The requirements of IET Regulations make it clear that circuits must be designed and the design data made readily available. A typical situation where rating factors need to be applied would be in the calculation of cable sizes for a lighting system in a large factory. Here many cables of the same size and loading may be grouped together in trunking and could be expected to be fully loaded all at the same time. With the modern trend towards energy saving and the installation of thermal insulation, there may be a need to derate cables to account for heat retention. The resistance of a conductor increases as the length increases and/or the cross-sectional area decreases. Associated with an increased resistance is a drop in voltage, which means that a load at the end of a long thin cable will not have the full supply voltage available.