American Wire Gauge (AWG) is the standard sizing system for round, solid, nonferrous electrical wire in North America. The scale covers everything from hair-fine 40 AWG instrument wire up to 0000 (4/0) used in service entrances. This reference computes the exact diameter, cross-sectional area, and a reference ampacity for every gauge in the range.
How it works
AWG is defined by a precise geometric progression, not an arbitrary lookup table. Two anchor points — 36 AWG at 0.127 mm and 0000 (4/0) at 11.684 mm — define the ratio, giving this exact formula:
d(mm) = 0.127 × 92^((36 − n) / 39)where n is the gauge number. The four oversized gauges 1/0, 2/0, 3/0, and 4/0 correspond to n values of 0, −1, −2, and −3. Cross-sectional area follows from the diameter:
A(mm²) = π / 4 × d²Two useful rules of thumb fall out of the formula:
- Every 6-gauge step roughly halves the diameter.
- Every 10-gauge step divides the cross-sectional area by about 10, which also divides the resistance per unit length by about 10.
The inverse scale — larger number = thinner wire
AWG is counterintuitive: a higher gauge number means thinner wire. This catches newcomers out every time. A few reference points to anchor your intuition:
| AWG | Diameter (approx.) | Typical use |
|---|---|---|
| 4/0 (0000) | 11.7 mm | Service entrances, large feeders |
| 4 | 5.19 mm | Large appliance circuits, sub-panels |
| 12 | 2.05 mm | Standard household 20 A branch circuits |
| 14 | 1.63 mm | Lighting circuits (15 A) |
| 22 | 0.64 mm | Control wiring, electronics |
| 28 | 0.32 mm | Ribbon cables, fine signal wiring |
| 40 | 0.08 mm | Fine instrument wiring |
Ampacity — two columns explained
The table shows two reference ampacity figures because the safe current depends heavily on how the wire is installed:
- Chassis wiring — a single insulated conductor in free air, where heat can radiate freely. Tolerates higher current.
- Power transmission — conductors bundled in conduit or cable trays, where heat accumulates because nearby conductors add to the thermal load. The safe current is considerably lower.
For a real circuit, these figures are starting references only. Your national electrical code — the NEC in the United States, the CEC in Canada — accounts for insulation temperature rating (60°C vs 90°C), ambient temperature, conductor material (copper vs aluminium), and installation method. Always size conductors to the applicable code, not to a reference table.
Common AWG selection mistakes
- Using the chassis ampacity figure for household wiring (bundled) — this can cause dangerous overheating.
- Confusing AWG with SWG (British Standard Wire Gauge) or metric mm² — these use different scales and are not interchangeable.
- Not accounting for voltage drop on long runs: a 14 AWG wire is fine for 15 A over 10 metres, but over 50 metres the voltage drop may require 12 AWG or thicker.