Fuses are installed to protect the power wires from overheating during overloads or malfunctions. The rating of the protection device in the simplest case should be less than or equal to the rated current load of the conductor. Each wire connected to the battery must be protected by a fuse
An overload occurs when the motor, inverter is running, or when more devices are plugged into the sockets at the same time. If the current in the circuit increases and for a long time keeps at the level of 110-150% of the rated current load of the conductor, then the wire and the protective device will heat up. If the operating mode does not change, the accumulated heat will damage the wire. To prevent this from happening, the wires must be protected from overload.
Most fuses will blow when the current is about 1.3 times their rating. Therefore, in order to limit the continuous current and not allow it to strongly heat the wire, the fuse rating is chosen equal to 80% of the current carrying capacity of the conductor.
In the tables, the current carrying capacity is indicated for conductors located in open areas with good air circulation. Heat dissipation is worse in cable ducts or inside partitions. Therefore, to a critical temperature, the wire will heat up even when less current flows through it. If the wire is laid in a cable duct or inside a partition, before choosing a protection device, its current-carrying capacity is reduced
Suppose we need to protect a power wire with a cross section of 25 sq. Mm against overload, the insulation of which can withstand a temperature of 105 C. According to the table, the maximum permissible continuous current for this wire is 170 A. The fuses are tripped at a current of 130% of the nominal. Therefore, to protect the wire, a fuse with a rating of 80% of 170A or 130A is needed. It will burn at a current of 1.3 x 130 A = 169 A.
The current flowing in the circuit heats not only the conductor, but also the fuse. To prevent the fuse from overheating, the continuous current must not exceed 80% of its rating. For a wire with a cross section of 25 mm2, we chose a 130 A fuse.The continuous current through it should not exceed 130 x 0.8 = 104 A. If the load in the circuit exceeds 100 A, it is necessary to increase the cross section of the power wire and select a larger fuse.