Wire Selection
Part of Power Transmission
Choosing the right conductor material, gauge, and insulation for every part of an electrical distribution system.
Why This Matters
Wire is the most abundant material in any electrical system, and wrong wire choices create problems that range from nuisance (lights that dim when a motor starts) to catastrophic (insulation fires from overloaded circuits). Every meter of wire in your system carries current, has resistance, and generates heat proportional to I²R. Selecting wire correctly keeps that heat within safe bounds while minimizing cost and waste.
Wire selection requires balancing three constraints: the wire must be large enough to carry the current without overheating (ampacity), it must be large enough to keep voltage drop within acceptable limits (impedance), and it must be appropriate for its installation environment (insulation rating). These three requirements often point toward different gauge choices — you always use the largest gauge indicated by any of the three constraints.
In a resource-limited environment, wire is precious. You will scavenge it from abandoned buildings, strip it from decommissioned equipment, and manufacture it from available metals. Knowing exactly what each gauge can handle prevents both dangerous undersizing and wasteful oversizing.
Conductor Materials
Copper is the standard for nearly all electrical wiring. Its properties make it nearly ideal:
- Resistivity: 1.68 × 10⁻⁸ Ω·m (lowest of common metals except silver)
- Easily soldered, mechanically ductile, resistant to corrosion
- Oxide layer (patina) is still conductive, unlike aluminum oxide
- Available from plumbing pipe, motor windings, electrical cable, telephone wire
Aluminum has 61% of copper’s conductivity, but is one-third the density. For the same weight, aluminum actually carries more current than copper — this is why high-voltage overhead transmission lines use aluminum (sometimes with a steel core for strength). For a rebuilding community:
- Use aluminum for long overhead spans where weight matters
- Avoid aluminum for any connection that may be disturbed, tightened, or loosened repeatedly
- Never connect aluminum directly to copper — galvanic corrosion creates high resistance at the junction
- Aluminum oxide (which forms immediately on exposed aluminum) is non-conductive — all aluminum connections must be made under compression or with anti-oxidant compound
Iron and steel wire can be used as a last resort for very low-current applications. Resistance is 6-7 times higher than copper, meaning iron wire of the same gauge carries proportionally less current and has much higher voltage drop. Use for fence wire, emergency ground connections, or signal circuits only. Never for power circuits where efficiency matters.
Scavenged magnet wire (enameled copper): Excellent conductivity, but the enamel insulation is only rated for low voltage (typically under 200V working voltage) and is not suitable for direct burial or exposure to moisture without additional protection. Ideal for transformer windings, motor coils, solenoids.
Understanding AWG (American Wire Gauge)
The AWG numbering system is counterintuitive: smaller numbers mean larger diameter wire. This is because the gauge number originally represented the number of drawing steps to reach that diameter.
Key reference points:
| AWG | Diameter (mm) | Area (mm²) | Resistance (Ω/100m) | Common use |
|---|---|---|---|---|
| 24 | 0.51 | 0.205 | 8.45 | Signal wire, low-current electronics |
| 20 | 0.81 | 0.519 | 3.38 | Instrument wiring, light control circuits |
| 18 | 1.02 | 0.823 | 2.13 | Extension cords, lamp cords |
| 16 | 1.29 | 1.31 | 1.32 | Light circuits up to 15A |
| 14 | 1.63 | 2.08 | 0.84 | Standard 15A house wiring |
| 12 | 2.05 | 3.31 | 0.53 | 20A circuits, heavy outlets |
| 10 | 2.59 | 5.26 | 0.33 | 30A circuits, AC units, water heaters |
| 8 | 3.26 | 8.37 | 0.21 | 40A circuits, large motors |
| 6 | 4.11 | 13.3 | 0.13 | Service entrance wire, 60A circuits |
| 4 | 5.19 | 21.2 | 0.083 | Service entrance, large feeders |
| 2 | 6.54 | 33.6 | 0.052 | 100A service entrance |
| 1/0 | 8.25 | 53.5 | 0.033 | 150A service |
| 2/0 | 9.27 | 67.4 | 0.026 | 200A service |
Metric wire: Many countries use square millimeters (mm²) for wire sizing. The approximate conversions: 2.5mm² ≈ 13 AWG, 4mm² ≈ 11 AWG, 6mm² ≈ 10 AWG, 10mm² ≈ 8 AWG, 16mm² ≈ 6 AWG, 25mm² ≈ 4 AWG.
Ampacity: How Much Current a Wire Can Carry
Ampacity is the maximum continuous current a wire can carry without exceeding its rated temperature. The wire heats from I²R losses; the insulation fails if it gets too hot.
Ampacity for copper wire in free air (approximate):
| AWG | Ampacity (free air) | Ampacity (conduit/bundled) |
|---|---|---|
| 14 | 20A | 15A |
| 12 | 25A | 20A |
| 10 | 35A | 30A |
| 8 | 50A | 40A |
| 6 | 65A | 55A |
| 4 | 85A | 70A |
| 2 | 115A | 95A |
| 1/0 | 150A | 125A |
The “conduit/bundled” values are lower because bundled wires cannot shed heat as easily. When multiple circuits run together in a conduit or cable bundle, each must be derated.
Temperature derating: The above values assume an ambient temperature of 30°C. For installations in hot environments (engine rooms, rooftops in summer), derate by 20-30% to stay within temperature limits.
Overcurrent protection must match the wire, not the load. If your circuit uses 14 AWG wire (rated 15A in conduit), the fuse must be 15A or less — even if the load only draws 5A. This protects the wire if a fault draws more current than the wire can handle. The fuse protects the wire; the wire protects the load.
