Installing Conductors in Existing Raceways

INSTALLING CONDUCTORS INTO EXISTING RACEWAYS

It is common for electricians to install conductors into a conduit raceway system that already has conductors installed in them. If this is the case, the amount of space being used by the conductors must be determined in order to know the amount of space available to insert more conductors.

Apply the following steps to determine the number of conductors that may be added to a conduit:

1. Determine the space occupied by existing conductors by using Table 5,
2. Determine the allowable fill (typically 40%) based on conduit type and the number of conductors already installed. See Table 4 for the type of conduit.
3. Subtract the total conductor area from the allowable fill to find the remaining space left.
4. Divide the remaining space by the area in square inches of the new conductors being installed. The area for conductors is found in Table 5

For Example: What is the remaining available area of a 2” Trade side EMT with (12) 8 AWG THHN Conductors? Use the steps above to solve.

Step 1: (1) single 8 AWG THHN has an approximate area of: 0.0366 in², Multiple 0.0366 x 12 = 0.4392 in²

Step 2: Since we have Over 2 Conductors, we must look at Chapter 9 Table 1, and determine the maximum fill is 40%. Next, we visit Chapter 9, Table 4, and find out what the 40% fill is, which happens to be 1.342 in².

Step 3: Using Step 2, 40% allowable fill of 1.342 in² and subtract the (12) conductors’ approximate area of 0.4392 in² from Step 1. (i.e.) 1.342 in² – 0.4392 in² = 0.9028 in² remaining space left.

To complete Step 4, let’s see how many more 8 AWG THHN Conductors we could add to this EMT raceway.

Step 4: Value from Step 3, which is 0.9028 in² / 0.0366 in² = 24.66, drop the .66 and you could place (24) more 8 AWG THHN Conductors in the Trade Size 2 EMT Raceway.

PRO TIP– Since all the conductors are the same you could have used Informative Annex C to determine the maximum number permitted, which is 36 conductors. To show our example calculation works, take our original (12) 8 AWG THHN Conductors and add the (24) more we added from our example for a total of 12 + 24 = 36.

Note: Just because we can put (36) 8 AWG THHN CU Conductors in a Trade Size 2 EMT raceway doesn’t mean that those conductors will have the ampacity necessary to do the job. If they are current carrying conductors (CCC), then 310.15(C)(1) will limit their current carrying ability.

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