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How to Use Armor Clad (AC) Cable

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Facts about armor clad cable and its application in homes and remodeling projects.

Armor Clad (AC) Cable was first listed in 1899 for the Sprague Electric Co. of New York. There were originally two experimental versions of this product, one called “AX” and the other “BX,” with the “X” standing for “experimental.” The “BX” type eventually won out and the name stuck. BX also became the registered trade name of armored cable for General Electric, who later acquired Sprague Electric.

AC Metallic Armored Cable

 

The first armor clad cable was called “BX” and it first appeared in the 1903 NEC, but didn’t start becoming popular until around 1930. AC cable is described in Art. 320 of the NEC. The armor of AC cable systems is tested for grounding and can provide a suitable equipment grounding path. AC cable made after 1959 requires an aluminum bonding strip under the armor to help improve the conductivity of this path. Although originally produced with steel armor, in the late 1980’s lightweight aluminum armored AC cable first became listed. (Source: NFPA)

Many of the problems associated with the original armored cable were due to installation problems, especially in cutting the metal sheath. Today manufacturing improvements have made armored cable very safe and reliable and goes by the trade name "AC" cable and is an acceptable alternative to metal conduit in most jurisdictions. Many people have shortened armor clad cable to armored cable leading to some confusion. There is also larger cable called metal clad cable or MC cable. This is used in larger electrical installations and not applicable to residential electrical systems.

Working with Armor Clad Cable

Using a hacksaw or electrician’s pliers to cut the metal sheathing has a tendency to damage the insulation on the wires inside the sheath or to leave very sharp edges on the metal which could easily create a short between conductors or between the armored jacket and the wires if the installation into the electrical box was not accomplished correctly. This has been eliminated by the creation of specially designed tools which cut the sheath without damaging the internal cables.

AC Cable Stripper

 

AC cable has some benefits and should be considered in areas where there is a relatively high possibility that damage could occur to non metallic sheathed cables from screws, nails or other sharp devices.

One area where cable damage is prevalent is in kitchen walls where upper cabinets are screwed into studs with, in many cases, three and four inch screws. It is much easier to penetrate a plastic covered cable such as Romex than it is an AC armored cable. Some other locations where AC cable is prevalent is in attics and inside walls with metal studs. Unfortunately many people think that because the cable is protected by metal, it can be used in areas where damage may occur.

Below is a list of where AC cable cannot be used:

1. Where subject to physical damage

2. In damp or wet locations

3. In air voids of masonry block or tile walls where such walls are exposed

4. or subject to excessive moisture or dampness.

5. Where exposed to corrosive fumes or vapors

6. Embedded in plaster finish on brick or other masonry in damp or wet locations

It is also very important that anti-short bushings be installed in the ends of all cables. The anti-short bushing protects the internal cables as they protrude out of the metal sheath from shorts occurring when the end of armored end of the cable is clamped into position in an electrical box or panel.

Many do-it-yourselfers neglect to install the anti-shorting bushings and end up with damage to the internal conductors of the AC metallic armored cable. Not only does it have the potential of creating short circuits at a later date but it does create a long term fire hazard.

Anti-short Bushings

 

In conditions where cabling is exposed, such as the last few feet of a run from an electrical box to a hot-water tank or electrical motor in a furnace the armor provides an added degree of protection for the wires that are carrying the power.

It should be noted, that the conductors in AC cable does not carry any more current than nonmetallic (NM) cables and AC cable must be properly terminated and clamped into metal boxes not plastic ones.

Incorrect Seating of Connector

Correct Seating of Connector

Other Types of Electrical Cable

NM Cable

NM cable stands for nonmetallic cable with 3 or 4 wires inside a PVC or similar sheathing material. It is relatively inexpensive and is extremely easy to work with. Although it is sometimes installed inside conduit NM cable is usually passed through studs and walls without any additional protective covering and is fine for many types of residential applications.

THHN

These wires are typically Thermoplastic High Heat-resistant Nylon-coated, or THHN for short. Due to their high thermal resistance, THHN conductors are used for appliance wiring as well as service entrances and panel feeds. The nylon jacket offers mechanical protection and resistance to petroleum by-products, chemical agents and oils. THHN insulation is flaming retardant PVC and the maximum continuous conductor temperature is 90°C for dry or damp locations and 75°C for wet locations.

A typical 100-amp service would use #2 THHN cables. They would then be rated at 125 amps. This would protect the wires if the amperage was a full 100 amps.

 

1 comment

Jerry Walch
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Posted on Feb 2, 2011

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Daniel Snyder

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