HomeMy WebLinkAboutLetter-4/22/02 (Vox Electric)
April 22, 2002
Oshkosh Inspector
1) Per your request Tony has recorded the Ohm readings on our parallel
feeder cables. See attached sheet signed by Tony.
2) We changed out the undersized J-Box on this run. We installed an
8"x8"x36" Hoffman Box.
3) We will be installing a 350 Amp breaker for this feeder, but I think we
could install a 400 Amp breaker.
Section 310-16 for 900 wire rating.
Section 310-15 for derating current carrying conductor.
Section 240-3 (B) for next breaker size.
Section 240-6 (A) for standard
size breaker.
4) 250 MCM single ground conductor was installed in each 3" sealtite.
Sorry for any inconvenience I may have caused you.
Sincerely,
Vos Electric, Inc.
tVm~
AI Mooren
\ ;
Technical Information & Technical Bulletins
@1998
B. DERATING FOR AMBIENT TEMPERATURE
All ampacity tables necessarily must use some ambient temperature as a basis and NEC Table 310-16
uses 300C (860F). At the bottom of the table, correction factors are given for situations where the ambient
is expected to be higher or lower than 300C (860F). Use of these factors is fairly straightforward; for
example, what is the ampacity of a 1/0 AWG, aluminum, type THHN conductor when the ambient
temperature is 1000F? The answer is found by taking the ampacity from the table and multiplying it by the
appropriate correction factor. In this example, we would have 135 x 0.91 = 122.8.5 amperes.
C. DERATING FOR NUMBER OF CONDUCTORS
Note 8 to the ampacity tables in the NEC contains the requirement columns for derating ampacity because
of adjacent current-carrying conductors. This note states that when the number of conductors in a raceway
or cable exceeds 3, the ampacities are to be reduced by the appropriate percentage. In the 1999 NEC, a
column of factors is given. For example, what is the ampacity of twelve No. 12 copper THHN conductors
installed in one conduit? From Table 310-16 the ampacity (in the table) is 30 amperes. From Note 8, the
derating factor for 12 conductors is 50 percent. 30 x 0.5 = 15 amperes per conductor.
D. DERATING FOR BOTH AMBIENT TEMPERATURE AND NUMBER OF CONDUCTORS
When derating for both conditions is necessary, then both calculations must be made. For example. what is
the ampacity of four 1/0 THW copper conductors when the ambient temperature is expected to reach
1100F? From Table 310-16 the ampacity is 150 amperes, derating factor for 1100F is 0.82, and derating
factor from Note 8 for 4 conductors is 80 percent. 150 x 0.82 x 0.8 = 98.4 amperes.
RECOMMENDED PULLING TENSION
FOR
PULLING CONDUCTORS IN RACEWAYS
The force required to pull a cable or cables into a raceway depends on the cable weight, the friction
coefficient and the raceway length. The following information is based on a study sponsored by the ICEA:
The maximum pulling tension placed on a cable should not exceed the following:
1. For cable equipped with a pulling eye or bolt attached to the conductor:
A. For copper of any temper and hard-drawn aluminum conductor 0.008 Iblcmil times the
circular mil area of the conductor(s), and,
B. For 3/4 hard drawn aluminum 0.006 Iblcmil times the circular mil area of the conductor(s).
2. For cable to be pulled with a cable grip over the sheath:
A. For cable with a lead sheath 1500 Ibs./square inch of lead cross-sectional area:
A = 1tt (D-t)
where A = cross-sectional area of lead (square inches)
t = sheath thickness (inches)
D = overall diameter of cable (inches)
o Southwiree
e Southwire is a registered trademar1< of Southwire Company.
One Southwire Drive
Carrollton. GA 30119 USA
770/832-4242
www.southwire.com
Technical Information & Technical Bulletins
B.
For a cable grip over nonlead jacketed cable, the maximum pulling tension should
not exceed 1000 Ibs. and, of course, may not exceed the maximum tension
calculated per 1 above. The pulling tension for a given installation may be
calculated from the following formulas.
T = L x W x f (for a straight section)
T = Pulling tension (Ibs.)
L = Length of Raceway Run (feet)
W = Weight of cable (Ibs./ft)
f = Coefficient of friction
(Generally taken as 0.5)
Tz = T1ela (for a raceway with a bend)
Tension for the straight section following the bend (Ibs.)
Tension for the straight section preceding the bend (Ibs.)
= Natural log base (2.718)
= Coefficient of friction
a = Angle of bend (radians)
(1 )
where
where
T =
T =
z
T =
1
e
f
(2)
As an aid in solving the above formula, Table 1 is offered listing the values of ela for
common angles.
