Making an underwriter's knot is a technique for tying lamp cord or appliance cord at the point where it joins the plug or socket. It is designed to reduce strain on the screw terminal connections — where the metal parts of the wires connect to the socket or plug — and prevent the wires from pulling free. An underwriter's knot may seem like an old-fashioned electrician's trick (and indeed it is), but it's necessary today because people still pull out plugs by pulling on the cord, and they still abuse lamp cords in one way or another.
With an underwriter's knot in place, the cord virtually cannot be pulled out of the plug or become disconnected from the socket.
How to Make an Underwriter's Knot
The photo above clearly shows an underwriter's knot before it is pulled tight. Note that the wire end that started on the left passes behind the cord before running through the loop on the right. The wire end that started on the right passes in front of the cord before running through the loop on the left. It doesn't matter which wire end passes on the front or back of the cord; it only matters that one goes in front and one goes behind. This ensures the knot will be symmetrical and will tighten into a compact knot. Here are the basic steps for making an underwriter's knot:
- Slide the body of the plug over the end of the cord before making the knot. If you're installing a lamp cord, insert the cord into the socket base and slide the socket down onto the cord.
- Separate about 4 inches of the two insulated wires, or conductors, within the cord. With lamp cord (sometimes called zip cord), simply pull the two wires apart so they separate in the middle. With round appliance cord, carefully cut away about 4 inches of the outer cord insulation, using a utility knife or a wire stripper. Do not cut into the insulation of the individual conductors.
- Bend one of the wires to form a loop so that the loose wire end passes behind the cord. Bend the other wire to form a loop, passing the loose end over the front of the cord.
- Feed the loose end of each wire through the loop of the opposite wire.
- Pull the two loose ends in opposite directions to tighten the knot.
Once the knot is complete, you can prepare the cord for the final connections by trimming each wire end to length and stripping 3/4 inch of insulation from the end, using wire strippers. It's also possible to trim and strip the wires before making the knot, but for beginners it's more foolproof to leave the wire ends long and trimming them after making the knot.
Origin of the Underwriter's Knot
The underwriter's knot is similar to a basic form of the "wall knot" described in "Ashley's Book of Knots," a book that was originally published in 1944 and remains the definitive guide for making knots of all kinds. The term underwriter's knot is generally believed to have come into use because this method was approved by insurance underwriters as a safer means for making electrical connections.