Electrical conductors are subject to stringent installation requirements, established in the NFPA National Electrical Code and the NYC Electrical Code, to which construction engineers must abide. There are many logical reasons for this.
A conductor in the open is vulnerable to physical damage, and at the same time it represents a high risk of electric shock or fire. Therefore, conductors must have both electrical insulation and physical protection; unless a conductor is armored or sheathed, physical protection is typically provided by electrical conduit.
The different types of electrical conduit in the market differ in terms of material used and flexibility: conduit can be either metallic or non-metallic, as well as rigid or flexible. Although each type is intended for different applications, there is some overlap between approved uses. Therefore, design engineers must often choose between many valid options for a given application. Sizing is very important: undersized conductors cannot accomplish their function, but oversized conductors represent a waste of capital.
This article will provide an overview of the main types of metallic electrical conduit and their applications. Keep in mind this is a general guide, not a replacement for NFPA and NYC codes. The technical requirements explained here are very general – make sure you check the applicable codes before specifying conduit in any project. There are five main types of metallic conduit, which are summarized in the following table:
|Electrical Metallic Tubing|
Rigid Metal Conduit
Intermediate Metal Conduit
Flexible Metal Conduit
Liquidtight Flexible Metal Conduit
Electrical Metallic Tubing (EMT)
EMT is a lightweight but rigid metallic raceway option. If offers less mechanical protection compared with IMC and RMC, but it has the advantage of being easy to bend, which is beneficial when construction engineers must build the electrical raceway around obstacles or corners. The most commonly used EMT materials are galvanized steel and aluminium.
Since EMT is not normally threaded at its ends, fittings use perpendicular screws or threaded compression unions. Set-screw fittings are cheaper, but compression fittings offer a tighter connection.
Electrical codes do not allow EMT in applications where electrical raceway is exposed to significant physical damage or corrosion, or in occupancies classified as hazardous locations.
Rigid Metal Conduit (RMC)
RMC is the heavy-duty option, with the thickest walls among all metallic conduit options. This type of conduit is the standard choice for demanding environments, offering both mechanical and chemical resistance. RMC is normally made from galvanized steel, stainless steel, red brass or aluminium. All types are suitable for corrosive environments, but additional protection may be required in the case of aluminium RMC.
RMC offers far greater mechanical resistance than EMT, but this comes with a much higher price tag. Working with RMC also involves more technical complexity, requiring specialized equipment for cutting and threading.
Intermediate Metal Conduit (IMC)
As implied by its name, IMC is the intermediate option, thicker than EMT but thinner than RMC. However, IMC uses a high-strength steel alloy to offer physical protection comparable to that of RMC, in spite of the reduced wall thickness. IMC can be used in the same applications where RMC is allowed, and it only has one limitation: while RMC trade sizes range from ½” to 6”, IMC only goes from ½” to 4”. Therefore, you must use RMC in heavy-duty applications where the specified conduit size exceeds 4”.
It is important to note that, although IMC is thinner than RMC, the external diameter is the same for both types of conduit. As a result, IMC has slightly more internal space to handle conductors.
Flexible Metal Conduit (FMC) and Liquidtight Flexible Metal Conduit (LFMC)
In the electrical trade, FMC is normally called “greenfield” or “flex”. The body of FMC uses an interlocked steel spiral to offer decent mechanical protection but also flexibility. FMC is typically used when raceway ends require flexibility for connection, or when a connection to vibrating equipment that may cause fatigue failure in a rigid connection. LFMC is basically FMC with a liquid-tight coating, typically made from a thermoplastic material.
Additional Recommendations from Construction Engineers
Keep in mind that conduit diameter is determined by conductor diameter, which in turn is determined by the load on the circuit. Therefore, energy efficiency measures can lead to conductor and conduit savings in new constructions. The savings from using a smaller conductor and conduit diameter may not be noticeable for a single branch circuit, but the savings add up in a large project such as a high-rise building.
MEP design software is also a very powerful tool to reduce conductor and conduit costs. When circuit routes are specified as short as possible, material requirements are reduced, along with the associated man-hours from associated construction engineers and others.