Accurate evaluation of Self Heating Effects in highly down-scaled devices becomes essential for improved performance and reliability. However, complex structure of BEOL causes analysis of SHEs to be difficult To remove the difficulty, based on Rent's rule to obtain interconnect density function, effective thermal conductivity of BEOL versus metal volume density and average aspect ratio (p) was calculated. With results above, TCAD simulation for SHEs was performed in 5 nm node nanowire FET. As a result, lowered thermal conductivity by complicated structure can bring underestimated SHEs through simulation.