U = R * I, where R is the resistance of the 8 meters of wire.
And, for example, if the total voltage drop is 0.4V, then the device would see 2.9V over it, with the negative input 0.2V higher than the power supply -, and positive input 0.2V lower than the power supply +.
Note that in reality cable resistance is not a finely controlled parameter - even if you buy wire rated to exact same cross-sectional area, resistance could easily vary by tens of %, so you get the exact answer by taking a measurement.
Also remember that contacts (connectors, screw terminals etc) also have resistance, something that might produce a significant voltage drop, too.