A discussion of the various kinds of conduit with guidelines to help you choose the right one
Conduit looks like plumbing pipe and serves to protect conductors from damage caused by moisture or impact. Several different conduit options are available; you will want to assess a job’s particular needs to determine which type is right for it.
Thinwall metal conduit is usually chosen for areas where it will be exposed, such as garages or utility rooms. Type EMT, as it is commonly known, is used for applications that do not require a lot of bending to fit around obstructions. Though EMT can be bent, it is much easier to buy prebent pieces. EMT is sold in lengths up to 10 feet; diameters range from 1/2 inch to 2 inches. (For more information, see How to Install Thinwall Metal Conduit.)
Flexible metal conduit looks like armored cable, but it contains no conductors. Also called Greenfield, it is the conduit of choice for large appliances such as water heaters and for areas where routing EMT would be too difficult. It can be used in outdoor and indoor applications. Flexible metal conduit is sold by the foot or in spools of 25 or 100 feet. Diameters are 1/2, 3/4, and 1 inch. (For more information, see How to Install Flexible Metal Conduit.)
Rigid nonmetallic conduit, made from PVC plastic, is a lighter alternative to metallic conduit for interior use. Schedule 40 conduit is sold in lengths of 10 feet. It should not be confused with PVC plumbing pipe, and it should only be used with housing boxes made from PVC as opposed to the nonmetallic boxes used with cable. (For more information, see How to Install Rigid Nonmetallic Conduit.)
Assessing the application of the conduit, and the number and size of the wires it will accommodate, will determine the conduit diameter you will need. The chart above offers some guidelines.