When routing power how is a safe current-carrying wire configuration determined with multiple wires?
Determine the cross-sectional area of the wire being replaced, and then divide that by the cross-sectional area of the smaller wire in order to determine how many smaller wires you'll need to equal the cross-sectional area of the larger wire.  If the quotient isn't an integer, ALWAYS round up to the next largest integer.  
Determine the resistance of one wire as you normally would, and then the resistance of the cable will be that of the resistances of the separate conductors all connected in parallel.  The voltage drop across the cable will then be the product of the current through the cable and its resistance.