Voltage Drop & Wire Gauge Calculator
The LED-strip and automotive wiring question, answered: enter the run length, the current and the gauge to get the drop in volts and percent, the 3% rule verdict and the gauge you should actually use. Enter the physical one-way length; the calculator doubles it, because the current goes out on one conductor and returns on the other.
Result
V drop = I × R × 2L = 5 A × 22.9 mΩ/m × 10 m = 1.15 V (9.56%)
| Quantity | Value |
|---|---|
| Voltage drop | 1.15 V (9.56%) |
| Voltage at the load | 10.9 V of 12 V |
| Wire resistance | 229 mΩ over 10 m round trip |
| Power lost in the wire | 5.73 W |
| Verdict | 9.56% exceeds your 3% limit. Use at least 14 AWG (2.5 mm²), shorten the run, raise the voltage, or feed both ends. The load sees 10.9 V. |
| Smallest gauge within the limit | 14 AWG (2.5 mm²) |
Method and when not to trust it. Copper resistivity 1.72×10⁻⁸ Ω·m at 20°C, round-trip length modelled. It ignores connector resistance (often bigger than the wire), temperature (hot wire drops more) and AC skin effect (irrelevant at DC). For precision work, measure at the load under full current.
Quick gauge guide (12V, 3% drop, round trip)
| Current | 2 m | 5 m | 10 m |
|---|---|---|---|
| 2 A | 20 AWG | 18 AWG | 16 AWG |
| 5 A | 18 AWG | 16 AWG | 14 AWG |
| 10 A | 16 AWG | 14 AWG | 12 AWG |
| 20 A | 14 AWG | 12 AWG | 10 AWG |
Worked example
A 5 m LED-strip run at 12V pulling 5 A over 18 AWG (0.75 mm²): round trip 10 m × 0.023 Ω/m = 0.23 Ω, so 5 A × 0.23 Ω = 1.15 V drop, or 9.6%. The verdict: undersized. Step up to 14 AWG (2.5 mm²) for 2.9%, or feed the strip from both ends. Open this example.
FAQ
What is the 3% voltage drop rule?
The standard low-voltage wiring convention: 3% on the branch, 5% in total. Beyond that, gear misbehaves: motors heat, LEDs dim and sensors drift.
One-way or round-trip length?
Round trip, because the current goes out and comes back. Enter the physical run length; this tool doubles it internally.
Why do connectors matter?
A cheap barrel connector can add 10–30 mΩ. At 10 A that is 0.3 V, more than meters of good wire. Crimp or solder high-current joints.
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