The selection of the correct diameter conduit requires the use of National Electric Code tables to find wire type diameters, fill amounts and conduit diameters. The National Electric Code has defined multiple tables based on the type of wire and conduit used.
Choose the table that matches the type of wire and the type of conduit being used. Calculate the diameter of the conduit by the total number of wires when placing the same gauge and type of wire in a conduit. Read the first column in the table to locate the wire gauge. Read across from the wire gauge to identify the maximum number of wires that one can place in a given conduit diameter. Choose a value greater than or equal to the number of wires chosen to place in the conduit.
When inserting wires of different gauges inside a conduit, determine the total diameter of the wires to obtain the diameter of the conduit. In this case, find the wire diameter for each of the gauge wire and type. Multiply the amount of wires for each gauge and type by the wire diameter, and sum up the results.
Locate the type of conduit none wishes to use. Read from the maximum allowable fill for the type of conduit and size of conduit. Select the first conduit fill amount that equals to or is in excess of the total fill earlier calculated.