Voltage loss over wire
This is what you actually have with a 2A load and 30ft of 18 Guage.







simulate this circuit &ndash; Schematic created using CircuitLab

As you can see, voltage is dropped over both the wires leaving you with 11.27V at the load assuming the load is constant.

However, loads are seldom constant, and that 6 ohms can easily, and randomly, increase to hundreds of ohms depending on what the load is doing. At that point the current falls and the voltage seen across the load becomes much closer to the supply voltage. As such, increasing the supply voltage to compensate for the worst case drop can cause over-voltage issues instead. The latter can be much more damaging.



The other issue you need to worry about is if you are transmitting signals between the local and remote device. Because of the voltage drop along the wires there is now a difference in reference levels at each end of the wire.







simulate this circuit

In the schematic above the low level digital signal is being sent back from the remote device, however, that signal is at a higher reference level than the local device. As such the local device sees a higher than normal low level on the wire. If that difference is high enough the logic may not work.

If the power currents are switching, the power switching noise, caused by the changes in the voltage drop, will be coupled directly into the signal line. If it is not too bad, that may be acceptable on a digital signal, but is a killer on an analog signal.