Electrical Wiring Methods Most People Don't Know About
As an electrician, I often come across homeowners who are unfamiliar with the different types of electrical wiring methods used in homes. In this article, I'll discuss several wiring methods that most people don't know about, along with their pros and cons.
Knob and Tube Wiring
Knob and tube wiring was commonly installed in homes built before 1930. This type of wiring consists of single black rubber insulated wires run through ceramic knobs mounted to wall studs and joists. The wires are separated from each other by about 1 inch using ceramic tubes.
The pros of knob and tube wiring:
- Allows good ventilation and cooling of conductors
- Less prone to damage from nails and screws compared to modern NM cable
The cons:
- Only allows for single conductor wires, limiting amperage capacity
- Outdated, no longer used in modern homes
- Can be unsafe if insulation breaks down or modified incorrectly
While I don't recommend leaving old knob and tube wiring in place, some homeowners choose to do so. If you have knob and tube wiring, I'd advise having it evaluated by a licensed electrician.
Armored Cable (AC)
Armored cable (AC), sometimes called BX cable, first became available in the 1920s. It consists of two or more insulated conductors wrapped in a spiral layer of steel armor. This armor provides extra protection against accidental damage compared to knob and tube and early NM cables.
The pros of armored cable:
- Sturdy steel armor resists damage
- Flexible and easy to install around corners
The cons:
- Heavier and more expensive than NM cable
- Requires special connectors to join cables and access wires
Armored cable is approved for some wiring applications, but NM cable has largely replaced it in modern residential wiring.
Non-metallic Sheathed Cable (NM)
Non-metallic sheathed cable (NM), commonly called Romex, has been used since the 1960s. It contains two to four insulated copper wires wrapped in a protective plastic sheath. Unlike armored cable, it does not contain any metal.
The pros of NM cable:
- Lightweight and inexpensive compared to armored cable
- Easy to install in walls, through joists, and around corners
- Different colored insulated wires for hot, neutral, and ground
The cons:
- Not protected by a metal shield like armored cable
- Plastic sheath can be damaged by nails and screws
NM cable is the most common wiring method found in modern homes. It comes in various sizes suitable for 15 to 50 amp electrical circuits.
Conduit Wiring Methods
Conduit refers to thin-walled tubing or pipe used to enclose and protect electrical wires. The conduit itself does not contain the conductors inside it. Common types of conduit include:
- Electrical Metallic Tubing (EMT) - thin steel tubing that can be bent by hand
- Rigid Metal Conduit (RMC) - thick steel threaded piping
- Flexible Metal Conduit (FMC) - bendable steel conduit
- Non-metallic Conduit - made of PVC or other plastics
The pros of conduit wiring:
- Very sturdy physical protection for wires
- Metal conduit can act as grounding path
- Allows easy wire changes by pulling new wires through conduit
The cons:
- More expensive and labor intensive to install than NM cable
- Requires conduit bodies, junction boxes, elbows, and fittings
- Metal conduit can corrode over time if not properly installed
Conduit is more common in commercial or industrial buildings but is also useful in certain residential applications.
In Summary
The key wiring methods I've described include:
- Knob and tube - oldest method no longer used today
- Armored cable - protect conductors but heavy and expensive
- Non-metallic sheathed cable - most common modern wiring method
- Conduit - provides sturdy physical wire protection
Understanding the different types of wiring allows you to make informed repair, replacement, and upgrade decisions for your home's electrical system. Let me know if you have any other questions!