By Mike Holt for EC&M Magazine
Based on the 2002 NEC

According to the National Electrical Code, the number of conductors in a raceway shall not be more than what can be installed or withdrawn without damage to the conductors or to the conductor insulation [300.17]. This can be achieved by limiting the total area of the conductors so that they do not exceed the percentage cross-sectional area of the raceway as listed in Table 1 of Chapter 9. Figure 1 Note: Graphics are not included in this newsletter.

Fill Number of Conductors Percent
1 conductor or cable 53%1
2 conductors 31%
3 or more conductors 40%
Raceway 24 inches or less 60%2
1 A multiconductor cable shall be treated as a single conductor [Table 1, Note 9].
2 Raceways having a maximum length not to exceed 24 in. [Table 1, Note 4], Figure 2.

The cross-sectional area of a raceway is dependent on the raceway’s diameter which is different for each type of raceway, see Table 4 of Chapter 9.

Example: The total cross-sectional area of a 1 in. raceway, Figure 3:
Electrical Metallic Tubing 0.864 in2
Flexible Metal Conduit 0.817 in2
Flexible Nonmetallic Tubing 0.785 in2
Intermediate Metal Conduit 0.959 in2
Rigid Metal Conduit 0.888 in2
Rigid Nonmetallic Conduit (Schedule 40) 0.832 in2
Rigid Nonmetallic Conduit (Schedule 80) 0.688 in2

The NEC does not contain any requirement that there be a maximum distance between junction or pull points, however we must consider the length of the conductors and there shall not be more then 360 degrees of total bends between raceway terminations [Table 1, Fine Print Note 1]. Figure 4

Sizing Raceway Using NEC Annex C
Annex C in the NEC identifies the number of conductors permitted in a raceway in accordance with the raceway fill limitations of Table 1 of Chapter 9 and the cross-sectional area of the given raceway. However, this Annex Table can only be used if all of the conductors are the same size and are of the same insulation type.

Table C1
How many 1/0 AWG THHN conductors can be installed in 2 in. electrical metallic tubing, Figure 5?
(a) 2 conductors (b) 3 conductors (c) 4 conductors (d) 7 conductors

• Answer: (d) 7 conductors, Table C1

Table C2A
How many compact 6 AWG XHHW conductors can be installed in 1-1/4-in. electrical nonmetallic tubing?
(a) 10 conductors (b) 6 conductors (c) 16 conductors (d) 13 conductors

• Answer: (a) 10 conductors, Table C2A

Table 7
How many 18 AWG TFFN fixture wires can be installed in in. liquidtight flexible metallic conduit?
(a) 40 (b) 26 (c) 30 (d) 39
• Answer: (d) 39, Table C7

Sizing Raceway Using Tables 4 and 5
When the conductors to be installed are not the same size and/or they do not have the same insulation type, the following steps should be used to size the raceway.

Step 1: Determine the cross-sectional area of “each conductor,” Figure 1.
Using Table 5, Chapter 9 for insulated conductors
Using Table 8, Chapter 9 for bare conductors [Note 3, Table 1]
Step 2: Determine the total cross-sectional area for “all of the conductors.”
Step 3: Size the raceway using Table 4, Chapter 9
40% for raceways containing three or more conductors [Table 1]
60% for raceways 24 in. or less in length (nipples) [Note 4, Table 1]

Raceway Example
Feeder conductors are installed in Schedule 40 rigid nonmetallic conduit. The raceway is over 200 ft long and it contains three 500 kcmil THHN, one 250 kcmil THHN, and one 3 AWG THHN conductor. What size PVC raceway is required for these conductors?
(a) 2 in. (b) 2-1/2 in. (c) 3 in. (d) 3-1/2 in.
• Answer: (c) 3 in.

Step 1: Determine cross-sectional area of each conductor, Table 5, Chapter 9.
500 kcmil THHN = 0.7073 sq. in. each
250 kcmil THHN = 0.3970 sq. in. each
3 AWG THHN = 0.0973 sq. in. each

Step 2: Determine the total cross-sectional area for all conductors
500 kcmil THHN = 0.7073 sq. in. x 3 wires = 2.1219 sq. in.
250 kcmil THHN = 0.3970 sq. in. x 1 wire = 0.3970 sq. in.
3 AWG THHN = 0.0973 sq. in. x 1 wire = 0.0973 sq. in.
2.6162 sq. in.

Step 3: Size the PVC raceway at 40% fill using Table 4 of Chapter 9.
2-1/2 in. = 1.878 sq. in., Too small
3 in. = 2.907 sq. in., Just right
3-1/2 in. = 3.895 sq. in., Larger than required

Nipple Example
What size rigid metal nipple is required for three 3/0 AWG THHN conductors, one 1 AWG THHN, and one 6 AWG THHN conductor? Figure 3
(a) 1 in. (b) 1-1/4 in. (c) 1-1/2 in. (d) 2 in.
• Answer: (c) 1-1/2 in. nipple

Step 1: Determine cross-sectional area of each conductor, Table 5, Chapter 9.
3/0 AWG THHN = 0.2679 sq. in. each
1 AWG THHN = 0.1562 sq. in. each
6 AWGTHHN = 0.0507 sq. in. each

Step 2: Determine the total cross-sectional area for all conductors
3/0 AWG THHN = 0.2679 sq. in. x 3 wires = 0.8037 sq. in.
1 AWG THHN = 0.1562 sq. in. x 1 wire = 0.1562 sq. in.
6 AWG THHN = 0.0507 sq. in. x 1 wire = 0.0507 sq. in.
1.0106 sq. in.

