Conduit Sizes 101
Conduits protect the conductors inside them from various environmental factors. They can also simplify construction and repair work, making it easy to access wires and move them through the building. Conduit sizes can vary in size from ¼ inch on up, with each conduit size having different threads per inch as well as inside and outside dimensions. The knockout size will vary with each conduit size as well. In this quick guide to conduit sizes, you will have a basic knowledge to work successfully with conduits. All sizing is determined by professional standards so you will find that it is constant from manufacturer to manufacturer.
When you are determining the conduit you need, there are several factors to look at. Be sure to look at all available tables and codes when determining what conduit is best for your application. There are also many different types of conduits (for example, rigid, intermediate metallic, aluminum, etc.) and each has different dimensions as well as different uses. Be sure to use the correct conduit for the job at hand.
The Fill Factor
The fill factor or conduit fill is the maximum amount of space that the conductors running through the conduit can occupy. This would be your most important factor in determining the conduit to choose for a job. There are strict regulations in dealing with the fill factor. The National Electrical Code of 1996 determines the total number of conductors or multi-conductor cables that can safely be in a conduit. A cross-sectional area of the conductors inserted would be measured and the code would determine the percent of the conduit which could be filled. A conduit can never be filled 100 percent as it is not safe. The code has determined how wires and bundles of wires can safely run through a given conduit. The table does not address any difference in high or low voltage cables so you can apply the table for both uses. Have this information readily available when shopping for conduits.
The ID and OD
These are roughly the dimensions that are acceptable for each size conduit. There will also be a maximum OD. This is a fixed number that can not be deviated from. For instance, a 1-inch rigid conduit will have an ID of 1.063 inch nominal while a 1 inch intermediate metallic conduit will have an ID of 1.12 inch nominal, yet both have the same knockout size of 1.375 inches. Their maximum OD will also vary with 1.331 inches for rigid and 1.295 inches for intermediate nominal.
It may sound backward but use the diameter of the conduit as a guide to the service needed. It is better to use a conduit that is too big than one that is too small. For example, for 60 amp service you would use a minimum 1-inch conduit, for 100 amp service you would use a minimum 1¼ inch conduit, and for 200 amp service you would use a minimum 2 inch conduit.
Choosing a Conduit for Underground Installations
When using conduit underground it is best to use a larger diameter conduit, typically larger than 2 inches. These typically hold items such as communication systems.