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Residential Wiring Best Practices

by / Tuesday, 15 January 2013 / Published in Healthy Building Inspections & Testing
Residential Wiring Best

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 is 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.

detail picture of Non-metallic cable- Healthy Building Science - Environmental Consultants

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.


detail picture BX wiring - Healthy Building Science - Environmental Consultants

BX is also known as Armor Clad or Flexible Metal Conduit cable.

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 a 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)

NM wire eaten by rodent - Healthy Building Science - EMF testing - RF testing

NM wiring shorted out - HBS - Environmental Testing

residential wiring eaten by rodents - HBS - Environmental Consultants


The time spent 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 Residential Wiring Errors here.


Healthy Building Science is an environmental consulting firm which provides EMF testing and  environmental testing services for commercial, multi-family buildings, offices, industrial and manufacturing workplaces, hospitals and medical facilities, and single-family homes in the greater San Francisco Bay Area and all of Northern California.



10 Responses to “Residential Wiring Best Practices”

  1. Marc Lamphier says :

    The notion that one has to protect themselves from alternating current electrical fields is one of those ideas that has a lot of popular belief but no scientific support. In particular the idea that wires behind your walls would even exert enough of an electrical field to even be detected is a bit fantastical. However, the shielding may be good for is cutting down on external noise if it is run next to communication cables, such as ethernet or coaxial. Although these are both designed to minimize electrical interference, they have their limits, and butted right up against a 110v alternating current wire could generate background noise.

  2. Alex says :

    You’ll need to ask your local code official this sort of electrical code question.

  3. Alex says :

    You’ll need to ask your local code official, but I believe as long as you used code-approved junction boxes on either end of the connection you should be okay. Freakin’ rats. They can cause so much damage. They chewed a wire harness in our van last year. $2000 down the drain. Now spraying fox fee around the parking area to deter the little buggers. Perhaps vehicles should have an MC option too?!?

  4. Dennis Slayton says :

    Question, since the BX is not in danger of nails, can I run BX to rewire all my outlets behind molding at floor level? If you say no, is it because there is not enough air space around the cable, if, so, I can redesign the molding. I am just trying to avoid cutting into the walls.


  5. […] are required to use armoured cable, which is rodent proof, the minimum standard for homes is Romex cable, a less-protected cable, which is susceptible to rodent damage. This lower standardcan cause […]

  6. John Udisky says :

    Can connect MC to nonMC wiring? I am planning on replacing rodent chewed non-MC cable on both ends of the run by using junction boxes to connect the different wires.

  7. Joey A. says :

    Noah gives great insight and counter argument to the perception of BX. Technology and sciences can advance an industry quickly and thusly these articles and authors’ understanding should be “up to date” as well. For the readers, like myself, who may be inexperience in this industry and try to asserctain as much “industry correct” information as possible, we thank all those who write about their experiences and offer additional resources and perspectives. Thank you OP and thank you Noah

  8. Noah Tsaying says :

    First – There hasn’t been any BX cable made in at least 50 years. The cable with a spiral tape sheath that looks like the old BX cable is designated type AC for “armored cable” not “alternating current” but it was a poor choice of acronyms and AC has failed to replace the BX acronym for much of the general public.

    What’s the difference between BX and AC? BX uses the spiral steel tape as a safety ground for BX which turned out to be a bad idea. as the cable ages the steel edges rust and the path for a fault current changes to the spiral path which is much longer and higher resistance. AC cable has a bare safety conductor and doesn’t rely on the spiral steel tape for the safety ground.

    BX also had a problem with the cut end of the steel tape damaging the insulation, especially older BX cables that pre-date thermoplastic insulation and contain varnished cotton cloth insulation. For AC cable the installer is required to protect the conductor from the cut end of the spiral steel tape with either a bushing or a cable end fitting that acts as a bushing.

    The combination of provoking faults and being unable to carry them well caused the industry to re-think. The choice came down to NM vs AC. AC cable is more expensive and more time consuming to install than NM and doesn’t really provide meaningful safety benefits

    Rodent chewed NM cable looks bad., AC that’s been repeadedly dampened by a roof leak or plumbing drip can still corrode and cause damaged insulation and so on and also look bad. Neither BX nor AC was ever nail-proof, but it is easier to put a nail through NM.

    The truth is that if you’re not a licensed P.E. consulting or specifying electrical materials for construction. A licensed P.E. specifying BX cable in this century would have lost his license long ago. You’re a homeowner with a nostalgic attachment to a style of wiring that appears like it should be safer and more durable than NM. Unfortunately installers omit bushings and damage insulation, more often than carpenters and rodents damage NM cables.

  9. Joe stone says :

    BX cable has been banned about 20 years ago and over a decade before your article was written. It has since been replaced by AC and more commonly MC cable.

  10. Jim Braninburg says :

    I’m a diyer..Great info..Thank you !

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