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Section VI
Related Structural Applications of Aluminum
Chapter 17
Rigid Aluminum Conduct
One of the earliest. and still most effective, methods of
installing concealed electrical cable within the structure
of a building, in the building foundations or the sub grade
is the use of rigid conduit. The many advantages rigid
conduit provides include,
A very high order of mechanical and flame protection
for the enclosed cables.
A high order of safety to personnel who might otherwise
accidentally come in contact with the ungrounded
portion of the electrical system.
An easy pulling, smooth, snag-free pathway for the
cables through an otherwise intricate, many-turned
run.
Pull-boxes to enable straight pulls as required.
An easy means for replacing conductors or pulling
in additional ones.
Permanent, built-in cable runways in cast concrete
structures.
Elements of Conduit Design
Important elements in the design of electrical conduit
He related to,
Adequate strength in relation to size for self-support
over reasonable lengths.
Smooth, round, burr-free interior.
Capability of being cut and threaded readily and bent
smoothly (no flattening) with normal field methods
and tools.
Provision and maintenance of good electrical conduc­
tivity through the conduit proper and across all
threaded joints.
Development of a compatible line of accessories, such
as elbows, couplings. unions, tees, pull and junction
boxes.
Freedom from destructive corrosion in the working
environment.
Provision for expansion fittings for long lengths oper­
ating under widely varying temperature conditions.
installation and construction requirements for electrical
conduit are set by the National Electrical Code (NEC).
These include rigid aluminum, galvanized steel, IMC
(intermediate metal conduit) and nonmetallic conduit, as
well as conduit fittings, conduit bodies and boxes in
aluminum, galvanized steel and PVC. Individual product
lines must be manufactured to conform to the standards
set forth by Underwriters Laboratories, in order to qualify
for UL Labels.
The factors involved in NEC rigid conduit specifications
relate to installation and operating requirements.
For example, the Code specifies maximum allowable
fill areas for given conduit sizes. Table 17-1, abstracted
from the 1987 NEC, shows the allowable fill area for
each conduit trade size. * Table 17-3 gives cross-sectional
areas for typical conductor sizes insulated with both
thermosetting and thermoplastic materials. Table 17-4
converts these area limitations to the number of typical
conductor sizes permitted in one conduit based on both
thermosetting and thermoplastic insulations. Tables 17­
Sa and 17-5b give dimensions of rigid aluminum, galvan­
ized steel, andIMC. Table 17-6 gives comparative weights
for the three types of metallic conduit.
The NEC also recognizes the effect of the conduit or
raceway on temperature rise of the encased conductor.
In Chapter 9 of this book, Tables 9-3, 9-4, 9-5, 9-6 and
9-7 show characteristics of wires and feeders in magnetic
and non-magnetic conduit or raceways. Metallic conduits
can contribute a small amount of heating due to hysteresis
(if the conduit is magnetic) and eddy current losses. Of
greatest importance in its effect on ampacity, however,
is the number of loaded conductors in a given conduit.
The tables referenced above give ampacity data for the
case of not more than three loaded conductors per
conduit. When the number of conductors increases to
from 4 to 6 the ampacity values are reduced to 80% of
the tabulated value. If there are 7 to 24 conductors in
the conduit, the allowable currents are to be reduced to
70% of the tabulated value. (The neutral conductors,
since they only carry the unbalanced currents in normally
balanced circuits, are not to be considered in determining
'" Table
17~2
shows the maximum number of compact conductors
allowable in conduit or tubing:.
17-1 related strudural applications of aluminum
TABLE 17-1 Conduit-Allowable Fill Area Square Inches Conduit
Trade Size
Internal
Total
1·Cond.
2-Cond.
3 or more
Inches
Area Sq. In.
53%
31%
40%
16
.28
.46
.80
1.08
1.78
2.54
3.91
5.25
6.74
10.60
15.31
09
.16
.27
.47
.63
1.04
1.46
2.29
3.07
3.94
6.20
8.96
%
1
1%
1%
2
2%
3
3%
4
5
6
I
I
I
I
i
30
.53
.86
1.50
2.04
3.36
4.79
7.38
9.90
12.72
20.00
28.89..
