Selecting the correct wire gauge is one of the most critical decisions in wire harness design. Undersized wire causes overheating, voltage drop, and potential fire hazards. Oversized wire wastes money, adds weight, and complicates routing. This guide provides everything you need to make the right choice for any application.
Whether you're designing automotive harnesses, industrial control cables, or custom electronics, the principles are the same: match wire capacity to current requirements, account for voltage drop over distance, and apply appropriate safety margins. Let's break down each factor.
Understanding AWG (American Wire Gauge)
The American Wire Gauge (AWG) system, established in 1857, is the standard for wire sizing in North America. The key thing to remember: AWG works inversely—smaller numbers mean thicker wire. A 10 AWG wire is much thicker than a 22 AWG wire.
AWG Quick Rules
- For every 6 gauge decrease, wire diameter doubles
- For every 3 gauge decrease, cross-sectional area doubles
- For every 3 gauge decrease, resistance halves
AWG sizes range from 40 (smallest, 0.08mm diameter) to 0000 or "4/0" (largest, 11.68mm diameter). For wire harness applications, you'll typically work with gauges between 24 AWG (signal wires) and 8 AWG (high-current power feeds).
| AWG | Diameter (mm) | Area (mm²) | Resistance (Ω/km) | Common Use |
|---|---|---|---|---|
| 24 AWG | 0.51mm | 0.20mm² | 84.2 | Signal, data cables |
| 22 AWG | 0.64mm | 0.33mm² | 53.0 | Sensors, switches |
| 20 AWG | 0.81mm | 0.52mm² | 33.3 | Low-power circuits |
| 18 AWG | 1.02mm | 0.82mm² | 21.0 | Ignition, LED lighting |
| 16 AWG | 1.29mm | 1.31mm² | 13.2 | Accessories, relays |
| 14 AWG | 1.63mm | 2.08mm² | 8.3 | Fuel pumps, fans |
| 12 AWG | 2.05mm | 3.31mm² | 5.2 | Main power feeds |
| 10 AWG | 2.59mm | 5.26mm² | 3.3 | Alternator, starter |
| 8 AWG | 3.26mm | 8.37mm² | 2.1 | Battery cables |
Wire Ampacity Chart
Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. Thicker wire has lower resistance, allowing more current flow without excessive heating. The chart below shows typical ampacity ratings for copper wire.
Important: Derating Factors
Free-air ratings assume a single conductor with unrestricted cooling. For bundled wires in harnesses, derate by 40-60%. High ambient temperatures require additional derating. Always apply a 1.25x safety factor for continuous loads.
| AWG | Free Air (A) | Chassis (A) | Bundled (A) | Safe Continuous (A) |
|---|---|---|---|---|
| 24 AWG | 3.5A | 2.0A | 1.4A | 1.0A |
| 22 AWG | 7A | 5A | 3A | 2.5A |
| 20 AWG | 11A | 7.5A | 5A | 4A |
| 18 AWG | 16A | 10A | 7A | 5.5A |
| 16 AWG | 22A | 13A | 10A | 8A |
| 14 AWG | 32A | 17A | 15A | 12A |
| 12 AWG | 41A | 23A | 20A | 16A |
| 10 AWG | 55A | 33A | 28A | 22A |
| 8 AWG | 73A | 46A | 40A | 32A |
| 6 AWG | 101A | 60A | 55A | 44A |
Values for copper conductor with 105°C insulation at 30°C ambient. Safe Continuous includes 1.25x safety factor for bundled wires.
Temperature Derating
For ambient temperatures above 30°C, reduce ampacity according to insulation type:
60°C Insulation (PVC)
- 40°C ambient: 82%
- 50°C ambient: 58%
- 60°C ambient: 0%
90°C Insulation (XLPE)
- 40°C ambient: 91%
- 50°C ambient: 82%
- 60°C ambient: 71%
150°C Insulation (Silicone)
- 40°C ambient: 96%
- 50°C ambient: 91%
- 60°C ambient: 87%
See our Silicone vs PVC vs XLPE guide for detailed insulation comparisons.
Voltage Drop Calculations
Voltage drop becomes critical in long wire runs. Even if a wire can handle the current thermally, resistance over distance reduces voltage at the load. This can cause motors to run slowly, lights to dim, and electronics to malfunction.
Voltage Drop Formula
Vdrop = Voltage drop (Volts)
L = One-way wire length (feet)
I = Current (Amps)
R = Resistance per 1000ft (Ω/kft)
Acceptable Voltage Drop
- <3% for critical circuits (ECUs, sensors)
- <5% for general loads (motors, accessories)
- <10% for non-critical loads (interior lights)
12V System Example
- 3% drop = 0.36V max (11.64V at load)
- 5% drop = 0.60V max (11.40V at load)
- 10% drop = 1.20V max (10.80V at load)
For 24V systems, double these values
| AWG | Max Distance (feet) for 3% Drop at Current | ||||
|---|---|---|---|---|---|
| 5A | 10A | 15A | 20A | 30A | |
| 18 AWG | 8.6ft | 4.3ft | - | - | - |
| 16 AWG | 13.7ft | 6.8ft | 4.6ft | - | - |
| 14 AWG | 21.7ft | 10.9ft | 7.2ft | 5.4ft | - |
| 12 AWG | 34.5ft | 17.2ft | 11.5ft | 8.6ft | 5.7ft |
| 10 AWG | 54.8ft | 27.4ft | 18.3ft | 13.7ft | 9.1ft |
| 8 AWG | 86.8ft | 43.4ft | 28.9ft | 21.7ft | 14.5ft |
Values for 12V DC copper wire. For 24V systems, double the distances. Red cells indicate wire is undersized for current even at short distances.
