NEC Conduit and EMT Fill Tables

Question: Please explain how to use the Tables in the National Electrical Code (NEC) to figure conduit fill. I have 3-12 AWG THWN, 4-10 AWG THWN, and 8-8 AWG THWN conductors. I plan to install them in EMT. What size EMT is required?

Answer: An Informational Note to 300.17 has a list of specific section numbers for raceways and other applications. Each raceway article in .22 (ie: 358.22) in the NEC indicates that the number of conductors shall not exceed that permitted based on the percentage of fill specified in Table 1, Chapter 9.

Table 1 shows that for over 2 conductors the percentage of fill permitted for all conductor types is 40 percent.

Step 1. Use Table 5 for the area of conductors: This table is based on conductor insulation and size. Find THWN under the Type heading, then go down the left hand column to 12 AWG, 10 AWG, and 8 AWG and move across to the center column for the approximate area in square inches.

12 AWG Type THWN = .0133 Sq. inches.
10 AWG Type THWN = .0211 Sq. inches
8 AWG Type THNN = .0366 Sq. inches

Step 2. Total the square inch areas:

3 – 12 THWN X .0133 = .0399
4 – 10 THWN X .0211 = .0844
8 – 8 THWN X .0366 = .2928

Total = .4171 Sq. inches

Step 3. Use Table 4 for the areas of conduit or tubing permitted: Find the table specific to Article 358 Electrical Metallic Tubing (EMT). Go down the fourth column (over 2 wires – 40%) to find a value that is at least4171 Sq. inches. Then go back horizontally to the second column to determine the minimum Trade Size.

1” = .346
1 ¼” = .598

The answer, then, is that trade size 1¼ EMT would be required.