RACEWAY FILL

hen conductors are installed in conduit or tubing (EMT) there is a limit on the number of conductors that may be installed. One reason for this limit is the practical limitations on inserting conductors into a conduit. The general rule is the pulling tension in pounds for a copper conductor must not exceed .008 times the circular mil area of a conductor. For Aluminum the multiplier is .006.

For a copper number 16 wire, this would limit the maximum tension to .008 x 2580 = 21 pounds.
For a copper No. 18 wire this would limit the maximum tension to .008 X 1620 = 13 pounds.

These limits are multiplied by the number of conductors for more than one conductor. This maximum tension is not a code rule but is the recommended pulling limits given in IEEE publication Std 241-1983. Simply put, the number of conductors is limited to ensure that the amount of tension is not too excessive so that the conductors are not elongated or pulled apart during installation.

The general rule for conduit and tubing fill is that the cross sectional area of 3 or more conductors shall not exceed 40% of the inside cross sectional area of the conduit or tubing. For a single conductor this area is increased to 53 %, and for just 2 conductors the area is reduced to 31 %. If a conduit is no longer than 24 inches, it can be classified as a nipple, and the allowable area for is increased to 60%.

For multiconductor cables installed in conduit or tubing, the actual dimensions of the cable shall be used to calculate the cross sectional area. If a cable is elliptical in shape, the cross sectional area must be calculated based on using the major diameter of the ellipse as a circle diameter. For rounding off, if the area of a cable or conductor is divided into the available area of a conduit or tubing and if a decimal fraction of. 8 or greater is obtained, then the next higher whole number is used.

Chapter 9 of the Code and Annex C contains Tables for most conduit, tubing and conductor cross sectional areas. Table 4 contains dimensions and percent areas for conduit and tubing. This table is generally used for EMT, Rigid, IMC, Flexible Metal Conduit, and Liquidtight Flexible Conduit. When all single conductors are the same size and insulation tables in Annex C or a computer program can be used to find the maximum number of conductors allowed in a raceway. Several free raceway fill computer programs are available at the electrician.com web site.

Example 1:

What size tubing is required for the following 30 conductors installed in Electrical Metallic Tubing?

4 TFFN NO. 18 CU
6 SF-2 NO. 16 CU
20 THHN NO. 14 CU

Answer

Step 1: Find the total cross sectional area of all the conductors.

Table 5 in Chapter 9 applies to insulation's TFFN, SF-2, and THHN.

Area of #18 TFFN is .0055 x 4 conductors equals .0220 square inches.
Area of # 16 SF-2 is .0139 x 6 conductors equals .0834 square inches.
Area of #14 THHN is .0097 x 20 conductors equals .1940 square inches.
Sum = .2994 square inches

Step 2: Find the size of Raceway.

Total area of the 30 conductors is .2994 square inches.
Turn to Table 4, "Dimension and Percent Area of Conduit and of Tubing," in the Section titled "Electrical Metallic Tubing." The question addresses 30 conductors that applies to the column "Over 2 Wires 40% Sq. inches." Read down this column until you find the number that is greater than .2994 (the total cross sectional area of the 30 conductors.) After finding .346, read across (left) to the "Trade Size Inches" column that indicates that a minimum One inch tubing is required.

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