Selecting the right wire gauge is one of the most critical decisions in harness design. Too small, and you risk overheating, voltage drop, and potential fire hazards. Too large, and you're wasting money on unnecessary copper while adding weight and bulk to your assembly.
This guide covers everything harness engineers need to know about wire sizing—from basic AWG fundamentals to advanced calculations for special applications. Whether you're designing automotive harnesses or industrial control cables, these principles apply.
Understanding AWG (American Wire Gauge)
The American Wire Gauge (AWG) system is counterintuitive at first: smaller numbers mean larger wires. A 4 AWG wire is much thicker than a 22 AWG wire. This dating back to the wire-drawing process—larger gauge numbers represent more drawing steps, resulting in thinner wire.
AWG Quick Facts
Every 3 AWG
Doubles/halves cross-sectional area
Every 6 AWG
Doubles/halves resistance
Range
0000 (4/0) largest to 40 AWG smallest
Common Harness Range
8 AWG to 24 AWG most typical
Outside North America, you'll encounter metric wire sizing (mm²). A quick conversion: 18 AWG ≈ 0.75 mm², 16 AWG ≈ 1.3 mm², 14 AWG ≈ 2.0 mm². Our wire gauge calculator handles both systems automatically.
AWG Reference Chart
Print this chart and keep it at your design station. These are the essential specifications for common wire gauges:
| AWG | Diameter (mm) | Area (mm²) | Resistance (Ω/km) | Weight (kg/km) | Typical Use |
|---|---|---|---|---|---|
| 4/0 (0000) | 11.68 | 107.2 | 0.161 | 953 | Main battery cables |
| 2 | 6.54 | 33.6 | 0.513 | 299 | High-current feeders |
| 4 | 5.19 | 21.2 | 0.815 | 188 | Alternator, starter |
| 8 | 3.26 | 8.37 | 2.06 | 74 | Motor power, HVAC |
| 10 | 2.59 | 5.26 | 3.28 | 47 | Heavy loads, 30A circuits |
| 14 | 1.63 | 2.08 | 8.28 | 18.5 | General power, 15A |
| 16 | 1.29 | 1.31 | 13.2 | 11.6 | Lighting, switches |
| 18 | 1.02 | 0.82 | 21.0 | 7.3 | Low-power accessories |
| 20 | 0.81 | 0.52 | 33.3 | 4.6 | Sensors, signals |
| 22 | 0.64 | 0.33 | 53.0 | 2.9 | Data, control signals |
| 24 | 0.51 | 0.20 | 84.2 | 1.8 | Telecom, electronics |
* Highlighted rows (14-18 AWG) are the most commonly used in automotive and industrial harnesses
Current Carrying Capacity (Ampacity)
Ampacity—the maximum current a wire can safely carry—depends on multiple factors: conductor material, insulation type, ambient temperature, and whether the wire is bundled with others. The following table shows conservative ratings for chassis wiring:
| AWG | Copper (Amps) | Aluminum (Amps) | ||
|---|---|---|---|---|
| Chassis | Bundled | Chassis | Bundled | |
| 4 | 135 | 80 | 105 | 62 |
| 8 | 73 | 46 | 57 | 36 |
| 10 | 55 | 33 | 43 | 26 |
| 14 | 32 | 17 | 25 | 13 |
| 16 | 22 | 13 | 17 | 10 |
| 18 | 16 | 10 | - | - |
| 20 | 11 | 7.5 | - | - |
| 22 | 7 | 5 | - | - |
Important: Derating Factors
These ratings assume 30°C ambient and 80°C insulation. For higher temperatures or tightly bundled harnesses, apply derating factors. A 10-wire bundle at 40°C ambient might only carry 50% of the chassis rating safely.
Voltage Drop Calculations
Voltage drop is often the limiting factor in wire sizing—especially for long runs in vehicles and machinery. Most applications require less than 3% voltage drop (0.36V on a 12V system, 1.44V on 48V).
Voltage Drop Formula
Vdrop = Voltage drop (V)
L = One-way length (m)
I = Current (A)
R = Resistance (Ω/km from table)
"I've seen so many headlight harnesses sized purely for ampacity. Sure, 16 AWG handles 10 amps safely, but on a 3-meter run, you're looking at 0.8V drop—that's 7% on a 12V system. Your customer's headlights are noticeably dimmer. Size for voltage drop first, then verify ampacity."
Hommer Zhao
Harness Design Engineer
Quick Reference: Maximum Wire Length for 3% Drop (12V System)
| AWG | 5A | 10A | 15A | 20A | 30A |
|---|---|---|---|---|---|
| 10 | 10.9 m | 5.5 m | 3.6 m | 2.7 m | 1.8 m |
| 12 | 6.9 m | 3.4 m | 2.3 m | 1.7 m | 1.1 m |
| 14 | 4.3 m | 2.2 m | 1.4 m | 1.1 m | - |
| 16 | 2.7 m | 1.4 m | 0.9 m | - | - |
| 18 | 1.7 m | 0.9 m | - | - | - |
Use our voltage drop calculator for precise calculations with your specific parameters.
