This chapter shows what resistance is and how it affects current flow. A load has a fixed resistance, so a reduced current due to cable resistance means that there must be less voltage across the load. In order to ensure that loads are not deprived of too much of their operating voltage due to cable resistance, the IET Wiring Regulations recommend that the voltage drop in a circuit should not exceed a certain percentage of the voltage at the origin of the circuit. Current-using pieces of equipment in installation circuits are connected across the supply and are therefore wired in parallel. The cable supplying such equipment, however, is connected in series with the parallel arrangement. The rate at which that energy is converted is called power. Two engineering pioneers, Watt and Joule gave their names to the units of power and energy.