/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgPulling Cables? Keep Them Slippery!PullingCablesKeepthemSlippery.htmHow far can you pull a cable in a conduit? It depends on the cable, the conduit and the lubrication.
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Pulling Cables? Keep Them Slippery!

How far can you pull a cable in a conduit?  It depends on the cable, the conduit and the lubrication.

“Cable lubricants are essential for reducing friction between the cable and the conduit,” says Dave Cooper, applications engineer. “Reducing pulling tension and sidewall pressure lets you pull cable farther, through more bends, without damaging the cable.”

Lubricant compositions range from wax emulsions to soap compounds to emulsions of polymers in water. Some lubricants do better with certain combinations of cable jacket and conduit materials.

Friction Numbers Vary

A cable and lubricant combination with a lower coefficient of friction will pull with less tension. That makes sense. But comparing lubricants can be tricky, because there are so many variables.

In one lubricant comparison, the coefficient of friction between a SOLONON cable jacket and PVC conduit varied from 0.06 to 0.18, depending on the lubricant, compared to a bare (unlubricated) value of 0.65. The table of jacket properties on page two of this issue lists a more conservative (higher friction) value of 0.35 for SOLONON in PVC conduit when it's adequately lubricated with a typical lubricant.

Lower friction reduces pulling tension, which decreases sidewall pressure in the bends. That protects the cable and increases overall reliability. Sidewall pressure can also stress the lubricant. Pulling through a bend, you may generate enough pressure to squeeze the lubricant into a very thin layer - 1 to 10 €m (0.04 to 0.4 mil). That makes the boundary, or thin-film, performance of the lubricant important. “The lubricant you choose must also be compatible with the jacket material,” says Cooper.

“If it isn't, it could affect cable life, especially at higher voltages.” UL-listed lubricants have been investigated for compatibility with common cable jacket materials.

How much do you need?

One rule of thumb for lubricant quantity is:

Q = 0.0015 x L x D
where Q is the quantity in gallons, L is conduit length in feet, and D is conduit nominal ID in inches.

You may need up to 50 percent more lubricant with stiff cable, dirty conduit, high conduit fills, multiple bends, or water in the conduit. Too much is probably better than not enough.

Put 1/3 to 2/3 of the lubricant into the conduit in front of the cable. Use a spreader to distribute lubricant evenly through the conduit. Unlubricated sections, especially bends, increase pulling tension.

Put the rest of the lubricant on the cable as it enters the conduit, applying most of it to the first half of the cable.

Cooper adds, “Different lubricants work differently. Whichever lubricant you choose, follow the manufacturer's instructions. That will let the lubricant work and give you a better pull.”

Some of the information in this article is based on a paper titled Friction Theory and Lubrication Techniques for Improved Cable Pulling, provided by American Polywater Corporation, Stillwater, Minn.