50 feet is the maximum recommended 0-10V dimming cabling length. Longer runs may work, but increase the chance of functional problems due to interference.
Here is the explanation why:
2 factors come into play:
1) Voltage drops from length
2) Interference picked up by length
For voltage drops, assuming worst case the longest run should be 123 feet to limit voltage drops to .1V.
Doing the math for 20AWG wire:
20AWG is 10.15ohms per 1000 feet
40mA is the max current
.1V is the max allowable voltage drop
.1V/40mA = 2.5 ohms
2.5ohms x 1000’/10.15 ohms = 246.3’ round trip
246.3’/2 = 123.15’ will result in a .1V drop worst case.
So that puts an upper limit, but more importantly a long conductor will pick up more and more induced voltage which is the issue. We found sources (http://sensortech.wordpress.com/2013/01/10/cable-length-for-analog-sensors/) and field experience suggests that 50 feet is a safe limit (ideally with shielded cable, or at least twisted pair).