code calculations
Sizing Raceway—Part 2
By Mike Holt, NEC Consultant
L
ast month, we covered how to size a raceway for
the installation of power cables. This month,
we’ll explain how to size raceways when the
installation of datacom wiring (i.e. data, signal,
control, fiber, etc.) comes into play.
Raceway fill requirements. As you may recall,
Sec. 300-17 notes the requirements of raceway
conductor fill. You can see other sections of the
Code that apply when working with datacom wiring
in Table 1, below.
Fig. 1. Where power and Class 1 circuit conductors are in a
raceway, you must determine the number of conductors in
accordance with Sec. 300-17.
Table 1. Code sections that deal with datacom wiring
required to be placed in a raceway.
Except for optical fiber cable, the NEC prohibits
the mixing of technology cables with power, Class 1,
or non power-limited fire alarm circuit conductors.
Example No. 1:
Q. What size electrical metallic tubing (EMT) is
required for two 12 strand nonconductive optical
fiber cables (diameter 0.25 in. or .0491 sq in. each)
and eight No. 12 THHN power conductors?
(a) ½ in. (b) ¾ in. (c) 1 in. (d) 1¼ in.
Fig. 2. Where nonconductive optical fiber cables are installed
with power conductors, size the raceway in accordance with
the requirements in Sec. 300-17.
A.
The answer is (b), ¾ in. (Fig. 2).
Step 1: Determine the cross-sectional area of each
cable and conductor.
Fiber (12 strand)5p2(diameter/2)
5 3.142 (0.250 in./2)2 50.0491 sq in.
From Table 5, Ch. 9, a No. 12 THHN50.0133 sq in.
Step 2: Determine the total cross-sectional area
for all conductors.
Fiber (12 strand)40.0491 sq in.22 cables 4
0.0982 sq in.
No. 12 THHN40.0133 sq in.28 wires40.1064 sq in.
Therefore, (0.133 1 .1064) 5 0.2046 sq in.
Step 3: Size the EMT raceway at 40% fill per Table
4, Chapter 9.
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Fig. 3. A multiconductor cable of two or more conductors
shall be treated as a single conductor for calcuating percentage of conduit fill area.
¾ in. EMT40.213 sq in.
Raceway fill suggestions. You are not required to
install the following technology cables in a raceway,
Continued on page 70
code calculations
Continued from page 66
Table 2. Code sections that deal with datacom wiring not
required to be installed in a raceway.
Fig. 4. Elliptical cable is considered a single conductor, and
fill is limited to 53%.
however, if you do install them in a raceway, do so in
accordance with the recommendations contained in
the “BICSI Cabling Installation Manual” (Chapters
4 and 5). BICSI stands for Building Industry Consulting Services International, Inc. You can contact the
group on the Internet at www.bicsi.org or by phone at
(800) 242-7405.(See Table 2, above.)
This installation guideline suggests you limit raceway runs to 100 ft, have no more than two 90° bends,
have a maximum pull force of 25 lb for Cat. 5 cable,
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June 2000 • EC&M • www.electricalzone.com
and have no more than 100 lb pull force for optical
fiber cable. In BICSI installation classes, students
use fish weight scales to get the feel as to the maximum pull tension (25 lb) recommended for highperformance cables (Cat. 5, 5e, 6, and 7).
Since most contractors have no idea how to limit
the pulling tension on technology cables in accordance with the BICSI standard, the generally accepted practice is to size the raceway so cables do not
exceed the percentage fill listed in Table 1, Chapter
9 of the NEC. Let’s look at a few more examples.
The first example discusses how to size a raceway
for “round technology cables.” Note 9, Table 1,
Chapter 9 states “a multiconductor cable of two or
more conductors shall be treated as a single conductor for calculating percentage of conduit fill area.”
Example No. 2:
Q. What size EMT is required for four Cat. 5 plenum
cables (diameter 1/6 in. or 0.167 in.), three 12 strand
nonconductive optical fiber cables (diameter ¼ in. or
0.250 in.), and two 24 strand nonconductive optical
fiber cables (diameter 7/16 in. or 0.438 in.)? (Fig. 3, on
page 66)
(a) ½ in. (b) ¾ in. (c) 1 in. (d) 1¼ in.
A.
The answer is (d), 1¼ in.
Step 1: Determine the cross-sectional area of each
cable.
Cat. 5 (4 pair)43.142(0.167 in./2)240.0219 sq in.
Fiber (12 strand)43.142(0.250 in./2)240.0491 sq in.
Fiber (24 strand)43.142(0.438 in./2)240.1439 sq in.
Step 2: Determine the total cross-sectional area
for all conductors.
Cat. 5 (4 pair)4 0.0219 sq in.24 cables40.0876 sq in.
Fiber (12 strand)40.0491 sq in.23 cables
4 0.1473 sq in.
Fiber (24 strand)40.1439 sq in.22 cables
4 0.2878 sq in.
0.5227 sq in.
Step 3: Size the EMT raceway at 40% fill per Table
4, Chapter 9.
1¼ in. EMT40.5980 sq in.
The next example discusses how to size a raceway
for “elliptical technology cables.” Note 9, Table 1,
Chapter 9 states “the cross-sectional area calculation
shall be based on the major diameter of the ellipse for
cables that have elliptical cross sections (not round).”
Example No. 3:
Q. What size Electrical Nonmetallic Tubing (ENT)
is required for one hybrid fiber/data cable? The
minor diameter of the ellipse is 3/16 in. (0.188 in.) and
the major diameter of the ellipse as a circle diameter
is ½ in. (0.50 in.). (Fig. 4, on page 70)
(a) ½ in. (b) ¾ in. (c) 1 in. (d) 1¼ in.
A.
The answer is (b), ¾ in.
Step 1: Determine the cross-sectional area of the
cable (based on the major diameter of the ellipse as
a circle diameter).
Hybrid cable43.142(0.50 in./2)240.1960 sq in.
Step 2: Determine the total cross-sectional area
for all conductors.
Hybrid cable40.1960 sq in.2140.1960 sq in.
Step 3: Size the ENT raceway at 53% fill (one
conductor fill) per Table 4, Chapter 9.
¾ in. ENT40.240 sq in.