Conductor Materials: Copper vs Aluminum
Copper is the default choice for most harness applications, but aluminum has its place—particularly in weight-sensitive or cost-constrained designs. Here's how they compare:
| Property | Copper | Aluminum | Winner |
|---|---|---|---|
| Conductivity | 100% (reference) | 61% | Copper |
| Weight | 8.96 g/cm³ | 2.70 g/cm³ | Aluminum (3.3× lighter) |
| Cost per kg | ~$8-10 | ~$2-3 | Aluminum |
| Flexibility | Excellent | Good (work hardens) | Copper |
| Corrosion | Good | Forms oxide layer | Copper |
| Termination | Standard crimping | Requires special terminals | Copper |
Aluminum wiring is increasingly used in EV applications where weight reduction directly impacts vehicle range. However, it requires careful design attention—aluminum connections must use compatible terminals and anti-oxidation compounds.
Insulation Types & Temperature Ratings
The insulation material determines where your wire can be used. Engine compartment wiring needs high-temp insulation; general interior wiring doesn't. Choosing the right insulation balances cost, flexibility, and environmental resistance.
| Insulation Type | Temp Range | Properties | Common Applications |
|---|---|---|---|
| PVC | -20°C to +80°C | Economical, flexible, good abrasion resistance | General purpose, interior |
| XLPE (Cross-linked PE) | -40°C to +125°C | Heat resistant, thin wall, lightweight | Automotive, under-hood |
| Silicone | -60°C to +200°C | Extreme flexibility, high temp, UV resistant | Aerospace, medical, engine bay |
| PTFE (Teflon) | -200°C to +260°C | Chemical resistant, low friction, thin wall | Aerospace, mil-spec |
| TPE | -40°C to +105°C | Oil resistant, flexible, recyclable | Robotics, industrial automation |
| FEP | -65°C to +200°C | Similar to PTFE, better for thin walls | Medical, data cables |
For waterproof applications, insulation material matters less than the connector sealing—but you still need insulation rated for the temperature and chemical exposure of your environment.
Wire Selection by Application
Different industries have established conventions and standards for wire selection. Here's a quick guide to typical choices:
Automotive (12V DC)
- Power: 10-14 AWG (headlights, motors)
- Accessories: 16-18 AWG (switches, lights)
- Signals: 20-22 AWG (sensors, CAN bus)
- Insulation: XLPE or TXL (thin wall)
Industrial Control
- Motor power: 8-14 AWG per NEC
- Control circuits: 16-18 AWG
- Signal wiring: 20-24 AWG (shielded)
- Insulation: PVC or XLPE (oil resistant)
Medical Devices
- Power: 18-22 AWG (low current)
- Signals: 24-28 AWG (sensors)
- Insulation: Silicone, FEP (sterilizable)
- Standards: UL 2556, IEC 60601
Aerospace
- MIL-W-22759/16 typical
- Sizes: 20-24 AWG (signals)
- Insulation: PTFE, Kapton
- Nickel-plated copper preferred
Common Mistakes to Avoid
After reviewing thousands of harness designs, these are the most frequent wire sizing errors we encounter:
Ignoring bundle derating
Apply 50-70% derating for bundled wires. A wire rated for 20A in free air might only safely carry 12A in a tight bundle.
Sizing only for ampacity
Calculate voltage drop first, especially for runs over 2 meters. You might need a larger gauge than ampacity alone suggests.
Using chassis ratings for enclosed spaces
Engine compartments and enclosed panels trap heat. Use bundled ratings or conservative derating factors.
Matching wire to fuse size exactly
Wire should handle more than the fuse rating to ensure the fuse blows before the wire overheats. Size wire for 125% of fuse rating minimum.
Forgetting temperature rise
Current flow generates heat. A wire at 80% ampacity might raise 15-20°C above ambient—factor this into insulation selection.
"The best wire size is often one gauge larger than your calculations suggest. That extra 20% capacity gives you margin for manufacturing variations, aging, and the inevitable scope creep when someone decides to add 'just one more' accessory to the circuit."
Hommer Zhao
Harness Design Engineer
Frequently Asked Questions
What wire gauge for LED lights?
Most LED lights draw 1-5 amps, so 18-20 AWG is typically sufficient. However, always check the actual current draw and calculate voltage drop for longer runs—LEDs are sensitive to voltage variations.
Can I mix wire gauges in a harness?
Absolutely. Use appropriate sizes for each circuit: heavier gauge for power, lighter gauge for signals. Just ensure proper protection (fusing) for each circuit independently.
Stranded vs solid wire for harnesses?
Always use stranded wire for harnesses. Solid wire is only for fixed installations. Stranded wire handles flexing, vibration, and routing around corners without breaking.
How do I convert AWG to metric (mm²)?
Quick approximations: 18 AWG ≈ 0.75 mm², 16 AWG ≈ 1.3 mm², 14 AWG ≈ 2.0 mm², 12 AWG ≈ 3.3 mm², 10 AWG ≈ 5.3 mm². Use our calculator for precise conversions.
Related Resources
About the Author
Hommer Zhao is a Harness Design Engineer with extensive experience in automotive, industrial, and aerospace wire harness applications. He specializes in electrical system design, EMC compliance, and helping customers optimize their harness designs for manufacturability and reliability.
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