Three-Step Wire Selection Process
For any circuit you are designing, apply these three checks in order and use the largest wire indicated by any check:
Check 1 — Ampacity: The wire must be rated for the full expected current. Add 25% margin for continuous loads (loads running more than 3 hours). A 10A continuous load needs wire rated for at least 12.5A — use 14 AWG minimum (15A rated) or 12 AWG for margin.
Check 2 — Voltage drop: Calculate expected voltage drop using V_drop = I × R_total. If it exceeds your limit (typically 3% for branch circuits), go up a gauge size and recalculate.
Check 3 — Mechanical requirements: Wire exposed to physical stress (outdoor overhead span, buried cable subject to digging, flexible cord to a portable tool) needs to be mechanically robust. Single-strand solid wire is stronger but less flexible than stranded wire. For runs subject to vibration or flexing, use stranded wire to prevent work-hardening failure.
Stranded vs. Solid Wire
Solid wire is one continuous conductor. Easier to strip and terminate, maintains its shape after bending, lower resistance than equivalent stranded wire. Best for in-wall circuits and conduit runs that do not flex.
Stranded wire consists of many fine conductors twisted together. More flexible, resistant to fatigue failure from repeated bending, easier to route through complex paths. Required for any application with continuous flexing (tool cords, panel feeders subject to vibration, wiring in moving machinery). Lower current capacity per unit weight than solid (same cross-section, lower actual contact area at terminals).
For a community grid: Use solid wire for all fixed, in-wall, or conduit installations. Use stranded wire for service drops (the overhead wire from pole to building, which moves in wind), transformer connections inside panels (where adjustments are made), and any motor circuit within 2 meters of the motor (motors vibrate).
Insulation Types and Ratings
Insulation determines what environments the wire can survive:
Thermoplastic insulation (PVC, polyethylene): Standard house wire. Rated for dry indoor use, typically to 60-75°C. Degrades with UV exposure (outdoor use) and becomes brittle at low temperatures. Common in nearly all scavenged building wire.
THHN/THWN: Thermoplastic with nylon jacket, rated for wet or dry locations (the ‘W’), up to 90°C. The most common modern building wire. Distinguished by its smooth nylon outer finish.
Rubber insulation (old wire): Found in pre-1960 buildings. More flexible than modern PVC, tolerates higher temperatures, but becomes brittle and cracks with age. Inspect carefully before reusing — cracked rubber insulation is a serious fire hazard.
Cross-linked polyethylene (XLPE or XHHW): Superior heat resistance (up to 90°C wet, 90°C dry), excellent for high-temperature environments and direct burial. Common in modern service entrance cable and underground feeder.
Silicone rubber: Excellent temperature range (-60°C to +180°C), flexible at low temperatures. Expensive. Use for wiring near heat sources (boiler controls, furnace wiring, engine-adjacent installations).
Bare wire (uninsulated): Used for equipment grounding conductors. Current code allows bare copper for grounding conductors in conduit. Never use bare wire for anything carrying live voltage — the first contact with any surface shorts and potentially ignites.
Direct Burial and Outdoor Wiring
For runs between buildings or to remote loads:
Option 1 — Overhead wire on poles: The safest approach for most building-to-building runs. Keep wire at least 5 meters above grade. Use weatherproof insulation. Minimum size for any overhead span that must support its own weight: 10 AWG solid copper (lighter wire sags excessively and may snap in ice or wind). For spans over 30 meters, use larger gauge wire or a steel messenger wire with the electrical conductors attached to it.
Option 2 — Direct burial cable: Use only wire rated for direct burial (UF cable, or XLPE-insulated wire in PVC conduit). Minimum burial depth for 120V: 30 cm. Minimum depth for 240V or higher: 60 cm. Mark the route with stakes or paint — future digging has destroyed many direct-burial runs. Conduit is strongly preferred over direct burial even for rated cable, because it allows the wire to be replaced without excavation.
Option 3 — PVC conduit: Run lightweight wire (THHN) through PVC schedule 40 conduit. The conduit provides mechanical protection and water sealing. Wire inside can be replaced. Conduit reduces heat dissipation, so use ampacity derating factors.
Protecting splice points: Any splice outdoors or in a damp location must be in a sealed enclosure. Wirenuts and tape are not adequate — water penetrates tape and corrodes wirenuts. Use weatherproof junction boxes with proper connectors, or use heat-shrink tubing with adhesive liner over soldered joints.
Making the Most of Scarce Wire
In a resource-limited environment, wire is valuable. Strategies for making it go further:
Increase voltage to reduce current. Using 240V instead of 120V for the same power load halves the current and allows wire gauges two sizes smaller. This is the single most effective wire-conservation strategy for long runs.
Central loads reduce total wire. Rather than running a separate circuit to every building, centralize high-draw loads (the forge, the grain mill, the charging station) in one location. Fewer long circuit runs means less total wire.
Salvage strategically. Old telephone cable contains many fine conductors that can be bundled in parallel to achieve higher current capacity. Old motor windings are high-quality magnet wire for transformer use. Old building wire (14-12 AWG) stripped from demolished structures is immediately useful for branch circuit wiring.
Aluminum for long overhead runs. For a high-voltage transmission span of several hundred meters, the weight advantage of aluminum is significant. The overhead line hangs from poles — aluminum weighs one-third as much as copper of the same resistance, reducing pole loads and sag. Accept the connection difficulties at the endpoints (use mechanical compression lugs, apply anti-oxidant grease to all aluminum connections) in exchange for the weight savings on the span.