Size wire so it neither overheats nor drops too much voltage: choose a cross-section from the ampacity table, derate it for bundling or insulation (×0.80 for 2 cables bundled, ×0.70 for 3), and check voltage drop with V_drop = (2 × L × I × ρ) / A, using ρ = 0.0172 Ω·mm²/m. A 20 A load on 15 m of 2.5 mm² copper drops 4.1 V, which is 34% on a 12 V system but 1.7% on 240 V, which is why low-voltage DC needs much heavier wire. The fuse rating must never exceed the wire’s ampacity, so upgrade an undersized wire rather than enlarging the fuse.
How to select the correct conductor cross-section for a circuit based on current load, run length, and installation method.
Undersized Wire Overheats While Oversized Wire Only Wastes Metal
A wire that is too small for its load heats up. At moderate overloads, insulation degrades over months. At severe overloads, insulation melts in minutes and the wire becomes a fire igniter inside walls and conduit where it cannot be seen. Wire sizing is not a regulatory formality — it is the calculation that prevents buildings from burning.
Wire that is too large wastes copper, adds weight, and costs more — but causes no safety hazard. When in doubt, go larger. The cost of oversized wire is small compared to the cost of a fire.
Current Capacity: The Ampacity Table
Ampacity is the maximum continuous current a conductor can carry without exceeding its insulation’s temperature rating. It depends on:
- Cross-section (larger = more current)
- Insulation material (PVC rated 70°C; XLPE rated 90°C)
- Installation method (clipped to surface radiates heat well; buried in insulation traps heat)
Copper conductor ampacity (approximate, 70°C PVC, in open air):
| Cross-Section | Current Capacity | Typical Use |
|---|---|---|
| 0.75 mm² | 6 A | Light-duty flex, lamp cord |
| 1.0 mm² | 10 A | Low-power circuits |
| 1.5 mm² | 16 A | Lighting circuits |
| 2.5 mm² | 25 A | General power circuits |
| 4.0 mm² | 32 A | High-power outlets, small appliances |
| 6.0 mm² | 40 A | Large appliances, short sub-circuits |
| 10 mm² | 54 A | Sub-panels, electric cookers |
| 16 mm² | 73 A | Large motors, long runs |
| 25 mm² | 95 A | Main supply cables |
| 35 mm² | 119 A | High-current mains |
Derating for Bundled or Buried Cables
Multiple cables bundled together or cables run inside thermal insulation cannot dissipate heat as well. Apply derating factors:
- 2 cables bundled: multiply ampacity by 0.80
- 3 cables bundled: × 0.70
- 4–6 cables: × 0.57
- Buried in insulation: × 0.50 or per specific cable datasheet
AWG Equivalents (American Wire Gauge)
AWG sizes run backwards — smaller number = larger wire:
| AWG | Metric Equivalent | Ampacity (approx.) |
|---|---|---|
| 18 | 0.75 mm² | 7 A |
| 16 | 1.5 mm² | 13 A |
| 14 | 2.5 mm² | 20 A |
| 12 | 4.0 mm² | 30 A |
| 10 | 6.0 mm² | 35 A |
| 8 | 10 mm² | 50 A |
| 6 | 16 mm² | 65 A |
| 4 | 25 mm² | 85 A |
| 2 | 35 mm² | 115 A |
Voltage Drop Calculation
Beyond current capacity, wire must be sized to limit voltage drop over long runs. Thin wire has high resistance; high resistance causes voltage to drop between supply and load.
Formula: V_drop = (2 × L × I × ρ) / A
Where:
- L = one-way length of run (meters)
- I = current (amps)
- ρ = resistivity of copper = 0.0172 Ω·mm²/m
- A = conductor cross-section (mm²)
- Factor of 2 accounts for outgoing and return conductors
Example: 20 A load, 15 m run, 2.5 mm² copper:
V_drop = (2 × 15 × 20 × 0.0172) / 2.5 = 10.32 / 2.5 = 4.1 V
On a 12 V system: 4.1 / 12 = 34% drop — completely unacceptable. On a 120 V system: 4.1 / 120 = 3.4% — borderline acceptable (3% preferred). On a 240 V system: 4.1 / 240 = 1.7% — excellent.
This calculation shows why low-voltage DC systems require much heavier wire for the same power.
Quick Reference: Maximum Run Length (3% Voltage Drop)
| Current | 12 V | 24 V | 120 V | 240 V |
|---|---|---|---|---|
| 10 A, 2.5 mm² | 2.7 m | 5.5 m | 27 m | 55 m |
| 10 A, 6 mm² | 6.6 m | 13 m | 66 m | 132 m |
| 10 A, 16 mm² | 17 m | 35 m | 175 m | 350 m |
| 20 A, 6 mm² | 3.3 m | 6.6 m | 33 m | 66 m |
| 20 A, 16 mm² | 8.8 m | 17.5 m | 88 m | 175 m |
The practical lesson: for 12 V DC systems, keep runs under 5 meters with heavy wire, or step up to 24 V to allow longer runs with the same wire.
Sizing for Fuse Coordination
Wire and fuse must be coordinated: the fuse must blow before the wire overheats.
Rule: Fuse rating ≤ wire ampacity
Never install a larger fuse than the wire can carry. Common mistake: a 4 mm² circuit (rated 32 A) protected by a 63 A fuse “because the load needed it.” The fuse will not blow before the wire burns. Upgrade the wire size instead.
| Wire Size | Maximum Fuse Rating |
|---|---|
| 1.5 mm² | 16 A |
| 2.5 mm² | 25 A |
| 4.0 mm² | 32 A |
| 6.0 mm² | 40 A |
| 10 mm² | 50 A |
Improvised Wire Sizing
When using salvaged or hand-drawn wire of unknown gauge, estimate cross-section by measuring diameter:
A = π × (d/2)² where d is measured with calipers.
A 2 mm diameter copper wire: A = π × (1)² = 3.14 mm² ≈ 2.5–4 mm² in practice. Use the lower ampacity table value for safety.
For hand-drawn wire from copper rod, the drawing gauge controls diameter. Keep notes on what gauge you drew and what load you assigned to it.