About this calculator
Wire is not a perfect conductor. It has resistance, which wastes voltage along the run and turns into heat in the conductor. Choose too thin a wire and you get dim lights, browning-out microcontrollers, and in the worst case melted insulation.
This calculator works from either end: pick an AWG size and see its diameter, resistance per metre and safe current, or state the current you need and get the gauge to use. It also computes voltage drop for a given run length.
How it works
American Wire Gauge runs backwards — a bigger number is a thinner wire. Each 6 gauges roughly halves the diameter, and each 3 gauges roughly halves the cross-sectional area and therefore doubles the resistance.
Resistance follows from the cross-section: copper has a resistivity of about 1.68 × 10⁻⁸ Ω·m, so resistance per metre is that divided by the conductor area. The calculator has the standard figures tabulated.
Voltage drop is Ohm's Law applied to that resistance, with one catch people forget: current has to get there and back. The resistance that matters is the round trip, so you double the one-way figure.
Current ratings are a thermal limit, not an electrical one. The figures here are chassis-wiring ratings — a single conductor in free air with 60 °C insulation. Bundle wires together, run them in conduit, or work in a hot enclosure and you must derate substantially.
Worked example
Running a 5 V LED strip drawing 2 A down a 3 metre cable. Is 22 AWG good enough?
- 22 AWG copper is 0.0530 Ω/m
- Round-trip resistance: 0.0530 × 3 × 2 = 0.318 Ω
- Voltage drop: 2 × 0.318 = 0.636 V
- The strip sees 5 − 0.64 = 4.36 V — noticeably dim
- Try 18 AWG: 0.0209 Ω/m → 0.125 Ω round trip → 0.25 V drop
22 AWG loses 0.64 V, more than 12% of a 5 V supply, and the far end of the strip will be visibly dimmer. 18 AWG cuts that to 0.25 V. On low-voltage runs, voltage drop bites long before the current rating does.
Practical notes
- For 5 V and 12 V systems, voltage drop almost always decides the gauge before ampacity does. A percentage of 5 V is a much smaller absolute margin than the same percentage of 240 V.
- Aim to keep voltage drop under about 3% of the supply for anything sensitive, and under 5% for lighting and motors.
- Derate by 20–40% for bundled cables, conduit runs, or high ambient temperature. Heat cannot escape from the middle of a loom.
- Stranded wire is slightly more resistive than solid of the same gauge for the same nominal area, but it survives flexing — use it anywhere that moves.
- Aluminium wire has about 1.6× the resistance of copper for the same gauge, so it needs to be roughly two gauges thicker for the same performance.
- These ratings are for the conductor. Connectors, crimps and screw terminals are frequently the real limit — and the real fire risk.
Frequently asked questions
What AWG do I need for 10 amps?
In free air with 60 °C insulation, 16 AWG is rated for about 13 A, so it covers 10 A. For anything bundled or enclosed, step up to 14 AWG. And check voltage drop separately if the run is long.
Why is a higher AWG number thinner wire?
The gauge originally counted drawing operations — how many times the wire was pulled through progressively smaller dies. More passes meant thinner wire and a higher number.
How do I calculate voltage drop in a cable?
Multiply the resistance per metre by the length, double it for the return path, then multiply by the current. A 3 m run of 22 AWG carrying 2 A drops about 0.64 V.
Does wire length affect current capacity?
Not the thermal rating, which is about heat per unit length. But length dictates voltage drop, and on low-voltage systems that usually forces a thicker wire than the current rating alone would.
What is the difference between chassis and power transmission ratings?
Chassis wiring assumes a single conductor in open air and allows a higher current. Power transmission ratings assume long bundled runs where heat accumulates, and are typically about a third as high for the same gauge.