Cable damaged during installation can cause service failures. Calculations should be made to indicate whether the pull looks easy or impossible. When a marginal situation is encountered, the entire pull should be reviewed. A final decision should be made based on factors known to the end user and installer. Pulling tensions may be evaluated by determining the maximum tension based on the pulling device used, and the maximum tension that can be applied to the conductors. The lesser of these two values is the maximum allowable tension. After calculating the pulling tensions, sidewall pressures may be calculated.
Do not exceed the allowable tension stated by the manufacturer of the pulling device or 10,000 pounds, whichever is less. Do not use metallic shielding wires, tapes or braids, or armor not designed for the purpose, in pulling tension calculations.
SINGLE CONDUCTOR:
T = S * A (pounds)
Where:
T = conductor tension, lbs
S = conductor stress, lbs/cmil (Table 1)
A = conductor area, cmil (Table 2)
MULTIPLE CONDUCTORS:
T = N * S * A (pounds) for 3 or less conductors
T = (0.8) * N * S * A (pounds) for more than 3 conductors
Where:
N = number of conductors
NOTE: Pulling different conductor sizes at the same time is not recommended if the conductor size or other cable characteristics are significantly different. If different size conductors must be pulled, it must be done with care.
TABLE 1
MAXIMUM ALLOWABLE CONDUCTOR STRESS
Cable Type |
Material |
Temper |
lbs/cmil |
All |
Copper |
Soft |
0.008 |
Power |
Aluminum |
Hard |
0.008 |
Power |
Aluminum |
3/4 Hard |
0.006 |
Power |
Aluminum |
AA-8000 |
0.006 |
URD |
Aluminum |
1/2 Hard |
0.003 |
Solid |
Aluminum |
Soft |
0.002 |
Note: 3/4 hard aluminum is allowed for power cable. The 1990 NEC defines use of AA-8000 for solid (8, 10 & 12 AWG) and stranded (8 AWG through 1000 kcmil) conductors.
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TABLE 2
CONVERSION FROM AWG to CMIL
(Solid and concentric stranded Aluminum and Copper conductors)
AWG |
cmil |
12 |
6,530 |
10 |
10,380 |
8 |
16,510 |
6 |
26,240 |
4 |
41,740 |
3 |
52,620 |
2 |
66,360 |
1 |
83,690 |
1/0 |
105,600 |
2/0 |
133,100 |
3/0 |
167,800 |
4/0 |
211,600 |
kcmil |
cmil |
250 |
250,000 |
300 |
300,000 |
350 |
350,000 |
400 |
400,000 |
500 |
500,000 |
600 |
600,000 |
750 |
750,000 |
800 |
800,000 |
1000 |
1,000,000 |