Engineering 2026-05-03 3 min read By Konvertibly Team

Wire Gauge Calculator: AWG, Ampacity, and Voltage Drop

Select the right wire gauge for your circuit using AWG sizing charts, calculate maximum current capacity, and determine voltage drop over long cable runs.

Wire Gauge and Current Capacity

Choosing the right wire gauge prevents overheating and voltage drop. The AWG (American Wire Gauge) system uses smaller numbers for thicker wires.

AWG Quick Reference

AWG 14 — 15A max — household branch circuits
AWG 12 — 20A max — kitchen outlets, garage
AWG 10 — 30A max — dryers, A/C units
AWG  8 — 40A max — ranges, water heaters
AWG  6 — 55A max — sub-panels, EV chargers
AWG  4 — 70A max — heavy loads
AWG  2 — 95A max — service entrance

Voltage Drop Formula

VD = (2 × L × R × I) / 1000
L = one-way length (feet or meters)
R = resistance per 1000ft (from AWG table)
I = current (A)

AWG 12 copper: R = 1.588 Ω/1000ft
100ft run, 15A:
VD = 2 × 100 × 1.588 × 15 / 1000 = 4.76V (out of 120V = 4%)
Rule: keep VD < 3% for branch circuits

Metric Wire Sizing (mm²)

1.5mm² → 15A (lighting circuits, UK)
2.5mm² → 20A (ring main, sockets)
4mm² → 27A (shower, cooker circuits)
6mm² → 36A (higher current appliances)
10mm² → 50A (main feeds)

Calculate wire sizing: Free Wire Gauge Calculator

Wire Gauge Reference (AWG and Metric)

AWG 14 (2.0 mm²): Max 15 A — US household circuit wiring
AWG 12 (3.3 mm²): Max 20 A — kitchen appliance circuits
AWG 10 (5.3 mm²): Max 30 A — dryers, small EV chargers
AWG 8 (8.4 mm²): Max 40 A — cookers, EV level 2
AWG 6 (13.3 mm²): Max 55 A — subpanels, larger EV chargers
Metric 1.5 mm²: 15 A (UK ring mains, lighting circuits)
Metric 2.5 mm²: 20 A (UK socket circuits)

Voltage Drop and Long Runs

Voltage drop across a cable = I × R = I × (ρ × 2L / A), where ρ is resistivity (copper: 1.72×10⁻⁸ Ω·m), L is one-way length, and A is cross-sectional area. Building codes typically limit voltage drop to 3–5% of supply voltage. At 230 V AC, 3% = 6.9 V. For a 10 A circuit with 20 m run, 2.5 mm² copper cable gives R = 1.72×10⁻⁸ × 40 / 2.5×10⁻⁶ = 0.275 Ω, voltage drop = 10 × 0.275 = 2.75 V (1.2% — acceptable). For the same run at 16 A, check with the larger load current. Long runs to outbuildings often require upsizing wire gauge one or two sizes to compensate for resistance.

Frequently Asked Questions

Why does smaller AWG number mean thicker wire?

AWG (American Wire Gauge) is based on the number of drawing steps needed to reach the wire diameter — more steps = smaller wire = higher gauge number. AWG 40 (0.08 mm) is fine wire for motors; AWG 0000 (4/0, 11.68 mm) is large cable for mains service. The unintuitive inverse relationship trips up engineers unfamiliar with the system. Metric wire sizes are described directly by cross-sectional area in mm², which is more logical.

What is the difference between solid and stranded wire?

Solid wire has a single conductor — lower resistance per mm², stiffer, better for permanent installations. Stranded wire has multiple thin conductors twisted together — more flexible, better for moveable cables (power tools, extension leads), more resistant to vibration fatigue. For equal current capacity, stranded wire requires slightly larger overall diameter due to air gaps between strands.

How do I calculate if a cable will overheat?

Cable ampacity (current-carrying capacity) depends on conductor size, insulation type (PVC, XLPE), ambient temperature, and installation method (in conduit, buried, clipped to surface). Cables in thermal insulation or bundled together have significantly reduced ampacity — a derating factor (typically 0.5–0.8) is applied. Always check the applicable wiring standard (NEC, BS 7671/IET Wiring Regulations) for the specific installation conditions.