Abstract: Cooling of nanoelectronic devices below 1mK is a challenging task, since the thermal coupling with the dilution refrigerator becomes weak at low temperatures and electronic devices are extremely susceptible to external heat leaks such as microwave radiation and electrical noise. Despite these technological challenges, there is a completely new world of physics, which can be explored once low temperatures are achieved. To reach such low temperature we developed a parallel network of nuclear refrigerators, which combines magnetic nuclear refrigeration technique with transport setup, by cooling of single measurement leads. In order to evaluate the cooling capacity of the setup, we developed various on-chip thermometry to read temperature below 10 mK. In particular, we focused on normal metal-Insulator-superconductor tunnel junctions, which are used as primary and secondary thermometer down to 7 mK. Furthermore, we implemented nuclear refrigeration of Coulomb blockade thermometer, proving cooling of the device down to 2.8 mK, which is the lowest temperature ever reported so far in a nanoelectronic device.