Cable damage from fishing is done before the circuit is ever energised. A run pulled hard around a metal edge, through a burr in a bored hole, or past the sharp lip of a knockout can lose sheath or conductor insulation at a single point, and that point is now sealed inside a wall where no inspection will ever reach it.
Tension is the underlying problem
Fishing cable through a finished wall almost always involves more force than a new-construction pull, because the route is indirect, the cable rubs against studs and insulation, and there is no way to feed it evenly. People end up pulling with both hands and a foot against the wall. High tension turns every edge along the route into a cutting tool and every bend into a place where the cable presses hard against something. The cure is to reduce force rather than to pull harder: more access holes, better lubrication where appropriate, and a fish tape or rod system that lets the cable follow a cleaner path.
Where the damage happens
Metal box knockouts with sharp edges, unreamed conduit ends, drilled holes with burrs left by a dull bit, the edge of a metal stud, the corner of a fire block, and the sharp lip of a hole in sheet metal ductwork if someone routed badly. Insulated cable dragged past any of these under tension will show a scrape, and sometimes the scrape reaches the conductor. Metal-framed walls are particularly unforgiving and require grommets or bushings in every opening the cable passes through, which is a requirement rather than a nicety.
Bend radius is a real constraint
Cables have a minimum bend radius, and pulling one around a tight corner exceeds it. Too tight a bend deforms the conductors, stresses the insulation on the outside of the bend and compresses it on the inside, and in flat cable can force the hot conductor against the bare grounding conductor. Bends made during a pull are worse than bends made deliberately afterwards because the cable is under tension while it bends. When a route requires a sharp change of direction, make an access opening at that point and feed the cable through in two stages rather than dragging it around.
Inspect before you terminate
The single most useful habit is to look at the cable after the pull and before landing it. Run the length that is accessible through your fingers, checking for scrapes, flattened sections and exposed conductor. If the pull was hard, be suspicious. Where damage is found near an end, cut back and re-pull if there is enough slack. Where damage is suspected in the middle of a concealed run, the honest answer is to pull the cable out and start again, which is painful but far better than energising a compromised conductor inside a wall.
Techniques that reduce the risk
Use a proper fish tape or glow rod rather than improvising with wire. Attach the cable with a pulling head or a taped taper so it does not snag. Make more access openings; patching drywall is quick and cheap compared with damaged cable. Ream bored holes and fit bushings or grommets in metal. Feed from the easier end and pull from the harder one, with a second person feeding so the cable is not fighting a coil. And avoid fishing through cavities crowded with insulation if a different route exists, because insulation multiplies friction dramatically.
What to check before you act
- Ream bored holes and fit grommets at every metal opening on the route.
- Add access openings rather than increasing pulling force.
- Inspect the full accessible length for scrapes before terminating.
- Feed from one end with a helper rather than dragging against a coil.
Common questions
Can I use lubricant on NM cable?
Lubricants are formulated for specific insulation types. Use a product compatible with the cable you are pulling and follow its instructions rather than improvising.
How do I test a cable I think I damaged?
Insulation resistance testing can reveal damage a continuity check will not, but it requires the right instrument and technique. If damage is suspected in a concealed run, replacement is the reliable answer.
The practical takeaway
Fishing damage is invisible and permanent, so the goal is to pull gently rather than to pull successfully. Cut more access holes, remove every sharp edge on the route, and inspect the cable before you land a single conductor.
References and further reading
- NFPA: National Electrical Code overview covering cable installation requirements
- OSHA: wiring design and protection including conductor insulation integrity
- CDC NIOSH: electrical hazard overview relevant to damaged wiring
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
