0mpilchfamilyBest Answer 5 years ago What are the specs of the LEDs? Take the specs. CAT5 uses 24AWG wire. Use that in this calculator and play around with the lengths till you find the max length that keeps the end voltage and current within the LEDs specs. So if the LED requires 3V @20mA. You'll still put 5V into the calc cause that's what your getting from the Arduino. And 0.20 A as the current then play around with the length.http://www.calculator.net/voltage-drop-calculator....Basically you'll be over 100 ft before it drops to 3V. Keep in mind you'll still need a current limiting resistor on the LED.