Wire Types and Insulation
An overview of conductor materials, insulation types, and how to choose the right cable for each application.
Why This Matters
Not all wire is the same. The conductor material determines resistance and flexibility. The insulation determines what environments and temperatures the wire can survive. The construction — solid vs. stranded, shielded vs. unshielded — determines mechanical performance. Using the wrong wire type in an application creates failures that may not be immediately obvious: insulation that cracks in cold weather, wire that breaks from vibration, insulation that melts near heat sources.
Understanding wire types lets you select correctly from available stock, substitute intelligently when the ideal is unavailable, and recognize when salvaged wire is suitable for a new application.
Conductor Materials
Copper
Copper is the dominant conductor material for a reason: it has the second-lowest resistivity of any common metal (after silver), is highly ductile, solders easily, and resists corrosion in most environments.
Resistivity: 0.0172 Ω·mm²/m at 20°C — the benchmark against which other conductors are measured.
Hard-drawn copper: Stiff, high tensile strength. Used in overhead lines and rigid conduit wiring where mechanical strength matters and movement is minimal.
Annealed (soft) copper: More flexible, lower tensile strength. Used in flexible cables, appliance cords, and any application involving movement or flexing.
Aluminum
Aluminum has about 60% the conductivity of copper by cross-section — a 16 mm² aluminum conductor carries roughly the same current as 10 mm² copper. However, aluminum is lighter and cheaper, making it dominant in large overhead transmission conductors and some main supply cables.
Disadvantages in small-scale wiring:
- Forms oxide layer that increases contact resistance at terminals
- More brittle than copper — breaks from repeated flexing
- Requires anti-oxidant compound at connections
- Cannot be soldered with normal methods
For most building wiring applications, copper is preferred. Aluminum is viable for large fixed cable runs (main supply cables) but not for flexible connections or frequently disturbed wiring.
Conductor Construction
Solid Wire
A single conductor strand. Stiff, maintains shape well, connects reliably to screw terminals. Not suitable for flexible applications — repeated bending breaks the conductor.
Best for: Fixed wiring in conduit, low-current signal wire.
Stranded Wire
Multiple thin conductor strands twisted together. Each strand is thin enough to flex independently without breaking. The more strands, the more flexible.
| Class | Strands | Flexibility | Typical Use |
|---|---|---|---|
| Class 1 | 1 (solid) | Rigid | Fixed conduit wiring |
| Class 2 | 7–19 | Semi-flexible | Fixed multi-core cables |
| Class 5 | Many fine | Flexible | Appliance cords, control cables |
| Class 6 | Very many | Highly flexible | Welding cables, trailing cables |
Terminal Problems with Fine Strands
Very fine stranded wire (Class 5-6) can splay at screw terminals, causing poor connections. Use ferrules (crimp-on metal sleeves) on the conductor end to consolidate strands before inserting into terminals.
Insulation Types
PVC (Polyvinyl Chloride)
The most common insulation for general building wiring. Inexpensive, flexible, chemically resistant, self-extinguishing (stops burning when flame is removed).
Temperature rating: 70°C continuous. Some formulations rated 90°C. Limitations: Becomes brittle below -20°C. Releases toxic hydrogen chloride gas when burning. Not suitable for high-temperature environments.
XLPE (Cross-Linked Polyethylene)
Better properties than PVC at a moderate cost premium. More resistant to heat, UV, and chemicals. Does not release chlorine compounds when burning.
Temperature rating: 90°C continuous (some grades to 105°C). Use: Underground cables, higher-temperature environments, premium building wiring.
Silicone Rubber
Flexible across a wide temperature range. Resists high heat, ozone, and UV.
Temperature rating: -60°C to +180°C continuous; short-term to 200°C. Use: Connections near furnaces, inside ovens, motor winding leads, anywhere near heat sources.
Rubber (Natural or Synthetic)
Traditional flexible insulation. Good electrical properties, maintains flexibility at low temperatures. Susceptible to oil and ozone degradation over time.
Use: Flexible cords, some outdoor applications.
Bare Conductor
Uninsulated wire. Used for ground conductors (which are intentionally connected to earth), overhead transmission lines (kept separated by distance and insulator hardware), and bus bars inside equipment.
Never substitute bare wire where insulated is required.
Cable Types by Application
| Cable Type | Description | Typical Application |
|---|---|---|
| Single-core PVC | One conductor, PVC insulation | Conduit wiring, internal panel wiring |
| Twin and earth (T&E) | Two insulated cores + bare earth, flat PVC jacket | Standard UK building wiring |
| SWA (Steel Wire Armored) | XLPE-insulated cores, steel wire armor, PVC outer | Underground, exposed outdoor, mechanical protection |
| Flexible (H05VV-F) | Stranded cores, PVC, round jacket | Appliance cords, portable tools |
| Coaxial | Center conductor, foam dielectric, braid shield, outer jacket | Antenna feeds, RF signals |
| Screened (shielded) | Signal conductors with metal foil/braid shield | Instrumentation, audio, data in EMI environments |
| High-temperature | Silicone or PTFE insulation | Furnaces, kilns, motor connections |
Identifying Salvaged Wire
When repurposing salvaged wire, identify before connecting:
- Measure diameter with calipers to determine conductor cross-section.
- Flex test — bend a short sample tightly. Insulation that cracks is aged PVC, no longer suitable.
- Conductor test — measure resistance per meter and compare to copper/aluminum tables.
- Smell test — PVC and rubber have distinct smells. Silicone rubber is odorless. Strong chemical smell may indicate degraded insulation compounds.
- Temperature rating — if markings are visible on the cable sheath, decode them. Common marking: H07V-R = European harmonized, 07=300/500V rating, V=PVC, R=rigid stranded.
When in doubt about an old cable’s rating, derate it — use it at no more than 60% of the ampacity you would assign to new wire of the same cross-section.