The basis for the design of the AC protective devices is the rated current $I_\text{B}$. For miniature circuit-breakers with characteristic B, the rated currents 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A are available. The rated current is calculated using the maximum output current of the connected inverter $I_\text{AC,max}$ and the rated factor $f_\text{B}$. $$ I_\text{B} = I_\text{AC,max} \cdot f_\text{B} $$
| Circuit breaker Characteristic | Rating factor $f_\text{B}$ |
|---|---|
| B | 1,3 |
| C | 1,1 |
The cable power loss $P_\text{ver}$ results from the lead resistance $R_\text{L}$ and the current flowing through the conductor $I_\text{L}$
$$ P_\text{ver} = R_\text{L} \cdot I_\text{L}^2 $$
The line resistance $R_\text{L}$ depends on the line cross section $A$, the line length $l$ and the material-dependent specific electrical resistance $1/\kappa$.
$$ R_\text{L} = \frac{l}{A} \cdot \frac{1}{\kappa} $$
Table 1: Overview of the specific electrical resistances of different materials
| Material | specific electrical resistance $1/\kappa$ in $\Omega \cdot \text{mm}^2 \cdot \text{m}^{-1}$ |
|---|---|
| Aluminium | $2,94 \cdot 10^{−2}$ |
| Copper | $1,75 \cdot 10^{−2}$ |
Relative losses $K$ result from power dissipation $P_\text{ver}$ and reference power $P_\text{ref}$.
$$ K = \frac{P_\text{ver}}{P_\text{ref}} $$
The relative loss can be used to calculate the dependent line cross section $A$. It is:
$$ A = \frac{l \cdot I_\text{L}^2}{\kappa \cdot P_\text{ref} \cdot K } $$
See also