---~~
..
i
I
i
!
i
,
12
.21
.34
.60
.82
1.34
1.92
2.95
3.96
5.09
8.00
11.56
i
,
I
TABLE 17·2 Maximum Number of Compact Conductors in Trade Sizes of Conduit or Tubing l:.l
Insulation Type
!
.!TTX!TT
Conductor
H 'H H
Size
~
AWGor
W H H
W H
I
k
'l
N ...W
N
eml
i
1 In.
6
4
2
1
110
210
3/0
I
I
410
250
300
350
400
500
600
700
750
1000
17·2 5
4
3
7
11/4
4
6
4
3
3
9
7
5
3
3
XIT
T
HIH
H W
W'
H
H
in~
13 11 12
9
8 8
7
6
6
4 4 5
4
3
3
3
3
3
3
N
1112 In.
X'T
T
X.T
T
X
I
I
T
T
Hi H H H'H H H I H H
H! W H H W H H , W H
Wi
N W
N W
N
Conduit Trade Size
2 in.
2112 In.
!
15 15
11 11 15 18 18
8 8 11 13 13
6 8 10 10 11 14 14
6
5 5 7 8 8 9 12 12
7 8 10 10
4 4 5 7
5 6 6 7 8 8
3 3
7
6
7
3 3 4 5 5
3 4 4 4 5
5
3
3 3 3 4 4
3 3 3 3 4
3 3
~I
X,T
T
X 'T
HIH
H
H
H H H
H W H H
W
N W
~IW
N
T
i
3 in.
3112 in.
4 in.
i
.
12 15 15
10 13 13
9 10 10.12 14 14
7 8
B 9 11 11
6 7
7 8 9 10 10 12 12
5 6 6 7 8 8 9 11 11
5 5 6 6 7 8 8 9 10
4 4 5 5 6 6 7 8 8
4 4 5 5 5 6 7
3 4
4 6
6 6
3 3 3 4 4
3 3 3 4 4 4 5 5 5
3 3 3 4 4 4
rigid aluminum conduit
TABLE 17-3 Conductor Cross-Sectional Areas (Based on Table 5, Chapter 9, 1987 NEC) film, and excellent ductility and machinability are basic
factors in its present wide acceptance. The following
material examines in some detail how these plus factors
compare with steel.
APPROXIMATE AREA. SQUARE INCHES INSULATED CONDUCTOR
'
WIRE ~-----------------1
SIZE,I
Types
Type
AWG.:
kemil
RHH &
RHW'
Type
THW
THHN
THWN
12
10
8
6
4
3
2
1
,038
,046
,085
.. 124
.161
.182
.207
.272
.311
.358
.415
,484
.592
,025
,031
,060
082
,109
.126
.147
.203
,237
.276
.329
.390
.488
,012
,018
,037
052
.084
.099
,684
.568
.762
.629
697
,832
1.026
1.158
1,225
1.291
1.421
1.548
1.953
2.275
2.593
2.901
-=~~~--~~~~
,
I/O
2/0
310
4/0
250
300
350
400
836
50O
600
700
750
900
900
1000
1250
1500
1750
2000
,
,983
1.194
1,336
1.408
1.478
1.617
1.753
2.206
2.548
2.890
3.208
,118
.159
.189
.226
.271
.328
A03
,467
.531
593
.716
,879
1.001
1.062
1.123
1,245
1.362
Type
XHHW' BARE
,017
,005
,022
,008
,046
,017
062
027
,042
.084
,099
.053
.118
.067
.159
.087
,109
.189
,226
.137
.271
.173
,219
.328
,
,403
.260
.467
.312
,531
.364
593
..416
_­
,520
.716
.904
.626
1.030
.730
1,094
.782
1.150 , .833
1.267
.933
1,389 1.039
1,767 1.305
2.061
1.561
2.378 1.829
2.659 2.087
i
Composition and Manufacture: Rigid aluminum con­
duit is usually extruded from the magnesium-silicide,
6063-Tl alloy, though other alloys can be used which
meet UL requirements.
