Your LED strip works at the bench. Then you mount it behind a shelf, run it off a 5 m reel, and the far end comes out pink.
You check the power supply: 12 V, 5 A, plenty. You check the wire: 18 AWG. The ampacity table says 18 AWG is good for 16 amps. Five amps is nothing. So why is the far end wrong?
Because ampacity and voltage drop are two different limits, and the table only tells you one of them.
The heat limit vs the delivery limit
Ampacity answers: will this wire melt or catch fire? It is about heat. 18 AWG in free air is rated around 16 A. That number assumes a short run, normal insulation, and an acceptable temperature rise.
Voltage drop answers: will the devices at the far end still get the voltage they need? It is about resistance. Every metre of copper has resistance, and the current you push through it turns into a voltage you lose along the way.
A wire can pass the ampacity check and fail the voltage drop check at the same time. That is the trap.
The numbers for a real strip
Take a common setup: 5 metres of LED strip drawing 5 A at 12 V, wired with 18 AWG.
The current does not travel 5 metres. It travels to the far end and back: 10 metres of round trip.
At 18 AWG, that run costs you roughly 9.6% of your voltage (our voltage drop calculator puts it there at 5 A). You paid for 12 volts and delivered about 10.8 to the last LEDs. A tenth of a volt at a time is fine; nearly a volt and a half is not.
And the current does not spread evenly along the strip. The first section carries the whole load; the last section carries the least. The far end sits at the lowest voltage in the system, which is why it changes colour first.
Why colour, specifically
An LED is a diode: it starts conducting at a threshold voltage (its forward voltage). Drop the supply and the current through it falls — fast. A small voltage change becomes a large brightness change, and on RGB strips the channels do not shift equally, so the mix drifts. Pink at the end is a red channel that kept working and a blue or green channel that faded.
The strip is not broken. The wiring is under-delivering, and the symptom just happens to be visible as colour.
What actually fixes it
- Go up in gauge. Every three steps (18 → 15 → 12) roughly doubles the copper and halves the resistance. Going 18 → 15 alone cuts the drop to about half.
- Inject power at both ends — or every metre or two on longer runs. Parallel feeding halves the effective round trip and evens out the current along the strip.
- Run higher voltage where the LEDs allow it. The same watts at 24 V is half the current, so the same wire drops half the volts.
- Keep the target in mind: under 3%. That is the number where the far end still looks and behaves like the near end. Our wire gauge chart carries both the ampacity columns and the drop maths on one page, because you need both to pick a wire honestly.
How to measure instead of guessing
You do not need to trust a table. Put a multimeter on the far end under full load — a strip with nothing connected pulls almost no current, so its voltage reads healthy and lies to you. With the strip lit at the brightness you actually run, measure:
- the supply voltage at the strip's input,
- the voltage at the far end,
- the difference as a percentage of the supply.
Above 3%: thicker wire or more injection points. Under 3%: you are done, and the colours at both ends should match.
The rule to take away
Ampacity tells you the wire survives. Voltage drop tells you the project works. Size for the drop, then sanity-check the amps. The wire that passes the heat test can still be the reason your build looks broken, and that is a wiring decision — not a mystery.
The free tools used here (wire gauge chart, voltage drop calculator, battery life and LED current calculators) all work without signup, because looking up the numbers should be faster than the mistake.
