A system for providing power from a power sourcing equipment, PSE, to a powered device, PD, over a communication cable, wherein the communication cable includes first and second power pairs of wires (162, 164) configured for supplying power to the PD, and at least one spare pair of wires (166, 168), which is not used for transferring power, the system comprising a cable resistance measuring mechanism for determining resistance (R P) of a pair of wires in the communication cable, the cable resistance measuring mechanism configured for: determining in said PSE a value of a first response signal (V 01) detected in said PSE at the spare pair of wires (166, 168) in response to a first reference signal (I REF1) supplied by said PSE over the spare pair of wires (166, 168), determining in said PSE a value of a second response signal (V 02) detected in said PSE at the spare pair of wires (166, 168) in response to a second reference signal (I REF2) supplied by said PSE over the spare pair of wires (166, 168), determining in said PSE a value of a third response signal (V 03) detected in said PSE at the spare pair of wires (166, 168) in response to a third reference signal (I REF3) supplied in said PSE over the spare pair of wires (166, 168), where the first, second and third reference signals (I REF1, I REF2, I REF3) differ with respect to each other and wherein the second reference signal corresponds to a function of first and third reference signals, and determining the resistance (R P) of a pair of wires in the cable based on values of the first, second and third response signals (V 01, V 02, V 03) and values of the first, second and third reference signals (I REF1, I REF2, I REF3), wherein the determined resistance (R P) is used to allocate by said PSE additional power to said PD to compensate for power loss over the communication cable, characterized in that the second reference signal (I REF2) is the square root of the first reference signal (I REF1) and the third reference signal (I REF3). determining in said PSE a value of a first response signal (V 01) detected in said PSE at the spare pair of wires (166, 168) in response to a first reference signal (I REF1) supplied by said PSE over the spare pair of wires (166, 168), determining in said PSE a value of a second response signal (V 02) detected in said PSE at the spare pair of wires (166, 168) in response to a second reference signal (I REF2) supplied by said PSE over the spare pair of wires (166, 168), determining in said PSE a value of a third response signal (V 03) detected in said PSE at the spare pair of wires (166, 168) in response to a third reference signal (I REF3) supplied in said PSE over the spare pair of wires (166, 168), where the first, second and third reference signals (I REF1, I REF2, I REF3) differ with respect to each other and wherein the second reference signal corresponds to a function of first and third reference signals, and determining the resistance (R P) of a pair of wires in the cable based on values of the first, second and third response signals (V 01, V 02, V 03) and values of the first, second and third reference signals (I REF1, I REF2, I REF3), wherein the determined resistance (R P) is used to allocate by said PSE additional power to said PD to compensate for power loss over the communication cable, characterized in that the second reference signal (I REF2) is the square root of the first reference signal (I REF1) and the third reference signal (I REF3).