Friday, December 30, 2016

Sizing Circular Raceways for Multiconductor Cables - Explained!

From time to time we get asked how to size a raceway where SE-R Cable is to be installed and how do we calculate the raceway fill requirements found in Chapter 9, Table 1 of the 2017 NEC*.

Determining the raceway fill for a complete raceway system (not for protection from physical damage) where Type SE-R Cable is to be installed is a simple calculation that often leaves electricians, engineers and inspectors confused. Again, keep in mind that this doesn't apply to short sections of raceway being used as physical protection for the SE-R Cable as denoted in Chapter 9, Table 1, Note 2 of the 2017 NEC.

All cable manufactures provide the diameter of their cables. This is usually found on their product information cut sheets. We will use our 4/0-4/0-4/0-2/0 AL SE-R from Encore Wire as an example for this article.

Since we are working with a cable assembly you need to determine what percentage of raceway fill the installer has to observe. Based on Chapter 9, Table 1, Note 9; a multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. Table 1 of Chapter 9 states that a single conductor (1) is to have a 53% maximum cross sectional area fill.

Once the fill percentage is established the user will draw their attention to the cable itself. In our published specifications reflects the diameter (straight across the cable from high point to high point) as 1.425 in. Based on the guidance provided in Chapter 9, Table 1, Note 9 of the 2017 NEC, the installer is going to use that diameter to perform a calculation nested within a normal mathematical formula.

Now, since we are looking for the area then we need to use that aforementioned formula to convert the diameter to an area. That formula can be done a few different ways. (𝝿/4) x D² is the base formula, in our case (𝝿/4) x 1.425² , which we added the diameter in place of the "D" in the formula. In complete value our formula is expressed as the following : 3.1415926536 ÷ 4 x 2.030625 = 1.5948491456 or simplified to 1.5948 which is now the approximate area for use with Chapter 9, Table 4.

As with any other raceway fill calculations, Table 4 comes into play. Once you have selected the raceway method you are installing, scroll over to the 53% fill column and find a raceway that will handle the 1.5948 in², which if the raceway was PVC Schedule 40 would result in a trade size 2 conduit. Again the benefit here is that the cable is considered a single conductor and can utilize the fill all the way to 53%.

Lastly, I also get questions on how to do this same calculation on a Type SE-U, NM-B, or UF-B cable and the calculation is just as simple. Chapter 9, Table 1, Note 9 also addresses "Elliptical" cross sectional areas as well. The NEC directs you to use the major diameter of the ellipse as a circle diameter just the same way we did for the previous Type SE-R example. The values for these types of cables are also found on the manufacturers product cut sheets or can be determined using a caliper tool where needed.

While this is not very complicated, it does generate many questions over the years on how to do the calculation, the hope is that this article will help eliminate the confusion on how to determine raceway fill for cables installed in circular raceways.

By Paul W Abernathy, CMI

CEO & President of Electrical Code Academy, Inc.

All Rights Reserved- Copyright 2016-2017

*NEC is a Registered trademark of the NFPA, National Fire Protection Association. We are not associated nor affiliated with NFPA.

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