Something you may notice for very long runs of LED strip is that they can be bright at one end and dim at the other. That’s because the strips are two long power rails with a bit of internal resistance and current through the LEDs at the far end have more of that resistance to travel through.

Here’s how LED strips are typically wired:

The vertical stack of LEDs is distributed along the strip somewhat, which is why you’re restricted to cutting the strip at regular intervals of every three or six LEDs.

Let’s simplify that by treating the LED circuits as lamps:

You can hover over a lamp to see where the current flows. The further you go from the power supply the greater the cumulative resistance of the power rails.

The expedient but costly solution is to reinforce the power rails in the strip by soldering some heavy hookup wire onto the strip at some of the cut points where you don’t actually cut it (once every metre should be plenty; more frequently would be unnecessarily tedious).

But if you happen to be running the strip in a loop, such that the ends end up somewhat close to each other, or if you’re willing to run one length of hookup wire alongside the strip, then there’s a simpler fix for that difference in brightness.

Connect one side of the power supply to the near end of the strip, and connect the other side of the power supply to the far end of the strip. Be careful to still connect minus to minus and plus to plus in the usual way, though. Like so:

This way the length of the circuit through each LED is (approximately) the same, and so the resistance is the same and they all come out the same brightness all the way along the strip.

You’ll see this in some prefabricated lighting strings which are not designed to be cut. They’ll have a third wire which is not be connected directly to the LEDs, but at the end it’ll be connected to one of the other wires, and that will complete the circuit from the far end back to the power supply to balance things out. If you cut those then they won’t work anymore, because you would need to reconnect two of the wires at the cut point.

It doesn’t matter if the power supply hookup lines are different lengths. Having the total length be unnecessarily long will be less energy efficient, but they affect all the LEDs equally regardless of whether or not both sides are the same length.

It might be tempting to link both ends of the strip together in tee intersections with the power supply. That should work, and you’ll get more light out of the system overall, but you may still see a bit of dimming in the middle of the loop.