Being an extrusion, aluminum conduit is completely
moisture-and vapor-tight. Also, extruded pipe provides
a smooth, uniform interior surface,
Weight Comparison: Aluminum conduit with aluminum
couplings weighs approximately one-third of its galvanized
steel counterpart, and one-half that of steellMC, This dif­
ference in weight is reflected in substantially greater ease
and cost savings in installation. For example, a IO-foot
standard length of 4-inch steel conduit weighs over 98
pounds and requires two men or a hoist to place it.
The same size aluminum conduit weighs only 34 pounds
and is handled easily by one man. The larger the conduit
size, the greater the savings in labor.
,
"'RHH and RHW without outer covering are the same as
THW,
the der'dting amounts given above, Note 10 to NEC
Tables 310-16 to 310-31 specifies when the neutral must
be counted as a current-carrying conductor),
Rigid Aluminum Conduit
Aluminum rigid conduit has been widely used during
the past five decades due to recognition that lightweight
aluminum conduit offers several advantages over steel"
including reduced costs of installation, increased cor­
rosion resistance and improved ground path.
Advantages of Aluminum Conduit
Aluminum's combination of light weight, relatively
high electrical and thermal conductivity, protective oxide
Electrical Characteristics
Aluminum conduit alloy 6063 has about 1I4 the elec­
trical resistance of the usual galvanized mild steel conduit.
In the installed condition, with couplings, elbows and
boxes, a run of aluminum conduit will show about
4-112 times greater electrical conductivity, and if the
installation is properly made will maintain its high value.
Protective Capability: In a conduit/cable system when a
phase-to-ground fault occurs, the conduit will normally
carry most of the fault current-which can be quite high
in value, Usually. the wiring system neutral is grounded
at one point. The conduit may be grounded at many
points. In any event, the fault current flowing in the
conduit raises its potential above ground by an amount
equal to the impedance drop to ground. The lower the
installed conduit impedance to ground the less danger
there is from fault! ground shocks, and in this respect
the advantage of aluminum conduit is obvious. The lower
resistance of aluminum conduit also means that ground
current fault relaying is more reliable.
It must be understood that rigid electrical conduit as
installed per NEC requirements is normally not supposed
to carry any ground currents; it is to act as a mechanical
protection and carry current only in the case of a fault.
The Code specifically· indicates that any neutral wires
shall not be instailed in electrical continuity wi th the
conduit and, if accidental continuities are found to exist
between the neutral and the conduit, such neutral faults
shall be cleared. This separation of neutral ground current
*1987 National Electrical Code, Section 250-21
17·3
related structural applications of aluminum
TABLE 17-4
Typical Number of Conductors Allowable in Trade Sizes of Conduit or Tubing
----------~----
Maximum Number of Conductors in Conduit"
Wire Size
AWG or
kernil
14
12
10
8
6
4
3
2
1
1/0
2/0
3/0
!
I
'!f.r
A B
1250
1500
1750
2000
B
A
1"
A B
10: 24 16 39
8' 18' 13 29
4
6 6: 11 11 18
,
5 5
1
9
3: 3
4,
2
1
4i
6'
1
!
1 1
2! 3, 4
1
1, 2
1 , 1 1
3
3
1 , 1 1
1' 2
6: 13
! 4: 10
I
i
!
4/0
250
300
350
400
500
600
700
750
800
900
1000
,,""
I
,
~I
1
1
1• 1
1' 1
!
1:
I
1
1
1'
1i
1
1
1
,
1%10
A B
69 40:
51 : 32
32 26
16 13
11 10
7 7
6 6
5 5
29
24
19
10
7
5
4
4
1! 3
1 2
1: 1
1 1
3
3
2
1
4
3
3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
1
1
1
1
1
1
!
