Skip to main content

1.2 Conductor Materials: The Electrical Core

The conductor is the functional heart of any wire harness. Its selection dictates not only the electrical performance (current capacity, signal integrity) but also the mechanical reliability of the termination. Design engineers often default to standard "hook-up wire" without considering the metallurgical and mechanical implications of the operating environment. A mismatch here leads to catastrophic failures: oxidation increasing contact resistance, fatigue breaks under vibration, or insulation meltdown due to improper current derating.

1.2.1 Copper Metallurgy and Plating

Pure copper is the standard conductor due to its conductivity, but it oxidizes rapidly. Plating is the mandatory defense against corrosion and is selected based on temperature and frequency requirements.

Plating Selection Matrix

Material

Temp Rating

Cost

Application Mandate

Bare Copper

Low

Lowest

Avoid in harness assembly. Oxidizes rapidly, causing high contact resistance at the crimp interface.

Tinned Copper

150˚C

Low

Standard Industry Baseline. Tin coating prevents oxidation, aids soldering, and is compatible with standard crimp terminals.

Silver-Plated

200˚C

High

High Frequency / High Temp. Excellent conductivity (Skin Effect) for RF signals. Mandatory for high-reliability aerospace.

Nickel-Plated

260˚C+

Highest

Extreme Environment. Used in engines and furnaces. Note: Harder material makes crimping more difficult; requires specialized tooling settings.

Process Note: Never mix plating types in a contact pair (e.g., Tin wire crimped to Gold terminal) without validation, as galvanic corrosion can degrade the connection.

1.2.2 Stranding Mechanics and Flexibility

Conductors are defined by their construction: Solid or Stranded. This choice is a trade-off between rigidity and flex-life.

A) Solid vs. Stranded

  • Solid Core: A single strand of copper. Low cost and rigid.
    • Risk: Extremely poor fatigue resistance. Prohibited in high-vibration environments (automotive, aerospace) as it will work-harden and snap.
    • Crimping: Difficult to achieve a reliable gas-tight seal; often requires soldering or IDC (Insulation Displacement Connectors).
  • Stranded: Multiple smaller strands twisted together. High cost but superior flexibility and fatigue resistance.
    • Mandate: Mandatory for any application involving motion, vibration, or complex routing.

B) Stranding Geometry (Type 2 vs. Type 3)

The arrangement of strands affects the wire's circularity and the quality of the crimp.

  • Bunched Stranding: Strands are twisted randomly in the same direction. The cross-section is irregular (not perfectly round).
    • Risk: Can lead to "hot spots" in crimps if strands shift unevenly.
  • Concentric Stranding (True Concentric): Layers of strands are twisted in alternating directions. The cross-section is essentially perfectly round.
    • Benefit: Provides the most consistent, gas-tight crimp. The uniform geometry ensures even compression forces during termination.
  • Rope Lay: Bundles of stranded groups twisted together. Used for large gauge power cables (e.g., 4/0 AWG) to maintain flexibility.

1.2.3 Current Capacity and Derating Rules

The "Ampacity" rating of a wire in a datasheet is typically defined for a single wire in free air at a specific ambient temperature (often 30˚C). In a harness, wires are bundled, trapping heat. Using the datasheet value for a bundled harness creates a fire hazard.

The Derating Mandate

Current capacity must be derated (reduced) based on two factors: Bundle Size and Ambient Temperature.

Practical Derating Factors (Bundle Size)

When wires are bundled, the inner wires cannot dissipate heat.

Number of Current-Carrying Wires

Derating Factor

Practical Example

1 (Free Air)

1.0 (100%)

10A rated wire carries 10A.

2 - 5

0.8 (80%)

10A rated wire limited to 8A.

6 - 15

0.7 (70%)

10A rated wire limited to 7A.

16 - 30

0.5 (50%)

10A rated wire limited to 5A.

Ambient Temperature Correction

As the environmental temperature rises towards the insulation's rating, the allowed current drops to zero.

  • Rule: If a PVC wire is rated to 105˚C and the engine bay is 85˚C, the wire only has a 20˚C thermal headroom. Current must be drastically limited to prevent melting.

Final Checklist: Conductor Selection

Mandate

Criteria

Verification Action

Plating Selection

Tinned Copper is the minimum standard for general interconnects.

Verify BOM calls for tinned/plated wire to prevent oxidation-induced crimp failure.

Vibration Control

Stranded wire is mandatory for all environments subject to vibration or flexing.

Solid core is blocked from use in automotive/industrial harnesses unless specifically justified.

Crimp Geometry

Concentric stranding preferred for high-reliability crimps (Class 3).

Visual inspection of crimp cross-section (micrograph) verifies consistent strand compression.

Safety Derating

Current capacity is calculated using Derating Factors for bundle size and temperature.

Design review confirms wire gauge is sufficient for the bundled load, not just the free-air rating.

Galvanic Match

Conductor plating matches the terminal plating (e.g., Tin-to-Tin).

Prevents long-term galvanic corrosion at the crimp interface.


Back to top