Armored Cable Today:
Safe, Reliable, and Economical

Today's BX and MC cable vs vintage BX Cable

Today's BX and MC cable vs vintage BX Cable

by George W. Flach, Electrical Consultant, New Orleans, LA

Prevailing misconceptions about the present-day armored cable most likely stem from imperfections inherent in the original BX. Improvements made in the product are readily apparent when today's armored cable (right) is shown side-by-side with the old BX.

Today's armored cable, historically known as BX and more correctly as Type AC, has proved to be a safe, reliable, and economical wiring method. However, there continue to be restrictions, and sometimes prohibitions, on its use in many areas. What is behind such restrictions and are they warranted?

These restrictions on the use of armored cable, for the most part, are the result of years of misconceptions. Today, many electrical inspectors are reconsidering their position on armored cable because of its long safety record, NEC recognition, and improvements made since its introduction.

Basic armored cable was developed in the early 1900s by Edwin Greenfield and Gus Johnson, who called their product BX cable. It has become a generic term for all armored cable. Some of the prevailing attitudes about the product most likely stem from imperfections inherent in the original BX.

For instance, the Type R rubber insulation used was subject to thermal-aging and cracking. The cotton-braided covering on conductors and overall braided or paper covering did little to prevent moisture damage to the insulation. When the bonding wire was finally added, it was flat and subject to breakage. In addition, it (and even the armor itself) was often mistakenly used as the neutral conductor.

The biggest problem was the lack of proper cutting tools. Old cutting methods, such as hacksaws or pliers for crimp cutting or twisting and breaking the steel armor, were very unsatisfactory and often led to nicks on the insulation and conductors, creating circuit opens or shorts.

As a result of these past deficiencies, the excellent safety record of armored cable attested to by decades of NEC recognition has been ignored. Improvements including newer, safer materials and expanded, application-oriented products, too often have gone unnoticed. The progress in the performance of armored cable is a direct result of changes made in its construction and installation techniques.

  • Insulation: The early rubber insulation has been replaced by thermoplastic insulation with excellent aging properties, thermal characteristics, and dielectric strength.
  • Wraps: Cotton braiding has been replaced with impregnated paper material that has good dielectric qualities and is moisture-resistant.
  • Anti-short bushings: Fibrous material used in early bushings has been replaced by thermoplastics that allow easy sliding and eliminate ripping, tearing, and deterioration.
  • Bonding wire: Easily breakable, flat bonding wire has been eliminated and replaced with a bonding wire that is in constant contact with the armor throughout the cable length.
  • Galvanizing: The early practice of galvanizing the sheet steel first, then cutting it into strips, left the cut edges unprotected and allowed them to rust. This technique has been di-opped in favor of galvanizing the steel after cutting, providing superior corrosion resistance.
  • Cutting techniques: A handheld roto-cutter (Seatek Co. Inc. pioneered the first commercially successful BX armor cable cutter in 1973. This patented tool is known as the Roto-Split) is now available to use in lieu of hacksaws, pliers, etc., for fast, automatically controlled cuts that significantly reduce the possibility of conductor damage.

In addition to these changes, armored cable is available now in various sizes and with multiple conductors for a variety of specialized applications, including modular wiring and fire-alarm circuitry.

Electrical inspectors in many areas of the country with strict building codes have recently begun to allow armored cable for a variety of applications when as little as two years ago some were not. Individuals such as Leo F. Martin, Deputy Commissioner for the City of Boston; Robert C. Duncan, Deputy Manager of Building and Safety for the Reedy Creek Improvement District and chairman of the Florida Chapter of the International Association of Electrical Inspectors (IAEI); and James G. Stallcup, Chief Electrical Inspector for the City of Fort Worth, TX, have rethought their positions.

Armored cable, today, is a proven and tested product with an exceptional safety record, duly recognized by the NEC and listed by independent testing laboratories. It's an option that electrical contractors find has many advantages.

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What's the Story addresses electrical design, construction, maintenance, and equipment questions and problems. Contributed articles will be considered; comments on published articles are encouraged. Write What's the Story, EC&M, 1221 Avenue of the Americas, New York, NY 10020.

REPRINTED FROM AUGUST, 1986 ISSUE
ELECTRICAL CONSTRUCTION AND MAINTENANCE 
@ Copyright 1986 McGraw-Hill, Inc. - All rights reserved.



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US Patent #817057 Flexible metallic tube 1906