Step 3: Size the nipple at 60% fill [Note 4, Table 1] using Table 4 of Chapter 9.
1-1/4 in. nipple = 0.9156 sq. in., Too small
1-1/2 in. nipple = 1.2426 sq. in., Just right
2 in. nipple = 2.0448 sq. in., Larger than required

Sizing Raceways for Low Voltage Cables
The raceway conductor fill limitations contained in Section 300.17 apply to the following signal systems:
Control and Signaling Cables, Figure 1 725.3(A) and 725.28
Fire Alarm Nonpower-Limited 760.28
Optical Fiber Cables (installed with power conductors) 770.6
Sound Systems (Audio) 640.23

Author’s Comment: Except for optical fiber cable, the NEC prohibits the mixing of technology cables with power, Class 1 or nonpower-limited fire alarm circuit conductors in the same raceway.

Example 1
What size electrical metallic tubing is required for the following cables/conductors [770.6]? Figure 2?
Two - Nonconductive optical fiber cable having a diameter of 0.25 in. and eight - 12 AWG THHN power conductors
(a) in. (b) in. (c) 1 in. (d) 1 inches

• Answer: (b) in.

Step 1: Determine the cross-sectional area of each cable and conductor.
Note: The area of a cable is determined by the formula = 3.14 x ( x diameter)2
Fiber [3.14 x (0.250 in./2)2] = 0.0491 sq. in. each
No. 12 THHN [Chapter 9, Table 5] = 0.0133 sq. in. each

Step 2: Determine the total cross-sectional area for all conductors
Fiber = 0.0491 sq. in. x 2 cables = 0.0982 sq. in.
No. 12 THHN = 0.0133 sq. in. x 8 conductors = 0.1064 sq. in.
0.2046 sq. in.

Step 3: Size the EMT raceway at 40% fill using Table 4 of Chapter 9.
in. = 0.213 sq. in.

Raceway Fill Recommendations
The following technology cables are not required to be installed in a raceway: CATV (Coaxial) Cable [Article 820] 90.3
Optical Fiber Cables 770.3
Power-limited Fire Alarm Cables 760.3
Network-Powered Broadband Communications Cables [Article 830] 90.3
Radio and Television Cables [Article 810] 90.3
Communications (Twisted Pair) Cables [Article 800] 90.3

However if any of these cables are installed in a raceway they should be installed in accordance with the installation recommendations contained in the Industry
BICSI Cabling Installation Manual. This installation guideline recommends that raceway runs:
Be limited to 100 feet,
Have no more than two 90-degree bends, and
Have a maximum pull force of 25 pounds for Category 5 cable and
Have a maximum pull force of 100 pounds for optical fiber cable.

Author’s Comment: In BICSI installation classes, students use fish weight scales to get the feel as to the maximum pull tension (25 lbs.) recommended for high performance Category 5, 5E, 6 and 7 cables.

Note: BICSI stands for Building Industry Consulting Services International, Inc., for more information, visit  HYPERLINK "http://www.bicsi.org" www.bicsi.org, or call 1.800.242.7405.

Since most installers have no idea how to limit the pulling tension on signal or communication cables, the generally accepted practice is to size the raceway so that the cables do not exceed the percentage fill listed in Chapter 9, Table 1 of the NEC.

Example
What size electrical metallic tubing is required for the following, Figure 4?
Four - Category 5 plenum cables (diameter 1/6th in. or 0.167 in.)
Three - 12 strand nonconductive optical fiber cable (diameter in. or 0.250 in.)
Two - 24 strand nonconductive optical fiber cable (diameter 7/16th in. or 0.438 in.)
(a) in. (b) in. (c) 1 in. (d) 1 inches

• Answer: (d) 1 in.

Step 1: Determine the cross-sectional area of each cable.
Category 5 (4 pair) = 3.14 x (0.167 in./2)2 = 0.0219 sq. in.
Fiber (12 strand) = 3.14 x (0.250 in./2)2 = 0.0491 sq. in.
Fiber (24 strand) = 3.14 x (0.438 in./2)2 = 0.1439 sq. in.

Step 2: Determine the total cross-sectional area for all conductors
Category 5 (4 pair) = 0.0219 sq. in. x 4 cables = 0.0876 sq. in.
Fiber (12 strand) = 0.0491 sq. in. x 3 cables = 0.1473 sq. in.
Fiber (24 strand) = 0.1439 sq. in. x 2 cables = 0.2878 sq. in.
0.5227 sq. in.

Step 3: Size the EMT raceway at 40% fill, Chapter 9, Table 4.
1 in. EMT = 0.5980 sq. in.

Raceway Sizing for “Elliptical Technology Cables”
What size electrical nonmetallic tubing is required for one hybrid fiber/data cable? The minor diameter of the ellipse is 3/16th in. (0.188 in.) and the major diameter of the ellipse as a circle diameter is in. (0.50 in.), Figure 5.
(a) in. (b) in. (c) 1 in. (d) 1 inches

• Answer: (b) 3/4 in.

Step 1: Determine the cross-sectional area of the cable (based on the major diameter of the ellipse as a circle diameter).
Hybrid cable = 3.14 x (0.50 in./2)2 = 0.1960 sq. in.

Step 2: Determine the total cross-sectional area of the cable
Hybrid cable = 0.1960 sq. in. x 1 = 0.1960 sq. in.

Step 3: Size the ENT at 53% fill (one conductor fill), Chapter 9, Table 4.
in. ENT = 0.131 sq. in., too small
in. ENT = 0.240 sq. in., just right
1 in. ENT = 0.416 sq. in., larger than required

Mike’s Comment: If you have any thoughts, feedback or comments on this article, please let me know.

Copyright © 2002 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)