Calculating Conduit Fill
Most electricians today carry around an Ugly’s book like this one. It has a lot of reference tables in it that we use everyday and is small and convenient to carry around as we go from job to job. The Uglys book has a section on conduit fill, taken from Annex C of the National Electric Code, that shows the maximum number of different types of conductors allowed in various types of conduit. So if you find the conductor you are using and the size and type of conduit you have, you can find just how many conductors of a certain size you can fit in a conduit. Now this is great, and it is a great help for a lot of use cases. But what if you have multiple wire sizes that you need to install in that conduit? How do you determine if your conduit can legally handle the amount of conductors you want to install. Now keep in mind that if you install 3 or more current carrying conductors in a conduit you will also need to derate those conductors, but that is a lesson for another day. Today we are discussing how to size the conduit for the number and size of conductors we want to install. And if the conductors are different trade sizes, then how do we determine that fill?
Table 1 of Chapter 9 in the NEC or National Electrical Code gives us the requirements for cross section conductor fill in conduit. If there is only 1 conductor in a conduit we are permitted to install a conductor that takes up 53% of the conduit cross sectional area. If there are more than 2 conductos in a conduit then the cross sectional area of the wire must not exceed 40% of the conduit cross sectional area. The is the rule unless the conduit is a nipple no longer than 24” and is installed between boxes, cabinets or a similar enclosure. In the case of a nipple the conduit fill can be increased to 60%.
But for the purpose of this article we will focus on a conduit that is longer than 24”, so it is not considered a nipple. Again, in that case the fill may not exceed 40% according to Table 1 of Chapter 9 in the NEC for conduits with over 2 conductors. Now that we know how much we can fill each conduit, how do we calculate the fill?
To answer this question we need to turn to Chapter 9 of the National Electric Code and look up Tables 4 and 5. Table 4 is broken down into 13 different conduit types, such as EMT, ENT, FMC, IMC etc… and gives us multiple cross sectional areas, such as 40%, 60%, 100% and so on for each of those types of conduit. We will focus for this article on the 40% fill in inches squared of ¾” EMT conduit.
Table 5 gives us the Area of different types of common wire in inches squared for each size of trade size wire. Starting at 18 AWG which stands for American Wire Gauge all the way to 2000 kcmil wire. Each section of Table 5 is broken down by insulation type because different insulations have different thicknesses and will impact the area of the conductor depending on the thickness.
Armed with the information in these three tables we can determine how to calculate conduit fill in almost any conduit. Here is a video tutorial to show you exactly how to use these tables to calculate conduit fill.