I
i
.... ~.
I
A
B
3%:11
BA
:
4"
B: A
B
A
5"
B
6"
B
A
I
1
6'
5i
1
1
A
:
I
2
1
1
1
1
1
1
1
3"
2%/'
65 154 9~
192
143
!
157
53 114 76,164117
:
43 73 61 104: 95 160127
1631
i
22 36 32 51! 49 79,66 106 85' 136:133
16 26 23 37 36 5~148 76 62 98 97 154 141
12 16 17 22 27! 35 36 47 47 60 73 94 106 137
10 13 15 19 23 29 31 39 40 51 63 80 91 116
9 11 13 16 20 25' 27 33 34 43 54 67 78: 97
94
70
44
22
15
9
8
7
5
4
3
3
1!
!
2"
A B
l"ht
A B
I
i
5
4
3
2
2
1
1
1
1
1
1
1
4
3
3
2!
1!
1'
!
I
I
8,
7,
6
5'
1
1
1
1
1
1
1
1
9 12' 14 18 19! 25, 25,
8 10 12 15 16, 21 21
7
8 10 13 14:I 17 18:
6
9 11 l2! 14 15
5
7
9, 10 12 13
7 8 10 10
6
4'
3
4' 5
61 7 8 9,
3! 4
3
7 8:
5 6
2
4
3
5 5'I 6 7:
4 41 5 6
1 2i 3
1
1
3
4 5
3 4
1
1
2
4 4
3 3
1 1 2, 2 3 3 4
1
1 1: 2 3
3 4
1
1 3
1 1
3 3
1
1 2
1 1
2 3
1:
1
1
2
1
1
1
1
1
1
1
1
, 1
1
1
1
1
7i
~I
32
27:
22
16
15
12
11
9
8
7
5
5
4
4
39
33
29
24
4!
5
3,
5
20
16
14
12
11
9
7
7
6
6
50'
42
35
29
24
20'
17
15
13
11
9
8
7
57
49
41
35
29
23
20'
lSi
16
14
11
7i
10
9
9
6
7
6!
8
3,
6
5
I ;1
4
4
4
72
61
51
42
35
28
24
21
19
16
13
11
11
10
9
8
A - THW. RHH, RHW (Without outer covering).
B - THWN and THHN. (XHHW in sizes #4 AWG through 500 kemil) see NEC,
Chapter 9. Table 3 lor other sizes and types,
flow from the conduit is very helpful in minimizing ac
electrolytic corro:;;;)Tl~ and interference with communi­
cation circuits.
Short Circuit Capability: Comparative short circuit tests'
on aluminum and galvanized steel conduits have been
made to determine their relative behavior under heavy
fault conditions. Fig, 17-1 shows the temperature rise vs.
time of 2-inch aluminum and steel conduits joined in series
and subjected to a short circuit current flow of 22,200
amperes RMS, The steel conduit at the end of 10 seconds
was buckling and dully glowing and its temperature rise
was about 4-112 times as great as that of the aluminum
-AlEE Paper DP 60-652, L. F. Roehman, 1960.
17-4
conduit. The steel couplings all smoked profusely and
showed thread damage. In contrast. the aluminum con­
duits still retained their gummed labels and showed no
signs of the heavy current passage after the test. Thus,
despite the considerably lower melt-point of aluminum
conduit. its ability to carry short circuit currents is greatly
superior to galvanized steeL
Circuit Voltage Drop: Aluminum conduit, being non­
magnetic, exhibits no hysteresis losses from alternating
current fields, The net effect is that voltage drop in a
typical three-phase feeder or branch circuit in aluminum
conduit may be from 10 percent to as much as 20 percent
lower than with a corresponding steel conduit. For con­
ductor sizes 250 kcmil and higher, this overall reduced
voltage drop may permit the use of conductors one size
rigid olum;num conduit
TABLE 17-5a Dimensions-ll.igid Aluminum and Galvanized Steel Conduit Trade
S;ze
,
0.0.
