The insurance margin consists of four parts: ● The basic derating is 25%, and the load may be reduced by more; ● The temperature range, the high temperature of the engine compartment, the derating is further 10%; , you can only lean upward; ● Consider I2t, you may move up a gear and use a larger fuse, otherwise the lifespan is not allowed. Due to the long body of commercial vehicles, a wire may be ten or twenty meters from front to back. It is also necessary to consider the failure of the insurance caused by the “long wire effect” to avoid the occurrence of short-circuit current that is not enough to fuse the insurance, resulting in continuous heating, and then the wiring harness burns out accident.