68 Modern Commercial Wiring
Copyright Goodheart-Willcox Co., Inc.
Wireways
Wireways are rectangular sheet metal enclosures with
removable covers. Fittings, such as elbows, clips, and end
pieces, enable the sections to be joined to form an overall
wiring system. Concentric knockouts are provided in each
side and at the ends.
When installed vertically, wireways are supported every
15′. Each wireway system must be complete prior to
installing the conductors. Entry into a wireway can be made
using other wiring methods, such as mineral-insulated
metal-sheathed cable (MI), metal-clad cable (MC), rigid
conduit, intermediate conduit, electrical metallic tubing,
electrical nonmetallic tubing, and rigid nonmetallic conduit.
The total cross-sectional area of all conductors in a
wireway must be less than 20% of the cross-sectional area of
the wireway. Splices and taps are permitted in the wireway,
but must take up less than 75% of the cross-sectional area.
See Figure 5-18.
Figure 5-17. Surface raceway can be used to supply power
outlets, lighting, and communication outlets. (Wiremold)
Sample Problem 5-1
Problem: What is the minimum cross-sectional
area for a wireway that will accommodate four 2/0
AWG THW, ten 6 AWG THWN, and ten 10 AWG
THWN insulated copper conductors?
Solution: First, determine the cross-sectional area
of all of the conductors. Cross-sectional areas of
conductors are found in Chapter 9, Table 5 in the
Code.
2/0 AWG THW: 4 × 0.2624 in2 = 1.05 in2
6 AWG THWN: 10 × 0.0507 in2 = 0.51 in2
10 AWG THWN: 10 × 0.0211 in2 = 0.21 in2
Total area = 1.77 in2
The wireway size must be large enough that 1.77 in2 is
less than 20% of the total area. If 1.77 in2 is 20%,
dividing 1.77 in2 by 0.20 will equal the area equivalent
to 100%. This is the minimum wireway cross section:
1.77 in2 ÷ 0.20 = 8.85 in2
Therefore, the wireway must have a cross section of
8.85 in2 or larger.
Approximate
Size Diameter Approximate Area
(AWG
or
Type kcmil) mm in. mm2 in.2
Type: RHH*, RHW*, RHW-2*, THHN, THHW, THW, THW-2, TFN, TFFN,
THWN, THWN-2, XF, XFF
RHH,* RHW,* 10 5.232 0.206 21.48 0.0333
RHW-2,* XF, XFF
RHH*, RHW*, 8 6.756 0.266 35.87 0.0556
RHW-2*
TW, THW, 6 7.722 0.304 46.84 0.0726
THHW, 4 8.941 0.352 62.77 0.0973
THW-2, 3 9.652 0.380 73.16 0.1134
RHH*, 2 10.46 0.412 86.00 0.1333
RHW*, 1 12.50 0.492 122.6 0.1901
RHW-2*
1/0 13.51 0.532 143.4 0.2223
2/0 14.68 0.578 169.3 0.2624
3/0 16.00 0.630 201.1 0.3117
4/0 17.48 0.688 239.9 0.3718
TFN, 18 2.134 0.084 3.548 0.0055
TFFN 16 2.438 0.096 4.645 0.0072
THHN, 14 2.819 0.111 6.258 0.0097
THWN, 12 3.302 0.130 8.581 0.0133
THWN-2 10 4.166 0.164 13.61 0.0211
8 5.486 0.216 23.61 0.0366
6 6.452 0.254 32.71 0.0507
4 8.230 0.324 53.16 0.0824
3 8.941 0.352 62.77 0.0973
2 9.754 0.384 74.71 0.1158
1 11.33 0.446 100.8 0.1562
1/0 12.34 0.486 119.7 0.1855
2/0 13.51 0.532 143.4 0.2223
3/0 14.83 0.584 172.8 0.2679
4/0 16.31 0.642 208.8 0.3237
250 18.06 0.711 256.1 0.3970
300 19.46 0.766 297.3 0.4608
(Copyright © 2013 NFPA)