TABLE I: Calculated (eta) Values
Bend Angle
in degrees
f = 0.40
f = 0.50
f = 0.75
15
30
45
60
90
1.11
1.23
1.37
1.52
1.87
1.14
1.30
1.48
1.69
2.19
1.22
1.48
1.80
2.19
3.25
The maximum sidewall pressure should not exceed 300 Ibs/ft of radius, i. e., the tension in the cable
immediately following a bend must not exceed 300 times the bend radius (in feet). For a sample
calculation, assume the following raceway plan.
200'
D~
i 900 ---,
70' 10'R
450
10'R C
B
A
Cable 3 1/C 500 kcmil, 0.175 wall, shielded, triplexed. 00 = 3.02"
Weight = 6.35 Ibs./ft. (copper conductor f = 0.5)
Pull from A to F
Normally the tension calculation is a progressive one as follows:
Tension at B (T1) = 200 x 6.35 x 0.5 = 635 Ibs. (from eq.1)
Tension at C (Tz> = T leta = 635 x 1.48 = 9401bs.
Tension at 0 (T) = 940 + (70 x 6.35 x 0.5) = 11621bs.
Tension at E (T4) = T 3efa = 1162 x 2.19 = 2545Ibs.
Tension at F (Ts) = 2545 + (100 x 6.35 x 0.5) = 2863 Ibs. total
,,~
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All Rights Reserved.
61-4
Technical Information & Technical Bulletins
@1998
Maximum permissible pulling tension on this cable equipped with a pulling eye bolt is
0.008 x 500,000 x 3 = 12,000 Ibs.
Sidewall Pressure (at C) = 940/10 = 94Ibs./ft.
Sidewall Pressure (at E) = 2545/10 = 255 Ibs./ft.
Because the sidewall pressure at E is rather high (through not outside design limits) it would be desirable
to investigate the results if the cable is pulled from F to A.
Tension at E (Ts) = 100 x 6.35 x 0.5 = 3181bs.
Tension at D (T7) = 318 x 2.19 = 6961bs.
Tension at C (T8) = 696 + (70 x 6.35 x 0.5) = 9181bs.
Tension at B (T9) = 918 x 1.48 = 13591bs.
Tension at A (T1o) = 1359 + (200 x 6.35 x 0.5) = 1994lbs. total
Pulling from F to A results in considerably less tension both at the bends and overall. While, in this case,
it would be acceptable to pull from either direction, it is prudent design to select the direction which results
in the least stress on the cable and equipment provided there are no extenuating circumstances such as
limited setup or working space at one end or the other.
During the pulling as well as other operations, it is frequently necessary to re-reel the cable or pass it over
pulleys or sheaves. To avoid damage to the cable, it is imperative that the cable not be bent over a radius
small enough to cause damage. This is especially important because this type of damage is frequently
concealed and therefore may very well go unnoticed until a proof test or service failure occurs. Tables
II, III & IV below give the minimum recommended bending radii for cables of various construction.
TABLE II
Non-Shielded & Non-Armored Power Cables
Cable 00, inches
Thickneness of Conductor
Insulation in Mils
Up to 1.000
\
1.001 to 2.000\
Over 2.000
Minimum Bending Radius as a Multiple
of Cable OD
155 and less
155-310
310 and over
4
5
5
6
7
6
7
8
TABLE III
Shielded or Armored Power Cable
Cable Type
Minimum Bend Radius as Multiple of
Cable 00
Flat Tape or Wire Armored
Tape Shielded
Wire Shielded
Interlocked Armor wlo Shielding
12
12
12
7
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~~ UUl1unj
@ Southwire is a registered trademark of Southwire Company.
One Southwire Drive
Carrollton. GA 30119 USA
770/832-4242
www.southwire.com
Technical Information & Technical Bulletins
TABLE IV
Control & Flexible Portable Cables
Cable Type
Minimum Bend Radius as Multiple of
Cable OD
Control Cables (19 conductor & larger
without armor)
8
Rubber jacketed, portable types used
on take-up reels or sheaves 0-5 KV
Over 5 KV
Control Cable (7 conductor & over)
6
8
20
These bending Radii are to be considered minimum recommended dimensions. When the cable is to be pulled
over or around these radii under tension, the maximum sidewall pressure of 300 Ibs. per foot must not be
exceeded. These radii are to be followed for static bends such as manhole training bends, etc.
ftt~ Southwire-
\~
MADE~
USA '=' Printed on Recycled Paper
Copyright 1998. Southwire Company.
All Rights Reserved.
61.5
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