Inches
.840
'h
-Y..
1.050
1.315
1.660
1.900
2.375
2.875
3.500
I
Wan 1.0.
Inches
Thickness .622
.824
1.049
1.380
1.610
2.067
2.469
3.068
3.548
4.026
5.047
.109
.113
.133
.140
.145
.154
.203
.216
.226
.237
.258
:
Length
WID Coupling
9'_11%"
9'-11%."
9'-11"
9'-11"
Threads
Per Inch
.
14
14
11 'h
11%
11'V2
1%
9'_11>#
1'12
9'-11"
111.6
2
9'-10'hu
2'12
8
!
9'-10%"
8
3
,
'"
31.6
4.000~'
9 -IOV.
8
4.500
9'-10%"
8
4
5
5.563
~10"
8
6 _ _-'--_-'6~.6~2.5
.~6.06'::5~.....L_-..:!.2:.::8~0_ _,
9'-1()':_.....L_--'S"--_ _
TABLE 17-5b
Dimensions-!MC Conduit Types I & II
Trade
Size
y,
%
1
1%
1%
2
2%
3
3%
4
0.0.
I
Inches
II
1.0.
inches
II
Wall Thickn...
I
I
II
.815
1.029
1.290
1.638
1.883
2.360
2.S57
3.476
3.971
4.466
.833
1.043
1.308
1.653
1.893
2.368
2.S63
3.468
3.988
4.4S8
.675
.879
1.120
1.468
1.703
2.170
2.597
3.216
3.711
4.206
.673
.873
1098
1.437
1.677
2.152
2.553
3.178
3.668
4.168
.070
.075
.085
.085
.090
.095
.130
.130
.130
.130
.080
.085
.105
.108
.10S
.108
.155
.155
.160
.160
or in some cases two sizes smaller is the governing concern.
where voltage drop
Corrosion Charaderislits 01 Rigid Aluminum Conduil
Alloy 6063, which is commonly used for rigid aluminum
conduit, has an industry maximum limit of 0.1 percent
copper coment and 0.35 percent for iron (Table 17-7).
Aluminum conduit has been used successfully for more
than 50 years in many marine and corrosive industrial
installations under conditions where galvanized steel coo­
duit should not be used.
The galvanizing on a steel conduit acts as an anodic
sacrificial metal coating and is continuously uwearing
out." On the other hand, aluminum quickly builds up its
refractory oxide layer which is relatively inert to most
chemicals except strong alkalies and acids. This tough,
protective skin automatically renews itself whenever the
bare metal becomes exposed.
Aluminum's resistance to atmospheric corrosion has
!
Length
WID Coupling
9'-11%."
I
9#-11 %"
9'-11"
9'-11"
9'-11"
Threads
Per Inch
14
14
11%
11 YzI
11%
9'-11%"
11 Y,
8
8
8
9'-11%"
8
9'-11'~
9'-10Y2"
9'-1OY,"
been demonstrated repeatedly by tests and research stud­
ies. Some of the results:
No signs of corrosion were evident on rigid aluminum
conduit when inspected 5 years after it was installed on
cooling towers. Severe combinations of moisture and
chemical-laden atmospheres at the installation had reo
quired frequent replacement of rigid galvanized conduit,
according to Paper CP 58-1072 of the American Institute
of Electrical Engineers.
Aluminum alloy 6063 was exposed to marine, industrial
and rural atmospheres. After exposures of I, 2, 3,4 and 5
years, there appeared no significant effects on yield and
tensile strengths.
Atmospheric corrosion rates of four solid metals, as deter­
mined by a IO-year study project of an ASTM committee.
are shown in Table 17-8.
Broad Industrial Application: Installations of rigid alu­
minum conduit usually require no maintenance painting
17-5
related structural applications of aluminum
TABLE 17·1
TABLE 17-6
Composition of Aluminum Conduit Alloy (6063) Weight Comparison per Hundred Feet*
Trade
Size
I
Rigid
Aluminum
Y,
28.1
%
37.4
54.5
71.6
88.7
118.5
187.5
246.3
295.6
305.2
478.9
630.4
1
1%
11;2
2
2,.
