Conduit sizing

Installing cables? Then you’ll probably be installing conduit, too. Here’s a five-step procedure for determining the correct conduit size . . . .

New conduits are sized based on their percent fill, jam ratio, and bend radius. New cables will usually require the installation of new conduits. Only with rare exception is any cable left exposed to the air (when it is “free-run”). The five steps for sizing conduit are as follows:

  1. Calculate total cable area.
  2. Calculate conduit fill.
  3. Calculate jam ratio.
  4. Resize conduit if jam ratio fails.
  5. Ensure cable bend radius is less than the conduit’s.

The NEC’s criteria is often used to size conduit. The design parameter for determining minimum conduit size is conduit fill. It’s denoted as a percentage. The conduit fill percentage is the ratio of the area of the cables inside to the inner area of the conduit. The calculation for a single cable inside of a conduit reduces to a simple equation:

Conduit Fill \% = \frac{d_{cable}^{2}}{d_{conduit}^2}\cdot 100

Here dcable is the cable’s outer diameter, and dconduit is the conduit’s inner diameter.

There are three criteria for maximum conduit fill provided by the NEC:

  • 53% for 1 conductor in a conduit
  • 31% for 2 conductors in a conduit
  • 40% for 3 or more conductors in a conduit

Multi-conductor cables are counted as a single conductor in accordance with Note 9 to Table 1 of Chapter 9. To illustrate that, I have produced the three figures below. By the time you have seen the third, you will think all but the first redundant. Click the image for a larger (readable) version.



Figure 1 – A conduit filled with either one insulated conductor or one multi-conductor cable. In both cases the fill limit is 53%.



Figure 2 – A conduit filled with either two insulated conductors or two multi-conductor cables. In both cases the fill limit is 31%.




Figure 3 – A conduit filled with either three insulated conductors or three multi-conductor cables. In both cases the fill limit is 40%.

The standard conduit type is rigid metal conduit, or RMC. It is often made of galvanized steel. It’s conduit-fill table is found in Article 344 of NEC 2014.

Most plants don’t use anything smaller than ¾ inch, and there are certain sizes that aren’t industry- or plant-standard, such as 2 ½ inches. A plant typically keeps all of the consumables and accessories associated with conduit installation — fittings, straps, bushings, nipples, etc — in stock. It’s easier and cheaper to stock only those items that they use most often.