The illumination level provided by the road and bridge lighting facilities is relatively low, and the human eye is in an intermediary visual state. At this time, the distinguishing object is not realized by the color difference between the two, but depends on the brightness difference between the object and the background. Taking road lighting as an example, the road lighting load on a bridge is uniformly distributed regardless of single-side lighting, interlaced lighting, symmetrical lighting, or central lighting. When the voltage loss is calculated, it can be assumed that the distribution load is concentrated on the distribution. The midpoint of the line segment is calculated according to the concentrated load. According to the requirements of the bridge power supply quality, the end voltage of the bridge lighting load line should not be less than 90% of the rated voltage, not more than 105% of the rated voltage; the end voltage of the power load line on the bridge should not be less than 90% of the rated voltage.

   For the general low-voltage lighting circuit, because of its higher voltage level requirements, the cross-section is generally selected first according to the allowable voltage loss conditions, and then the heating conditions and mechanical strength are verified. For low-voltage power lines, because of their large load currents, the cross-section is first selected according to the heat conditions, and then the voltage loss and mechanical strength are verified.