3
,
3;1
4
5
6
!
I
Galvanized
Steel
80.3
106.4
154.5
203.7
251.0
338.0
541.0
697.0
837.0
1003.0
1343.0
1823.0
IMe
Type I
Percentage Limits
per Industry
57.0
Standards
78.0
112.0
144.0
176.0
235.0
393.0
483.0
561.0
625.0
i
I
Copper
Silicon
Iron
Magnesium
Manganese
Chromium
Titanium
Zinc
Others
Aluminum
*Nominal
0.10
0.20
0.35
0.45
0.10
0.10
0.10
0.10
0.15
max.
to 0.6
max.
to 0.9
max.
max.
max.
max.
max,
Remainder
• Alloys with up to 0.40% copper are ac·
ceptab!e to Underwriters' laboratories,
Inc.. for use in rigid aluminum conduit
and fittings.
or protective treatment, as may be required on steel cOn­
duit. Because its resistance to corrosion is greater than
steel's, it is the choice for many severely corrosive in­
dustrial environments such as: sewerage plants, water
treatment stations, filtration plants and chemical plants.
Exposure to Galvanic or Electrolytic A ttack: Aluminum
conduit should not be direct-buried in earth due to un­
predictable soil conditions of moisture, possible presence
of strong electrolytes and stray electrical currents. If it is
necessary to bury aluminum conduit, in accordance with
the NEC, it should be thoroughly coated with coal-tar
epoxy Or given a layer of half-tapped approved tape. Also,
factory-applied PVC coatings are commercially available.
Galvanic attack can occur when the aluminum forms
the anode of a battery created by twO dissimilar metals
in contact with an electrolytic medium. Stray alternating
or direct currents can aggravate the galvanic action,
eventually destroying the buried or embedded conduit.
tives (to speed setting time), use of sea or brackish water.
use of unwashed beach sand or other saline aggregate and
similar materials.
2. A tow resistance conduit run should be maintained.
3. The circuit neutral should be grounded at one point
and be insulated from and have no electrical contact with
the conduit. The conduit should not carry any of the
ground currents associated with normal operation.
Hazardous Locations: Requirements of Article 500 of the
NEC covering conduit installations in hazardous locations
can be easily met with properly installed rigid aluminum
conduit. The characteristics of aluminum conduit facilitate
the installation of a tight explosion-proof system.
Installation 01 Aluminum Conduit"
Easier, safer workability adds to the many advantages
of rigid aluminum conduit at every step of installation.
Conduit Embedded in Concrete: In accordance with the
NEe, aluminum conduit should not be embedded in con­
crete without approved protection because of the possibil­
ity of galvanic or electrolytic corrosion. Stray currents
can aggravate the corrosion, severely damaging the con­
duit. If it is necessary to embed aluminum conduit in
concrete, it should be given the same protection recom­
mended for direct·earth burial.
Additionally, to mitigate the corrosion of all metallic
conduit embedded in concrete:
Cutting: Fast, easy cuts are handled well by standard
cutting tools, which are generally used. Power saws and
cutting wheels work fast and neatly on I-II2-inch and
larger conduit; for smaller sizes (as for smaller steel con­
duit), an IS-tooth hacksaw is recommended for easy, neat
cuts. Reaming, as required by the NEC, can be easily
accomplished by conventional means.
I. The concrete should contain no extraneously added
chloride. Such chlorides can originate from concrete addi­
See also ChapleT 11 sections describing methods of installing
cable in conduit and calculations of pulling tension.
17·6 Threading: Standard dies, used according to accepted
good practices, thread aluminum rigid conduit faster than
(j
rigid aluminum conduit
C
F
Steel
1600
BOO
1400
100
1100
600
'"
1000
.!!1
tr
'"
;;
"
500
~
~
'"
n
E
~
800
400
600
300
100
400
Aluminum
100
200
0
0
I !
