Conductor Selection
Part of Basic Electrical Circuits
Choosing the right wire gauge, material, and insulation for electrical conductors in any application.
Why This Matters
Wire selection determines whether an electrical system runs safely and efficiently or overheats, loses excessive voltage, and eventually catches fire. Too thin a wire for the current it carries generates heat that degrades insulation, wastes energy, and can ignite surrounding materials. Too thick a wire wastes scarce metal and is difficult to work with.
In a rebuilding context, copper wire is a critical resource. It takes significant energy and skill to smelt and draw copper wire, and the supply from salvage will eventually be depleted. Selecting the minimum adequate wire size for each application conserves this resource without compromising safety. Knowing how to substitute alternative materials—iron wire, aluminum, graphite—when copper is unavailable extends the community’s electrical capabilities.
Conductor selection also involves the insulation: the material surrounding the conductor that prevents short circuits and protects users from shock. Knowing which natural and improvised materials can serve as insulation, and their limitations, is as important as knowing which metals conduct electricity.
Conductor Materials
Copper is the standard for electrical conductors: low resistivity, easily drawn into wire, adequately strong, and resistant to corrosion.
Aluminum has about 60% of copper’s conductivity but is lighter and more abundant. Used widely for overhead power lines. Requires larger diameter to carry the same current as copper. Oxidizes at connections, creating high-resistance joints—connections must use anti-oxidant compound or be properly sealed.
Iron and steel are poor conductors (about 12% of copper’s conductivity) but were historically used when copper was unavailable. Salvaged fence wire, baling wire, and structural wire can carry current at low voltages if sized generously. Iron corrodes readily—keep connections clean and coated.
Silver is the best conductor (106% of copper) but prohibitively scarce. Historical use in telegraph relays and precision instruments.
Resistivity comparison (lower is better):
| Material | Resistivity (nΩ·m) |
|---|---|
| Silver | 15.9 |
| Copper | 16.8 |
| Gold | 22.1 |
| Aluminum | 26.5 |
| Iron | 97.1 |
| Steel | 100–200 |
| Nichrome | 1000–1500 |
| Graphite | 3000–60000 |
Wire Gauge Systems
Wire diameter is specified by gauge systems that run counterintuitively: a higher gauge number means a thinner wire.
American Wire Gauge (AWG):
| AWG | Diameter (mm) | Max current (amps) | Resistance (Ω/km) |
|---|---|---|---|
| 10 | 2.59 | 30 | 3.3 |
| 12 | 2.05 | 20 | 5.2 |
| 14 | 1.63 | 15 | 8.3 |
| 16 | 1.29 | 13 | 13.2 |
| 18 | 1.02 | 10 | 21.0 |
| 20 | 0.81 | 5 | 33.5 |
| 22 | 0.64 | 3 | 53.3 |
| 24 | 0.51 | 2 | 84.9 |
| 28 | 0.32 | 0.5 | 213 |
Metric standard: Wire specified directly in millimeters diameter. 1.5 mm² cross-section is common for household wiring in metric countries (roughly equivalent to AWG 14).
Field estimation without gauges: Pass the wire through a series of drilled holes. The smallest hole it passes through gives the diameter. A vernier caliper gives direct measurement.
Calculating Wire Size
Determining safe current capacity: The key question is: will this wire overheat at the current it will carry?
- Determine the maximum continuous load current
- Add a safety margin: wire rated for at least 125% of continuous load
- Select wire gauge from the table above
Determining acceptable voltage drop: For long runs, the wire resistance creates voltage drop: V_drop = I × R_wire = I × (length × resistance per meter × 2)
The factor of 2 accounts for both the outgoing and return conductors.
Maximum acceptable voltage drop is typically 2–5% of supply voltage. More than this causes lights to dim noticeably, motors to run hot, and sensitive electronics to malfunction.
Wire sizing example: A water pump motor draws 8A and is located 50 meters from the battery bank (12V supply). Maximum acceptable voltage drop: 3%.
Maximum drop = 12V × 3% = 0.36V Total wire length = 50m × 2 = 100m Required resistance = 0.36V / 8A = 0.045Ω for 100m Resistance per meter = 0.00045 Ω/m = 0.45 Ω/km
From the table, AWG 10 (3.3 Ω/km) is far too high. Need much lower resistance.
Recalculate: Need resistance ≤ 0.45 Ω/km. No standard wire gauge achieves this at reasonable size.
Solution options:
- Increase supply voltage to 24V (cuts current by half, cuts drop by 75%)
- Use much larger wire (4 AWG or thicker)
- Add a voltage regulator at the motor
This example illustrates why long-distance low-voltage systems are inefficient—a practical argument for higher voltage distribution.
Insulation Materials
Commercial wire insulation is plastic (PVC, polyethylene, XLPE). These are salvageable but cannot be fabricated from scratch. For field-made conductors, historical insulation methods apply:
Rubber: Natural rubber latex from Hevea trees, vulcanized with sulfur, is excellent insulation. Resistant to moisture, flexible, rated for hundreds of volts. The standard before synthetic plastics.
Gutta-percha: Related to rubber, historically used for undersea telegraph cables. More rigid than rubber, extremely water-resistant.
Linseed oil–soaked cloth: Layers of cloth tape soaked in boiled linseed oil and dried make serviceable low-voltage insulation. Multiple layers provide some mechanical protection.
Shellac-coated cotton: Cotton thread or tape coated with shellac (dissolved flake shellac in alcohol) dries to a hard, moderately water-resistant coating. Used for coil winding and control wiring at low voltages.
Oiled paper: Layers of paper saturated with mineral oil or linseed oil. Used as insulation in early power transformers and cables. Adequate for dry, protected indoor installations.
Ceramic standoffs and insulators: For carrying bare wire at high voltages, ceramic insulators support the conductor while preventing current flow to structure. Clay-fired ceramic insulators can be made locally.
Voltage limits for improvised insulation:
| Material | Safe voltage limit |
|---|---|
| Dry cloth tape | 50V |
| Shellac-cotton | 150V |
| Multiple linseed oil cloth layers | 250V |
| Natural rubber | 1000V+ |
| Ceramic insulators | 10,000V+ |
Never use improvised insulation for voltages exceeding these limits without extensive testing.
Joining and Splicing Conductors
Mechanical splice: Twist conductors together with at least 5–6 tight turns. Wrap with insulating tape. Adequate for dry locations; adequate for low-voltage DC.
Brazed or soldered splice: Mechanically join first, then apply solder or brass brazing. Far more reliable than mechanical splice alone. Soldered joints must be kept dry as solder wicks are moisture channels.
Terminal connections: At equipment terminals, loop wire around the terminal screw in the direction the screw tightens (clockwise). Tighten firmly. Aluminum wire requires anti-oxidant compound at all connections.
Crimped connections: A crimping tool compresses a metal ferrule over stripped wire ends. Strong, vibration-resistant, and faster than soldering when a proper crimping tool is available.
Signs of a failing connection:
- Discoloration or scorching around the joint
- Measurable voltage drop across the connection
- Intermittent circuit operation correlated with vibration or temperature changes
- A joint that is warm to the touch when the circuit is loaded
Regular inspection and retightening of connections is preventive maintenance that catches failures before they cause fires or equipment damage.