The book demonstrates calculation techniques of industrial frequency magnetic field intensity in surrounding space from currents four-core wires and cables of electric circuit. Calculations for defining the character of changes in magnetic field intensity in surrounding space from currents four-core wires and cables with square cores layout for balanced and open-phase electric circuit conditions. Analytical expressions using probabilistic characteristics were obtained, and calculations results allowing to evaluate electromagnetic compatibility conditions for technical means and electromagnetic security for operating personnel with respect to permissible values of industrial frequency magnetic field intensity in space around wires and cables office (industrial) premises electric supply systems are provided. Additionally, study results of electric and magnetic fields in cable channels and electric supply distribution units rooms are demonstrated. The book is intended for engineering personnel dealing with design and operation of electric supply system in office, living and industrial buildings, environmental problems and life-protection.