Residential Wiring Best Practices
PART 2: Rewiring Options, Benefits and Drawbacks
In Part One I discussed knob and tube wiring and the potential problems and issues that may arise if your home still contains legacy knob and tube. Perhaps your home still contains knob and tube, or you are considering remodeling or planning new construction. In this article I will explain current options and residential wiring best practices. I will also explain the pros and cons for the two most popular wiring types, Romex and BX, and why I ALWAYS use BX in any new wiring I install myself, and insist it be used in any project in which I am managing or consulting.
Since the 1960s, Knob and Tube has not met current electrical code thus any renovations or new construction built in the last 50 years should contain more modern wiring.
Today, residential construction primarily uses three types of wiring, Non-Metallic (NM) cable being the most common; followed by BX (also know as Armor Clad, or Flexible Metal Conduit); and individual conductors run inside conduit.
1) NM Cable: Non-metallic cable is a composite cable consisting of 1 or more “hot” conductors, a neutral conductor, and a ground wire. All the conductors are individually insulated and the entire bundle is sheathed in PVC plastic to make a nice, neat package.
NM cable is flexible, heat & fire resistant, easy to install, and relatively inexpensive.
NM cable is by far the most common type of wire used in single-family residential applications today, and been used extensively for the past 40 years.
NM cable has several advantages over knob and tube. The wire is protected by 2 layers of insulation, with each individual conductor being insulated and the entire bundle also being insulated. This makes insulation failure a rare occurrence and reduces the risk of accidental electrocution and arcing. The plastic used is resistant to moisture and microbes, and is very durable. The plastic sheathing also has a very high melting point thus is heat and flame resistant. Splices are done in junction boxes, thus protected from the elements and pests. Plus all the wires needed for a circuit are right there together in one neat package.
One added benefit of NM cable over knob and tube is that knob and tube emits fairly high levels of low frequency alternating current magnetic fields, due to the separation of the hot and neutral conductors. The magnetic field strength around knob and tube carries a good distance from the wires, and is proportional to the distance between the hot and neutral conductors. Inside NM cable, the hot and neutral conductors are side by side, thus NM emits a much smaller, more localized and weaker magnetic field. The magnetic field strength drops very quickly with distance from the NM cable.
2) BX: BX is also known as Armor Clad, or Flexible Metal Conduit cable. BX contains one or more individually insulated “hot” conductors, one individually insulated neutral conductor, and a ground wire, which may or may not be individually insulated.
BX is sheathed in a flexible metal spiral made either from aluminum or galvanized steel. BX provides all the advantages of NM cable plus some additional benefits.
The flexible metal sheathing is easy to install, about as easy as NM cable . It resists puncture by nails and screws, and, as I will describe later, is more resistant to being chewed through by rodents and other pests than NM cable and knob and tube.
BX is as good as NM cable from a Magnetic Field (MF) standpoint, but has the added benefit of shielding Electric Fields (EF). The metal sheathing absorbs the electric fields emitted by the wires and shunts it to ground. The plastic sheathing of NM cable does not shield electric fields.
The main drawback is that BX is more expensive. It is about 35-40% more for the cable than BX. It is also slightly more difficult to install, as cutting the metal sheathing takes slightly more time, than cutting NM cable .
3) Individual conductors run inside conduit: Individual conductors run inside conduit is used primarily in industrial and commercial applications, and is rarely used in residential wiring. It consists of individual, insulated conductors pulled from a spool through conduit, either metal or PVC. This is a more difficult and time consuming wiring method, but is more efficient in commercial applications where wiring runs must be exposed and thus must be inside some form of conduit.
If the conduit used is PVC, the benefits are the same as NM cable , low Magnetic Fields (MF) but no Electric Field (EF) shielding. If the conduit used is galvanized steel, the benefits are the same as BX, good resistance to puncture, low MF emittance, and good EF shielding.
Why I ALWAYS choose BX over Non-Metallic (NM) cable : A real world example.
Sometimes individual conductors run inside conduit is the only option, but when I have a choice between NM cable and BX, I always choose BX as a residential wiring best practice. There are two main reasons:
1) BX shields electric fields. Many of my concerned clients have spent a lot of effort and money to shield against and reduce their exposure to electric fields. Some even go as far to turn off the circuits to their bedrooms when they sleep to reduce their exposure. If construction, remodels and renovations are done with BX, the electric field exposure is significantly reduced. The cost is greater, but for a 250 foot roll of BX, it is only about $30 more expensive than NM cable. So for a small job the extra cost may only be $100 or so, and even for a large job the extra cost should not exceed $1,000.
2) Perhaps you are not concerned with electric fields. Here is another reason to use BX over NM cable that can save you well over $1,000. BX is resistant to nails and screws and rodents and other pests. If you have to tear out a wall to replace a wire that has been punctured by a nail, supposing you were hanging a picture, this repair could cost well over $1,000.
Here is a true story from when I worked as an electrician: I was working on a job where whenever the client switched on the recessed lights in the living room ceiling, the circuit breaker would trip. Diagnosing this problem required over 2 hours crawling around in an attic and isolating the area where there was a short circuit. I was frustrated to find TWO separate areas and narrowed down the area enough to open up the ceiling and see the problem. A rat, or other rodent, had completely chewed through the insulation of the NM cable, allowing the hot and neutral wires to touch and create a short circuit. This had happened in TWO places in the living room ceiling! (See Pictures)
The time spend diagnosing the problem, locating the short, cutting the wall, replacing the wire, patching the wall, and painting was well over $2,500. If the architect, owner, or contractor had insisted on BX instead of NM cable, this repair would likely not have been necessary.
This is why I always use BX on my own jobs and recommend it on any project in which I am consulting. Residential Wiring Best Practices is part of a series. See the earlier blog on knob-and-tube common wiring errors here.
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