Problems extending a 12V power lead to an RGB-LED strip
The voltage drop over 20 meters of cable will vary based on the load, which you will no doubt be varying based on the number and color of leds you turn on. The change in voltage will cause issues with the led controllers. Use a local regulator, or capacitors, or both. Or use standard AC extensions and keep the power supply local to the strip. 

The diameter of the cable will affect it, because a smaller diameter wire has a higher resistance and a higher resistance means a bigger voltage drop as the current increases.