Automotive Wire Sizing
Automotive applications have specific requirements per ISO 6722 and SAE J1128 standards. Most passenger vehicles use wires between 0.22mm² and 50mm², with current limits ranging from 3A to 150A depending on insulation type.
Automotive Wire Selection by Load
| Application | Typical Current | Recommended AWG | Wire Type |
|---|---|---|---|
| Sensors & switches | 0.5-2A | 22 AWG | TXL, GXL |
| Fuel injectors | 1-3A | 22 AWG | TXL |
| Ignition coils | 3-5A | 18 AWG | GXL, TXL |
| LED lighting | 2-5A | 18-16 AWG | TXL, SXL |
| Halogen headlights | 10-15A | 14-12 AWG | GXL |
| Fuel pump | 8-15A | 14 AWG | GXL, SXL |
| Cooling fans | 15-30A | 12-10 AWG | SXL |
| Starter motor | 150-400A | 4-1 AWG | Battery cable |
For more on automotive harness requirements, see our Automotive Wire Harness Cost Breakdown.
Industrial Wire Sizing
Industrial applications often involve longer distances, higher ambient temperatures, and more demanding environmental conditions than automotive. Per IPC/WHMA-A-620, conductors 22 AWG and larger should be high-conductivity copper, while 24 AWG and smaller may use high-strength copper alloy for flex life.
Control Panel Wiring
- Signal wires: 22-24 AWG stranded
- Control circuits: 18-16 AWG
- Motor power: Per NEC ampacity tables
- Min bend radius: 4x cable OD
High-Voltage Applications
- EV battery cables: Per ISO 19642
- Min signal wire: 0.35mm² (22 AWG)
- Starter motors: 6.0mm² minimum
- Insulation: 600V withstand required
For mining and harsh environment applications, see our guide on Wire Harness for Mining: IP69K Requirements.
Wire Gauge Selection Process
Follow this systematic process to select the correct wire gauge for any application. The goal is to find the smallest (most cost-effective) wire that meets all requirements.
Determine Maximum Current
Calculate the maximum current the circuit will draw. For motors, use locked-rotor current. For resistive loads, use steady-state current. Add 25% safety margin.
Select Initial Gauge by Ampacity
Using the ampacity chart, find the smallest gauge with adequate current capacity. Use bundled ratings for harnesses, free-air for single conductors.
Calculate Voltage Drop
For the selected gauge, calculate voltage drop at maximum current and wire length. If drop exceeds limits (<3% critical, <5% general), upsize the wire.
Verify Environmental Compatibility
Ensure the wire and insulation type suit the environment: temperature range, chemical exposure, UV exposure, flex requirements, and abrasion resistance.
- • Engine compartment: -40°C to +125°C, oil resistant
- • Interior: -40°C to +85°C, low smoke
- • Outdoor: UV resistant, wide temperature range
Final Verification
The final wire gauge is the larger of the ampacity selection and voltage drop selection. Document your calculations for traceability.
“Wire gauge selection is where theory meets reality. I've seen designs fail because engineers calculated ampacity perfectly but forgot about voltage drop in a 5-meter run. Always calculate both, always add safety margin, and when in doubt, go one gauge larger—the cost difference is minimal compared to a field failure.”
Frequently Asked Questions
How do I select the right wire gauge for my application?
Select wire gauge based on three factors: 1) Current capacity—wire must handle maximum current plus 25% safety margin, 2) Voltage drop—keep under 3% for critical circuits, 5% for general applications, 3) Environmental conditions—temperature, vibration, and chemical exposure may require upsizing or special insulation.
What is the ampacity of 14 AWG wire?
14 AWG copper wire has an ampacity of 15-32 amps depending on insulation type and installation method. In free air with good ventilation, it can handle up to 32A. In bundled harnesses with restricted cooling, derate to 12-15A for continuous use. Always verify against specific wire specifications.
How do I calculate voltage drop in a wire?
Use the formula: Vdrop = (2 × Length × Current × Resistance) / 1000, where length is one-way distance in feet and resistance is ohms per 1000 feet. For 12V systems, keep total drop under 0.36V (3%) for critical circuits or 0.6V (5%) for general applications.
What is the difference between AWG and metric wire sizes?
AWG (American Wire Gauge) is used in North America and works inversely—smaller numbers mean thicker wire. Metric sizes express cross-sectional area in mm². Common conversions: 18 AWG ≈ 0.75mm², 16 AWG ≈ 1.0mm², 14 AWG ≈ 2.0mm², 12 AWG ≈ 3.3mm².
Can I use aluminum wire instead of copper to save cost?
Aluminum wire costs about 30% less than copper but has 61% of copper's conductivity. This means you need a larger gauge (typically 2 sizes up) for equivalent current capacity. Aluminum also requires special termination techniques. It's suitable for long runs and non-critical circuits, but copper is preferred for most harness applications. See our cost-saving tips for more guidance.
Sources & References
- Engineering Toolbox. "AWG Wire Gauge Sizes." engineeringtoolbox.com
- PowerStream. "Wire Size Chart and Current Load Limits." powerstream.com
- SCONDAR. "Standards for Wire Selection in the Wire Harness Industry." scondar.com
- Southwire. "Voltage Drop Calculator." southwire.com
- ISO 6722:2006 "Road vehicles — 60 V and 600 V single-core cables."
- IPC/WHMA-A-620D. "Requirements and Acceptance for Cable and Wire Harness Assemblies."