During cable installation, cable is typically routed through conduit or trunking. As the cable is being pulled through conduit, friction between the cables and the conduit walls can make the cables difficult to pull. Predicting how many cables can be pulled through the conduit can prevent cable failure and wasted time. Additionally, higher current signals running through cable can generate heat. If too many of these cables are placed within a conduit, then the heat generated can be a source for fire.

If you cut a piece of conduit in half, you can see that the thickness of the conduit wall material causes the inside and outside diameters to vary. In the AV industry, conduit is referred to by its "trade size." This may refer to inner or outer diameters of the pipe; typically, the approximate outer diameter is called "trade size."  Some common types of conduit are electrical metallic tubing (EMT), intermediate metal conduit (IMC), rigid metal conduit (RMC), flexible metallic conduit (FMC), and  polyvinyl chloride (PVC) conduit.

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Regionally Specific: US Customary Conduit Sizes
Some typical US Customary trade sizes for conduit are ½, ¾, 1, and up to 4. 
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Regionally Specific: Metric Conduit Sizes
Outside the US, conduit sizes are typically measured in millimeters and are based on the outer diameter. Some common  metric outer diameter sizes are 20 mm, 25 mm, 32 mm, 40 mm and 50 mm.
Conduit Dimensions Chart
Refer to the chart below to see how the inside and outside diameters of trade size conduits differ.
Note: the above dimensions are intended as an example. Actual conduit dimensions may vary by manufacturer.

There is a defined maximum amount of space that cables can occupy inside the conduit. The requirements for the amount of space varies by country. Check with the local authority having jurisdiction (AHJ) for the applicable guidelines. A conduit's inner diameter measurement is used to calculate the inner area of the conduit. The inner area is important because it is this space that determines how many cables you can run through the conduit. One important concept to remember is that the total circular area of all cables that will be pulled through the conduit is taken into account.

Advanced Concept
Advanced Concept Alert: Conduit Capacity
Performing the calculations for conduit capacity and other related formulas are beyond the scope of this course. You can learn more about calculating conduit capacity in InfoComm’s classroom and online courses. More