I
( 2-
4
!
I
5 6 ! 10
Seconds
inch conduit ,22,200 amperes)
Fig. 17-1. Measured temperature rise vs time for aluminum
and steel conduit under shari circuit conditions.
steel and with less effort. Hand threaders produce good
threading on smaller sizes. Power threaders, however. can
be used on all sizes and operated at maximum drive speeds.
When using top drive speeds. and for longer !oollife. dies
with 300 to 350 rake angles should be used. Standard
cutting oils of known good quality are recommended for
uniform threads.
Bending: Sweeps, elbows, cross-overs. offsets-every type
of bend is easily made in rigid aluminum conduit. Stan·
dard hand and power benders produce smooth and exact
bends in all conduit sizes.
Standard EMT benders can be used on I-inch or smaller
aluminum conduit for one-shot bends. Use an EMT
bender one size larger than the conduit. Mechanical or
power benders can be used on all sizes, provided they
have shoes and action similar to those of an EMT bender.
Joining: Adequate electrical conductance in a conduit sys­
tern requires tight joints. To simplify field joining. both
threads of every length of rigid aluminum conduit should
be lubricated in the field if not lubricated at the factory.
(On field-cut threads. of course, a reliable quality lubricant
containing zinc or graphite should be used). Proper lubri­
cation aids in assuming tight joints. a system that can be
easily dismantled. and a permanent, low-resistant electrical
ground path.
Pulling: Modern equipment and faster, safer techniques
that work well with aluminum conduit have been intro­
duced for fishing and pulling through all types of wire­
ways.
Propelled Lines: C02 or air propelled lines of nylon or
other plastic shoot through all sizes or rigid aluminum
conduit.
Small Conduit: In sizes up to i-il2-inch and on shorter
runs - up to 100 feet - polyethylene fish tapes can be
used effectively. Also recommended are round. flexible.
speedometer-type steel cables. Use of flat steel tapes
should be avoided' since they tend to jam in the bends
or, if not used carefully, scrape and cut conduit walls.
17.7
related structural applications of aluminum
TABLE 17-8
Atmospheric Corrosion of Solid Metals over a 10 Year Period
Atmosphere
~
Desert
Rural
Coastal
Location
Aluminum*
___._ j.________ 1
Zinc ....
Lead
0.000
0.001
0.004
0.028
0.031
0.025
0.005
0.023
0.020
0.052
0.047
0.046
0.010
0.042
0.021
0.068
0.190
0.190
0.009
0.019
0.022
0.016
0.017
0.027
Phoenix, Arizona
Industrial
State College, Pa.
Key West, Fla.
La Jolla, Cal if.
New York, N.Y.
Industrial
Altoona, Pa.
Coastal
____L_
Copper
I
Corrosion rate shown in average mils per year. Table based on ASTM data (Committee report).
* Aluminum 1100. Aluminum conduit is usually made from aluminum alloy 6063, generally considered to be equivalent in
corrosion resistance to aluminum 1100.
**Prime western zinc.
Larger Conduit: For pulling large conductors through
larger conduit or longer runs, polypropylene rope is
recommended. Steel pulling cables, especially when old
or frayed, can damage steel or aluminum conduit.
Fittings for Rigid Aluminum Conduit: Although galvan­
ized or plated steel fittings are permitted by the 1987
NEC and can be safely used with rigid aluminum conduit,
good aluminum fittings result in a superior installation.
An all-aluminum conduit run has better conductivity
and provides safer ground protection. And aluminum's
17-8
non-magnetic property, in fittings as well as conduit,
reduces ground current losses and voltage drops.
Expansion loints: Linear expansion of rigid aluminum
conduit is not a factor in most installations. If a straight
run is unusually long or subjected to extremes of tempera­
ture, expansion fittings might be needed. A good general
rule: use an expansion joint if the degree-feet of a run
may exceed 10,000. (Degree-feet is the length of the run
in feet mUltiplied by the temperture rise, OF.)