Branch Wiring Types

Branch wiring is the wires that run from electrical panels to outlets, switches and other devices in our homes. If you removed the drywall from your walls and saw electrical wires, most of it would be branch wiring. This type of wire, or cable, has evolved over the years. There are generally three types and I’ll describe them below:

Knob and Tube Wiring

Knob and tube wiring is a method of wiring that was installed in homes between about 1880 and the 1940’s. In order to route the wiring through the home holes would be drilled in the boards (joists, studs, etc.) in the home and a ceramic ‘tube’ inserted in the hole, then the wire would be run through the tube. If the wire was run along a board every so often a ceramic ‘knob’ would be attached to the wood and the wire would be attached to the knob. This is how this wiring type got to the name ‘knob and tube’.

Knob and tube wiring in homes can be problematic today because over time the insulation may have become brittle and extending knob and tube wiring should be done using modern wiring methods. With brittle insulation, if the wire is moved or bent, the insulation can crack and fall off. This can present a shock or fire hazard. If you need to add wiring to your home and the existing wiring is knob and tube wiring you need to transition to modern wiring techniques. In doing so you have to work with the existing knob and tube wiring and doing so makes the first problem mentioned all the more likely. Lastly, in an attic space with knob and tube wiring, you shouldn’t add insulation on top of knob and tube wiring. It should be encapsulated in some appropriate way or replaced.

Wire Inside a Conduit

In some older homes, 1950’s-ish, and in some jurisdictions today, wiring in a home is ran through a protective enclosure called a conduit. Conduit is made out of rigid, or sometimes flexible, metal or plastic. It’s essentially a pipe for electrical wires. Conduit is first installed between junction boxes of the electrical system and then wires are pulled or pushed through the conduit.

NM Cable or Romex®

NM Cable, often called Romex (R) which is a registered trademark.

NM, or Non-Metalic, cable is the most common type of wiring in use today in homes. It is essentially a set of wires encapsulated inside a protective sheathing. Originally the sheathing was made of a fabric material, today that sheathing is plastic. The wires inside the sheathing are themselves enclosed inside a coating of plastic insulation, except for conductors intended to be ground wires. Those are bare, with no insulation around them.

Today the sheathing around NM cable is color coded to the size of the wire inside. This started around 2001. A white sheathing indicates 14 gauge wire inside, yellow indicates 12 gauge wire, orange – 10 gauge, black – 6 or 8 gauge, gray – for larger gauges. Note that older NM cable in homes generally has the same color sheathing for all sizes of wire.

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