200 wire runs — how far AWG 12 carries 15 A
For every meeting of 20 gauges and 10 currents: the resistance, the voltage drop, and how far you can run before losing 3%. AWG numbers are not a table to memorise but a sequence whose diameter is calculated.
AWG is a sequence, not a table
AWG pins number 36 at 0.127 mm and stretches thirty-nine equal steps up to number 0. Each step multiplies the diameter by the 39th root of 92, so jumping six steps quadruples the cross-section — 4.02 times, to be exact. The numbers shrink as the wire thickens because the count runs backwards.
Current goes out and comes back
When you count voltage drop, the run counts twice. Ten metres from battery to lamp means twenty metres of copper for the current to cross. Miss this and your answer is exactly half of the truth — and you will happily pick a wire half as thick as you need.
Thicker wire cools worse inside
Heat is made in proportion to the cross-section, but it escapes through the surface. That is why a wire twice as thick does not carry twice the current. The figure here scales with area to the power 0.65, fitted to ordinary wire whose insulation tolerates 60 °C — a deliberately cautious basis, so it reads lower than published tables.
The lower the voltage, the worse it hurts
Lose the same 0.36 V and it is 3% of a 12 V system but 0.16% of a 230 V one. That is why car wiring looks so thick, and why a long 12 V light run dims at the far end. This page works out the length that stays under 3% for each voltage separately.
Two families in circulation
The United States names wire by AWG number; most of the world names it by cross-section in mm². The two never line up exactly, so each page shows the nearest match by area. When substituting, err toward the thicker one.
Find it by gauge and current
Length within 3% · Europe 230 V
AWG 0000107.219mm² · 192.1A
AWG 0067.431mm² · 142.1A
AWG 053.475mm² · 122.2A
AWG 233.631mm² · 90.4A
AWG 421.151mm² · 66.9A
AWG 613.302mm² · 49.5A
AWG 88.366mm² · 36.6A
6mm²6mm² · 29.5A
AWG 105.261mm² · 27.1A
4mm²4mm² · 22.7A
AWG 123.309mm² · 20Akitchen and air conditioning
2.5mm²2.5mm² · 16.7Asocket circuits
AWG 142.081mm² · 14.8Ageneral outlets
1.5mm²1.5mm² · 12Alighting circuits
AWG 161.309mm² · 11A
AWG 180.823mm² · 8.1Alamp cord
0.75mm²0.75mm² · 7.6A
AWG 200.518mm² · 6A
AWG 220.326mm² · 4.4A
AWG 240.205mm² · 3.3Asignal wire
How to read this
- The length is the distance from source to device. The return trip is already counted.
- Figures are for copper. Aluminium of the same size has about 1.6 times the resistance.
- The "current it handles" is a cautious estimate. Real installations follow the local code table.
- A wire running hot and a lamp dimming at the far end are two different problems. Pick a gauge that answers both.
Frequently asked questions
Q. Why do bigger AWG numbers mean thinner wire?
Because the count starts at the thin end. Number 36 is 0.127 mm, and thirty-nine equal steps thicken it up to number 0.
Q. How many mm² is AWG 12?
3.31 mm², from a diameter of 2.053 mm. No table needed: 0.127 × 92^((36−12)/39) gives it.
Q. Do I double the length when computing voltage drop?
Yes. Current goes out and comes back, so a device ten metres away sits at the end of twenty metres of copper. Skip this and your answer is half.
Q. Where does the 3% figure come from?
It is common design practice. Stay under it and lamps do not visibly dim and motors do not lose their pull.
Q. Why is car wiring so thick?
Because it is 12 V. Losing the same 0.36 V is 0.16% at 230 V but a full 3% at 12 V.