Thin wire is the quiet killer of DIY projects. The build works on the bench, then the LED strip dims at the far end, the motor stalls under load, or a connector gets warm for no obvious reason. Nothing is broken. The wire is just too thin for the current it carries over that distance.
Here is the whole decision, in the order it actually matters.
The two rules that matter
Voltage drop, not wire temperature. People pick wire gauge by the ampacity chart, which is a fire-safety number for mains wiring in a wall. On a 5 V or 12 V project, the failure you actually hit is drop: the resistance of the run eats the volts before they arrive. A 5 V LED strip on a 5 m run of thin wire can arrive with 3.9 V and visibly dim.
The 3% rule. Keep the total drop, out and back, under 3% of the supply. For 5 V that is 0.15 V. For 12 V it is 0.36 V. Below that, and most electronics behaves like it does on the bench.
The math, once
Copper has a resistance per metre that depends only on its cross section:
drop = 2 × length × current × resistance_per_metre
The 2 is there because current travels out along one conductor and back along the other. Both halves drop volts, and both count.
A worked example: 5 A over 5 m of 18 AWG.
- 18 AWG is 0.021 ohms per metre
- drop = 2 × 5 m × 5 A × 0.021 = 1.05 V
On a 12 V rail that is 8.8% and clearly too much. Moving to 14 AWG (0.0083 ohms per metre) gives 0.42 V, about 3.5%, close enough that the next size up removes all doubt.
Gauge, current and distance, together
The useful way to hold this in your head is that every three gauge steps roughly halves the resistance. So a run that is twice as long needs about three sizes thicker for the same drop. That is also why long runs surprise people: doubling the distance is not a 2x problem in practice, it is a step change in what wire you need.
Practical bands for low voltage projects:
- Small signals and sensor runs: 22 to 26 AWG is fine, drop is irrelevant at milliamps.
- Short runs of a few amps (LED strips, small servos): 18 to 20 AWG.
- A metre or two at 5 to 10 A: 16 AWG at minimum.
- Anything longer, or anything that a motor starts and stalls on: 14 AWG and up, and think about where the power enters the strip.
The traps that bite
Daisy chaining power into an LED strip. The wire between segments carries the current of every segment after it. Feed the strip from both ends, or inject power every few metres, and the last pixel stops being a dimmer cousin of the first.
Connectors, not wire. A cheap barrel jack or a Dupont run can add more resistance than two metres of decent cable. If a joint is warm, that is where the loss is. The paper amp rating is a starting point, not a guarantee.
Battery projects. The wire from cell to regulator is often the thinnest part of the build and the one carrying the highest current. Shorten it before you thicken it, and keep the ground return in mind: the drop is in both legs.
Grounding on mixed-voltage builds. A shared ground that has visible drop between two boards turns into signals that read wrong, not just power that sags.
Do the sum for the exact build
Rules of thumb get you close. For a specific rail and length, put the real numbers in and read the answer: the voltage drop and wire gauge calculator takes the volts, the one-way length, the current and the gauge, and returns the drop in volts and percent, the 3% check, and the smallest gauge that passes. Every result is a link, with the inputs in the query string, so you can send the exact scenario to whoever is wiring it.
Free, no signup, no ads. It is one of the bench tools we keep for the builds we write about weekly at TinkerNews.
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