Using an electrical conduit is the required method of protecting electrical wires in exposed situations where NM cable cannot be used. Electrical conduit, whether it be rigid metal (EMT), rigid plastic (PVC), or flexible metal (FMC), is limited as regards the maximum number of electrical wires that can be run inside the conduit. This rating, called conduit fill capacity, is specified by the National Electrical Code and followed by most local codes, which serve as the governing law in any given area.
Defining a fill capacity and limiting the number of conducting wires is a matter of safety. Electrical wires heat up slightly under the flow of current, and restricting the number of wires allowed in the conduit is a means of limiting heat build-up and ensuring that heat inside the conduit can dissipate.
It's critical to adhere to conduit fill capacity. Too many wires carrying too much current carries the danger of generating enough heat to melt the vinyl insulation on the wires and cause a serious fire hazard.
The fill capacity is based not only on the conduit type and size but also on the type and size of the wire itself. The first step in finding the fill capacity is properly identifying the conduit material.
Electrical metallic tubing (EMT), sometimes called "thin-wall," is the most common type of rigid metal conduit used in residential construction. Because it is metal, it can act as the ground connection when it is connected to metal electrical boxes and metal fittings. The tubing comes in 10-foot lengths, which can be joined together with couplings or elbows, conduit bodies, and other fittings.
EMT couplings hold tubing pieces together and also bond the ground connection between the conduit sections. Couplings come in setscrew and compression types. Setscrew couplings have two screws, one for each piece of the conduit. Compression couplings have retainer rings and screw-on ends that tighten down on the conduit for a tight connection. EMT tubing connects to metal electrical boxes with locknut connectors to make a tight connection that is also electrically conductive.
Rigid polyvinyl chloride (PVC) is a plastic conduit that is often used for underground installations and in wet areas. This conduit comes in 10-foot lengths and has a built-in coupling formed on one end, called the female side. PVC conduit pieces typically are joined permanently with PVC solvent glue, similar to PVC plumbing pipe connections. Since PVC has thicker walls than metal conduit, it can hold fewer wires. PVC also is nonconductive, so it cannot serve as a grounding conductor.
PVC conduit comes in two common grades, specified by the thickness of the conduit wall:
- Schedule 40 PVC is a thinner-walled conduit that is used for most simple underground installations, such as running a feed through the yard to an outlet on a shed.
- Schedule 80 PVC is a thicker-walled conduit that is used in high-traffic areas, such as under parking lots and driveways.
Because of its greater wall thickness, Schedule 80 conduit has a smaller interior diameter and therefore can hold fewer wires than a Schedule 40 conduit of the same nominal size.
A flexible metal conduit (FMC), also called Greenfield, is made of spiral bands of metal and is flexible enough to be bent around corners and other obstacles. This conduit is used for devices that may need to be moved around easily, such as drop-in fluorescent lighting. It is also used for short wiring runs that will be exposed, such as connections between wall-mounted boxes or switches and garbage disposal units or hot water heaters. FMC comes in rolls and can be cut to any length needed to complete the job.
Allowable Conduit Fill Capacities
The allowable number of wires that can be placed inside a conduit varies according to the type and size of the conduit and also on the size of the conducting wires. Wire size is defined by the American Wire Gauge, or AWG, number. The smaller the AWG number, the larger the wire diameter. For THHN insulated wire, the most common type of wire used in the conduit for household circuits, the fill capacities are:
|Size and Type of Conduit||14 AWG Wire||12 AWG Wire||10 AWG Wire||8 AWG Wire|
|1 1/2-inch EMT||84||61||38||22|
|1/2-inch PVC—Sch 40||11||8||5||3|
|3/4-inch PVC—Sch 40||21||15||9||5|
|1-inch PVC—Sch 40||34||25||15||9|
|1 1/2-inch PVC—Sch 40||82||59||37||21|
|1/2-inch PVC—Sch 80||9||6||4||2|
|3/4-inch PVC—Sch 80||17||12||7||4|
|1-inch PVC—Sch 80||28||20||13||7|
|1 1/2-inch PVC—Sch 80||70||51||32||18|
What About Using NM Cable Inside Conduit?
The National Electrical Code does not forbid you from running NM cable inside conduit in exposed locations, but in practice, this is rather difficult to do and most electricians will instead run TNNH wire, if possible. The one exception is when NM or UF (underground feeder) cable emerges from a hidden location and must run across an exposed area. In this situation, an electrician sometimes extends the sheathed cable through the conduit to its connection points. For example, UF cable emerging from the ground and running up the side of a building can be run through conduit over the exposed portion of its run.
NFPA 70®: National Electrical Code®. National Fire Protection Association.