IF IT BREAKS, YOU PULLED TOO HARD
Pulling calculations start with conductor tension. Computers speed calculations.
- Published in Southwire's Power Cable Update newsletter in March 1997
- Reprint permission granted
How much tension will your cable stand? It depends on the conductor. Conductors are generally the only part of a power cable that can take pulling forces without damage.
"Never pull on shielding or neutral wires," says Sid Ticker, senior applications engineer. "Don't use the armor either, unless it's specifically designed as a pulling element."
Tension Varies With Cross-Section
For single-conductor cables, maximum pulling tension varies with the cross-sectional area of the conductor and with the conductor material.
- Tension = S x A
- Where S = Maximum allowable stress (see table) and
- A = conductor area in cmil
For example, with a copper 250kcmil conductor, multiply 250,000 by 0.008. That allows 2,000 pounds maximum tension.
For three or fewer conductors you can multiply the single-conductor tension by the number of conductors. For four or more conductors, use only 80 percent of the total conductor stress. Conductors should all be the same size. It's a bad idea to pull different conductor sizes together.
Other Factors May Impose Limits
What can you pull with that much tension? It depends. There are four key factors to consider: clearances, jam ratios, conduit fill and sidewall pressure.
Start with a diagram of the pull. First find the conduit size you'll need by calculating clearances, jam ratios and conduit fill. Then calculate the tension it will take to pull the cable through the conduit layout.
Many of the pulling calculations can be done on a PC with a specialized program. The computer lets you easily compare the same pull going in both directions.
"If there's a sharp bend in the conduit, tensions and sidewall pressures may work better if that bend comes at the end of the pull, rather than at the beginning," says Ticker.
If your calculated pulling tension exceeds the maximum allowable cable tension, or the maximum rating of the pulling device, you need to revise your pulling layout.
After you have a usable pulling tension, calculate the sidewall pressure at every bend. You may find additional limitations there.
"Preplanning is key to successful cable pulls," says Ticker. "Careful attention to detail will help prevent cable damage."
For more information on cable-pulling, refer to Chapter 7 of the Southwire Power Cable Manual.
Maximum Allowable Conductor Stress (S) |
Material |
Cable Type |
Temper |
Lb./cmil |
Copper |
all |
soft |
0.008 |
Aluminum |
power |
hard |
0.008 |
Aluminum |
power |
3/4 hard |
0.006 |
Aluminum |
power |
AA-8000 |
0.006 |
Aluminum |
URD |
1/2 hard |
0.003 |
Aluminum |
solid |
soft |
0.002 |
|
