Here is How (and Why) to Tie an Underwriter's Knot

Strain Relief That Holds

Underwriter's Knot
Underwriter's Knot. Photo: Timothy Thiele

An underwriter's knot is a technique for tying lamp cord or appliance cord at the point where it joins the plug or socket, designed to reduce strain on the screw terminal connections and prevent them from pulling free. It anticipates the situation in which people pull cords from wall sockets by tugging on the cord rather than by gripping the plug as is recommended.  

The method appears to have come into use in about 1944, when it was first known as an ​The term "underwriter's knot" is thought to have come into use because this method was approved by insurance underwriters as a safer means for making electrical connections.

 

An underwriter's knot is a simple and effective way to support the cord within the plug and give the connection some durability. With the underwriter's knot in place, the cord virtually cannot be pulled out of the plug.

Here is how to make the knot: 

  1. Slide the body of the plug over the cord before making the knot.
  2. Separate about 4" of the two conductors within the cord. On lamp cord (sometimes called zip cord), this involves simply pulling on the two wires until they separate in the middle. On round appliance cord, it usually means cutting away about 4" of the outer cord insulation.
  3. Strip about 3/4" of insulation from the ends of the individual conductors, using a combination tool or wire stripper.
  4.  Take each of the two wires and form a loop in each. Then take the free end of each wire and insert it through the loop of the opposite wire.
  5. Pull the knot tight.

Test to make sure the knot is large enough that it can't fit through the opening on the plug body.

 If the knot is too small, you may need to tie ​a second knot. Or, you can add a clamp to the base of the plug cap. Either method will secure the cord and keep it from being pulled from the plug connections.

Last, make the wire connections to the plug insert. You have just completed an underwriter's knot.

 

  • NOTE: The underwriter's knot is also useful in making lamp cord connections to a lamp socket, such as when rewiring a lamp with new cord, or when replacing a faulty socket. Before connecting to the screw terminals on the socket, the wires are tied in an underwriter's knot. After making the screw terminal connections, the socket sleeve is pulled down over the wire connections and knot, and the lamp is reassembled.