A Power Over Ethernet (PoE) system, or other power over data lines system, includes Power Sourcing Equipment (PSE) providing combined data and voltage over wires to a Powered Device (PD). Since cable length and PD load currents may not be known, there is a variable voltage drop along the cable between the PSE and PD. Prior to the PD being fully powered up, a test is performed by the PSE to determine the actual resistance or voltage drop of the cable, and the results are stored in a memory accessed by the PSE upon powering up. The PSE uses the stored information to adjust its voltage source to provide a target voltage at the PD input during full power operation. This may obviate the need for a voltage regulator at the PD. The test may only be conducted when the PSE is initially powered up or may be conducted periodically.