Pulling conductors

Once the raceway is installed, the conductors must be pulled into it without damage. On small branch circuits, a fish tape and a careful helper are enough; on large feeders, a pull involves reels weighing hundreds of kilograms, a powered puller, a crew in radio contact and a calculation of the forces on the cable. A damaged conductor may pass inspection and fail years later, so the techniques in this lesson matter long after the job is finished. The lesson covers planning a pull, the tools, conductor identification, estimating pulling tension, lubricants, the pull itself, and testing afterwards.

Planning the pull

Before pulling, confirm that the raceway is complete, clean and correctly sized: every coupling tight, every bushing in place, every end reamed (Lesson 2), and the fill and total bends within the code limits (ELEC-220, Lesson 2). Then decide:

• Which conductors go in each raceway, in what sizes, colors and lengths, from the panel schedules and feeder schedules (Lesson 11). Measure or calculate the run length, then add enough for terminations and a margin.
• The pull direction. Tension builds up through the run, and bends multiply it, so a run with bends near one end is usually pulled so that the bends come early in the pull, where tension is still low: feed the cable in at the end nearest the bends and pull from the far end.
• The equipment: fish tape or blower and line, rope of adequate strength, a hand-pulled or powered puller, sheaves or pulling elbows at pull points, reel stands or a cable cart, lubricant, radios.
• The crew: someone feeding at the reel end, someone at the puller, and people at intermediate pull points, all in communication. Everyone knows the signal to stop.

Tools for pulling

• Fish tapes: steel, fiberglass or nylon tapes pushed through a raceway to pull in a rope or small conductors. Non-conductive tapes are safer near energized equipment.
• Blower and vacuum systems: a vacuum or compressed air pulls or pushes a foam piston (a "mouse") attached to a light line through the raceway, quickly and over long distances. The light line then pulls in a stronger rope.
• Pull rope: low-stretch rope rated well above the expected tension. Stretchy rope stores energy and can whip violently if it breaks.
• Cable pullers (tuggers): capstan-type machines that wind the rope around a rotating drum, controlled by an operator who regulates tension by how hard the rope is held. Larger pullers have tension gauges and automatic limits.
• Basket grips (wire mesh grips) and pulling eyes attach the rope to the conductors; for groups of conductors, the conductor ends are staggered and taped smoothly so the head of the pull passes through fittings without catching.
• Sheaves and pulling elbows guide the cable at pull points and at the raceway entrance, protecting it from sharp edges and reducing friction.
• Reel stands, jacks and spindles let large reels turn freely, so the cable feeds without being dragged or kinked.

Conductor identification

Conductors are identified by color so that each phase, neutral and bonding conductor can be traced. Codes fix some colors: the neutral (identified conductor) must be white or gray, and the bonding or equipment grounding conductor green (or green with yellow stripes, or bare). Phase colors are partly code and partly convention:

In the United States, the NEC requires white or gray for the grounded (neutral) conductor and green, green with yellow stripes or bare for equipment grounding conductors, and requires that the identification method for ungrounded conductors be posted at each panel (210.5(C) and 215.12(C)). By common convention, 208Y/120 V systems use black, red and blue, and 480Y/277 V systems use brown, orange and yellow, with gray neutrals for 277 V.

Large conductors are often available only in black, so they are identified at every termination and accessible point with colored tape or labels. Where several systems share a building, conductors of different systems must be distinguishable, and neutrals of different systems kept separate and identified.

Estimating pulling tension

For long or heavy pulls, estimate the tension before pulling, to choose the equipment and pull direction and to check that the conductors will not be damaged. For a straight section, the tension added is roughly the weight of the cable times the coefficient of friction times the length:

At a bend, the tension leaving the bend is the tension entering it multiplied by an exponential factor that depends on the friction coefficient f and the bend angle θ in radians:

With lubricant, f is typically 0.3 to 0.5; a 90° bend (θ = 1.571 rad) with f = 0.4 multiplies tension by about 1.87. The order of sections matters: tension is multiplied at each bend, so a bend near the end of the pull, where tension is already high, adds much more than the same bend near the start.

Another hazard is jamming. When three conductors of the same size are pulled into a raceway whose inside diameter is about three times the conductor diameter (a ratio between about 2.8 and 3.2), the conductors can wedge side by side in a bend and jam, producing enormous tension. Checking this ratio and choosing a different raceway size avoids the problem.

Lubricants

Pulling lubricants (wire lubes) reduce friction, often halving tension compared with dry pulling. Use a lubricant compatible with the conductor insulation and jacket, as stated by the lubricant and cable manufacturers. Apply it generously to the conductors as they enter the raceway and, for long pulls, pre-lubricate the raceway by pulling a lubricant-soaked swab through it. Clean up spills, because lubricants make floors slippery.

Making the pull

At the feed end, set the reels so that the conductors feed straight into the raceway without crossing or kinking, with a sheave or feeding tube to guide them over the edge. Pull steadily without stopping if possible, because restarting from rest requires more force than continuing (static friction is higher than sliding friction). The person feeding watches for crossovers, insulation damage and reels that bind, and calls a stop immediately if anything looks wrong. At intermediate pull points, helpers guide the cable around the corner and keep it from rubbing on edges. Never exceed the rope's or puller's rating, and stand out of the line of the rope under tension: a broken rope can snap back with dangerous force.

For branch circuits, the same principles apply on a smaller scale: pull from boxes with enough slack, keep conductors from twisting, and leave at least the minimum free conductor length at each box for connections (commonly about 150 mm or 6 inches, with more in large boxes and panels).

After the pull

Leave enough length at each end for routing and termination, cut conductors cleanly, and protect the ends from moisture until they are terminated. Label every conductor at both ends with its circuit or feeder designation. Before energizing, feeders are tested: an insulation resistance test with a megohmmeter between each conductor and ground and between conductors, at a test voltage suited to the insulation rating (for example 1,000 V DC for 600 V cable), confirms that the insulation was not damaged during the pull. Low or unequal readings between phases indicate a problem that must be investigated before the feeder is put in service. Results are recorded for the commissioning documents.

In the United States, insulation resistance testing of feeders is commonly specified in Division 26 sections, often referencing the NETA Acceptance Testing Specifications (ANSI/NETA ATS) for minimum values and procedures.

Pulling in cable tray and long runs

Cable laid in tray is installed differently from cable pulled through conduit. For short runs, cables are simply laid in by hand. For long runs, rollers are placed every few meters along the tray, with sheaves or radius rollers at every bend so that cables do not drag over rungs or bend below their minimum radius. The cable is pulled with a rope and a puller, or by hand with a team spaced along the route, and then lifted off the rollers into its final position, tied and dressed. On very long underground or tunnel runs, intermediate pullers or cable pushers (which grip the cable and drive it forward) share the load so that no section exceeds its tension limit.

Minimum bending radius applies to every cable during pulling and in its final position. Manufacturers state it as a multiple of the cable diameter (for example 8 or 12 times for many power cables, more for shielded medium-voltage cables). Exceeding it, for example by pulling around a sharp edge without a sheave, can damage insulation and shielding in ways that only appear later as failures. Check the radius of every sheave, conduit body and bend the cable will pass through.

Coordination and records

Large pulls are planned with the general contractor and other trades, because they need space for reels and pullers, may block corridors, and sometimes require openings in floors or walls. Record the pull: date, conductors, reel numbers (so cable lots can be traced if a defect appears), measured or estimated tension, lubricant used, and the insulation resistance test results. These records become part of the turnover documents and are invaluable if a feeder fails.