drop under 3% per NEC recommendation.
- *Longer runs require larger gauge wire to compensate for resistance.
The AWG System
American Wire Gauge is the standard wire sizing system in North America. The numbering is backwards from what you'd expect — a lower number means a thicker wire. AWG 4 is much thicker than AWG 14. The system dates to the number of drawing dies the wire passed through during manufacturing: more draws = thinner wire = higher number.
Each 3-gauge step doubles the cross-sectional area and halves the resistance. Each 6-gauge step doubles the diameter.
Common Wire Sizes and Ampacity
| AWG | Diameter (mm) | Ampacity (Cu, 60°C) | Typical Use |
|---|---|---|---|
| 14 | 1.63 | 15A | Lighting, general outlets |
| 12 | 2.05 | 20A | Kitchen, bathroom, garage |
| 10 | 2.59 | 30A | Dryer, water heater |
| 8 | 3.26 | 40A | Range, large appliances |
| 6 | 4.11 | 55A | Sub-panels, large A/C |
| 4 | 5.19 | 70A | Service entrance |
| 2 | 6.54 | 95A | Service entrance |
Voltage Drop Calculation
Vdrop = (2 × L × I × R) / 1000
Where L = one-way length (feet), I = current (amps), R = resistance per 1000 ft. The factor of 2 accounts for the round trip (hot and neutral/ground).
Example
A 20A circuit with 12 AWG copper (R = 1.588 Ω/1000ft) running 75 feet:
- Vdrop = (2 × 75 × 20 × 1.588) / 1000 = 4.76V
- On a 120V circuit: 4.76/120 = 3.97% — exceeds the 3% recommendation
- Solution: upsize to 10 AWG (R = 0.999 Ω/1000ft): drop = 3.00V = 2.5% — acceptable
Copper vs. Aluminum
Copper conducts 61% better than aluminum, so aluminum wire must be 1–2 gauge sizes larger. Aluminum is cheaper per amp of capacity and lighter, making it common for service entrance cables and long feeder runs. But aluminum expands more with heat, requiring special connectors and anti-oxidant paste to prevent loose connections and fire hazards.
NEC Code Essentials
- Wire ampacity ratings depend on insulation temperature rating (60°C, 75°C, or 90°C)
- Conductor fill limits for conduit (NEC Table 310.16)
- Derating required when more than 3 current-carrying conductors share a conduit
- Ground wire must be sized per NEC Table 250.122 based on circuit breaker size
Calculate the right wire gauge for your circuit
Try the Free Wire Gauge Calculator →Frequently Asked Questions
What wire gauge do I need for a 20-amp circuit?
12 AWG copper minimum per NEC. For runs over 50 feet, consider upsizing to 10 AWG to minimize voltage drop.
How does the AWG numbering system work?
Smaller numbers = larger wire. Each 3-gauge decrease doubles the cross-sectional area. Common: 14 AWG (15A), 12 AWG (20A), 10 AWG (30A).
What is voltage drop and why does it matter?
Voltage reduction as current flows through resistance. NEC recommends ≤3% for branches and ≤5% total. Excessive drop causes dimming, overheating, and malfunction.
What is the difference between copper and aluminum wire?
Copper conducts 61% better. Aluminum must be 1–2 gauges larger for the same current but costs less. Aluminum requires special connectors and anti-oxidant compound.
What wire gauge is used for household wiring?
14 AWG for 15A circuits, 12 AWG for 20A, 10 AWG for 30A, 8/6 AWG for 40–50A. Service entrance uses 2/0 or 4/0 aluminum for 100–200A service.