Copper is a very good conductor of electricity, but it isn’t perfect. It has a certain amount of resistance, determined primarily on its cross-sectional area (but also by its purity and temperature). This wiring resistance is “seen” by the amplifier output as part of the load; if a cable with a resistance of one ohm is connected to an 8- ohm speaker, the load seen by the amplifier is 9 ohms. Since increasing the load impedance decreases current flow, decreasing power delivery, we have lost some of the amplifier’s power capability merely by adding the series resistance of the cable to the load. Furthermore, since the cable is seen as part of the load, part of the power which is delivered to the load is dissipated in the cable itself as heat. (This is the way electrical space- heaters work!) Since Ohm’s Law allows us to calculate the current flow created by a given voltage across a given load impedance, it can also give us the voltage drop across the load, or part of the load, for a given current. This can be conveniently expressed as a percentage of the total power. Hope that helps.