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 PULLING
TENSIONS
The
information provided herein may serve as a guide to installing
cables in ducts or conduits and is based in part on industry studies. Where experience has justified it,
we have included our own figures.
Two
tension calculations are required for each cable installation.
First must be calculated the MAXIMUM ALLOWABLE TENSION
for the particular cable that is to be installed.
This value is dependent upon the method
of attaching to the cable, the allowable sidewall
bearing pressure and the construction of the cable.
Second,
knowing the weight of the cable and the details of the conduit run the ESTIMATED PULLING TENSION that can occur
during installation is calculated and compared
with the MAXIMUM ALLOWABLE TENSION.
The
following gives details for calculating each of the above
tension values.
Maximum
Allowable Tension
(1) Based on pull by conductor:
Tm = .008 x n x CM (applies to both annealed copper and hard drawn
aluminum conductors)
Tm = maximum allowable tension in lbs.
n = number of conductors in cable (assumes equal tension in each
conductor)
CM = circular mil area of each conductor.
(2) Based on pull by Kellems grip over lead sheath:
Tm = 4712 x t(D-t)
D = outside diameter of cable in inches
t = lead sheath thickness in inches.
(3) Based on pull by Kellems grip applied over:
Non-shielded, jacketed cables - 2000 lbs.*
Shielded, jacketed cables - 1000 lbs.*
* Do not exceed tension limit of Condition 1 above.
(4) Based on pull by Kellems grip applied directly on the insulation or outer
Permashield® layer of Kerite Double Permashield® cables after removing the
shielding: 3,000 lbs. per inch of conductor diameter.*
* Do not exceed tension limit of Condition 1 above.
(5) Based on maximum allowable side bearing pressure when pulling around a
conduit bend:
(a) Single conductor or multi-conductor
Tm = 450 x D x R
Tm = maximum allowable tension on cable in lbs.
D = outside diameter of cable in inches
R = radius of bend in feet
(b) Three conductor twisted
Tm = 225 x D1 x R
(c) Three 1/C cables in parallel
Tm = 675 x D1 x R
For (b) and (c)
Tm = maximum allowable tension on three cables
D1 = diameter of one individual cable in inches
R = radius of bend in feet
The
actual allowable tension will be governed by the lowest of
the above calculated tensions for the method of pull
selected.
Example:
Determine the maximum allowable pulling tension on three
1/C 500 kcmil copper, 15kV, 175 mil, 100% insulation
wall, copper tape shield, PVC jacketed cable, paralleled, to be pulled around a 3 ft. radius bend
by Kellems grip applied over outer jackets.
Limit
by Condition 1
Tm
= .008 x n x CM
Tm = .008 x 3 x 500,000
Tm = 12,000 lbs.
Condition
2 does not apply
Limit
by Condition 3-shielded, jacketed-1,000 lbs.
Condition
4 does not apply.
Limit
by Condition 5-side bearing pressure
Tm
= 675 x D1 x R
D1 = 1.51
Tm = 675 x 1.51 x 3
Tm = 3,058 lbs.
The
maximum pulling tension is limited by Condition 3 -1,000
lbs.
Example:
Determine the maximum allowable pulling tension on three
1/C 500 kcmil copper, 15kV, 175 mil, 100% insulation
wall, copper tape shield, PVC jacketed cable, paralleled, to be pulled by conductor around a 3 ft.
radius bend.
Limit
by Condition 1
Tm
= .008 x n x CM
Tm = .008 x 3 x 500,000
Tm = 12,000 lbs.
Conditions
2, 3 and 4 do not apply.
Limit
by Condition 5-side bearing pressure
Tm
= 675 x D1 x R
D1 = 1.51
Tm = 675 x 1.51 x 3
Tm = 3,058 lbs.
The
maximum pulling tension is limited by Condition 5 -3,058
lbs.
Estimated
pulling tension must be calculated to ensure it does not
exceed the maximum allowable pulling tension.
Estimated
Pulling Tension
Pulling
tensions anticipated for an installation are governed by cable size and weight, length of run, number and
angle of bends. Usually only approximations
can be made, the following simple assumptions provide safe guideline limits.
Calculation
of Tension
(1)
Straight horizontal run:
T
= W x L x n x C.F.
where:
T = tension in lbs.
W = cable weight in lbs./ft.
L = length of run in ft.
n = number of cables
C.F.= coefficient of friction
The coefficient
of friction will vary between 0.3 for well lubricated cables pulled
into new, smooth wall conduits to 0.5 for lubricated cables pulled
into rough or dirty conduits or ducts.
(2) Pulls around
static bends:
Multiplying
factors, shown below, must be used to estimate the increase in tension
due to pulling around bends. The tension at the point just ahead
of the bend is multiplied by the appropriate factor from the table
below, the product being the tension that exists immediately past
the bend. This factor must be applied in the calculation of the
estimated pulling tension at each point where the cable encounters
a bend as it is pulled through the duct or conduit run.
Multiplying
Factor
Angle of Bend in Degrees
Coefficient
of Friction |
15 |
30 |
45 |
60 |
75 |
90 |
| 0.30 |
1.08 |
1.17 |
1.27 |
1.37 |
1.48 |
1.60 |
| 0.40 |
1.11 |
1.23 |
1.37 |
1.52 |
1.69 |
1.87 |
| 0.50 |
1.14 |
1.30 |
1.48 |
1.69 |
1.92 |
2.19 |
Example:
Determine the tension required to pull three 1/C 500 kcmil
copper, 15kV, 175 mil insulation wall, copper tape
shield, PVC jacketed cable, in a horizontal duct as shown below.
For pull
A to B:
TB
= W x L x n x C.F.
W = 2.346 lbs./ft. of 1/C cable
L = 350 ft.
n =3
C.F.= 0.4 (assume average condition of duct wall)
TB = 2.346 x 350 x 3 x 0.4 = 985 lbs.
For pull
B to C:
TC
=TB x Multiplying Factor for 90° Bend
TC = 985 x 1.87
TC = 1,842 lbs.
For pull
C to D:
TD
= W x L x n x C.F. + TC
TD = 2.346 x 50 x 3 x 0.4 + 1.842
TD = 141 + 1,842 = 1,983 lbs.
Alternative
Set-up
Assume
pay off reel at D.
For
pull D to C:
TC
= W x L x n x C.F.
TC = 2.346 x 50 x 3 x 0.4
TC = 141 lbs.
For
pull C to B:
TB
=TC x Multiplying Factor for 90° Bend
TB = 141 x 1.87
TB = 264 lbs.
For
pull B to A:
TA
= W x L x n x C.F. + TB
TA = 2.346 x 350 x 3 x 0.4 + 264
TA = 985 + 264 + 1,249 lbs.
Result:
Pull by Kellems grip over the jacket is not allowed (1,000
lbs. maximum versus calculated pulling tension of
1,983 lbs. or 1,249 lbs. depending upon direction of pull). Pull by conductor is allowed. Tension is less
when pay-off reel is at the "D" end nearest
the bend location.
The
above calculations are based on the use of an approved pulling compound on the entire surface of the cable
with approximately 1/16 " layer of compound
to be applied as the cable enters the duct or conduit.
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