Different types of metal conduit
Metal conduit can come in many shapes and materials. It can be made of galvanized steel or stainless steel. Conduit is not usually made from other types of metals.
Metal conduit can sometimes be used to ground a circuit, but its length is very limited. It is safer to use cables with a ground wire.
Rigid Metal Conduit (RMC)
RMC is a thick-walled threaded tubing. It can be made from coated steel, stainless or aluminum. Connect the conduit to the main tubes by attaching connectors. Rigid metal conduit offers protection against impacts and other damage. For short runs, it can be used to ground wires. RMC's thicker walls protect the cables from electromagnetic interference (EMI), which can cause damage to sensitive equipment.
To protect the insulation from abrasion, it is important to reamed the ends of rigid conduit. To prevent galvanic action, fittings must match the conduit tubing's metal.
Two main drawbacks exist when using rigid metal conduit. The first is that all connections are threaded. This means that more labor is required to install the system. RMC also becomes more expensive. The conduit's thick walls make it more difficult to install and therefore costlier.
Electrical Metallic Tubing (EMT).
EMT is a thin-walled, unthreaded metal tubing that is generally made from coated steel. However, it can also be made from aluminum. EMT is used for electrical conduit in commercial buildings but not in residential buildings. EMT is attached using clamp-type fittings, which slide onto the tubing. Then they are secured with a set screw. Electrical Metal Tubing costs 40% less than GRC and is 40% lighter.
This image shows the clamp fittings and electrical metal tubing. Metal channel framing supports the conduits.
EMT offers good protection to the cables within, but it shouldn't be used in areas that are susceptible to severe damage like power plants or near vehicles. EMT is not recommended for areas where there are corrosive fumes or vapors. Special corrosive-resistant tubing, gasket clamps, and other suitable equipment are needed.
As with RMC, all edges must be removed from the cut ends of tubing.
EMT can be dangerous for electricians who aren't familiar with the process. The clamps may loosen over time and cause the set screws to come undone. To prevent this from happening, you can use a driver or torque wrench to ensure that the screws are in the right tension.
Intermediate Metal Conduit (IMC)
Intermediate Metal Conduit (IMC), as its name implies, has walls that are thinner and thicker than RMC but more dense than EMT. IMC weighs between RMC or EMT. Intermediate Metal Conduit can be threaded, but it can also unthreaded and used in conjunction with clamp-type fittings. IMC can be coated and is usually made of steel.
Flexible Metal Conduit (FMC)
Flexible metal conduit (FMC), is usually available in sizes between 3/8" to 3", although larger sizes are sometimes possible. This hollow tube can be made by coiling steel strips or self-interlocked aluminum. FMC is available in two thicknesses: a full wall or standard. FMC can also be made extra flexible by most manufacturers for tighter bend radiuses. However, this is not UL approved.