The method of calculation provided by this recommended practice applies to the following conditions : Continuous currents and thermal equilibrium. Current may be either direct. or alternating. Alternating current may be single or multiphase@ at frequencies up to 800 cycles per second: above 800 cycles@ the AC resistance must be corrected for skin and proximity effects. Insulated wires in air subject to convection and radiation. Heat transfer from the wires to the surroundings by conduction is negligible. Specific environmental conditions such as altitude@ ambient air temperature@ type of convection@ etc@@ must be fixed for each calculation. Wire construction. Conductors of circular cross-section ; fabricated of any metal@ solid or stranded. If stranded@ the conductor should form a compact circular unit. Close-fitting insulation@ concentric with the conductor@ consisting of one or more layers of different materials.