Wire Sizing

Part of Electrical Wiring and Installation

How to select the correct conductor cross-section for a circuit based on current load, run length, and installation method.

Why This Matters

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	63 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.