Here is some history about Type AC and NM Cable that you may find interesting. It is extracted from a 2012 Study on Damaging Type NM Cable from NFPA…Enjoy!
Armored cable (AC) was first Listed in 1899 for the Sprague Electric Co. of New York, and was originally called “Greenfield Flexible Steel-Armored Conductors,” after one of its inventors, Harry Greenfield. There were originally two experimental versions of this product, one called “AX” and the other “BX,” with the “X” standing for “experimental."
The “BX” version became the one that eventually got produced, and hence Investigation of Damage and Degradation on Breakdown Voltage of NM Cables the name “BX” stuck, which also became the registered trade name of armored cable for General Electric, which later divested this business to Sprague Electric. Armored cable, or BX, first appeared in the 1903 NEC, but did not start becoming popular until around 1930, and is still a popular wiring method today. AC cable is described in Article 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 in accordance to NEC requirements.
Nonmetallic-sheathed cable, or NM for short, was first Listed and described in the NEC in 1926, but it was invented a few years earlier by Rome Wire Company in 1922 in Rome, NY, and marketed under the trade name “Romex®.” Romex® is now a registered trademark of Southwire Company of Carrollton, GA.
NOTE: It is officially Type NM Cable or Nonmetallic-Sheathed Cable versus using the tradenames…
Early NM cable had their individual conductor insulation wrapped in a cotton braid that was impregnated with either a varnish or tar-like substance for moisture protection. Around 1950, synthetic spun rayon was being permitted to replace the cotton thread in the jacket braid. Then in the early 1960’s, thermoplastic began replacing the braided jacket altogether, and by about 1970, most all NM cable had a compounded PVC outer jacket, even though a braid was still permitted until 1984. Also in 1984, NM-B cable was developed and required to have 90°C rated individual conductors, and a 75°C outer jacket.
Until the early 1960’s, most NM cable for residential use did not have a grounding conductor. However, changes in the 1962 Code that mandated equipment grounding for all branch circuits popularized the use of NM cable with ground. Earlier versions of NM cable with ground permitted the grounding conductor to be one or even two sizes smaller than the current carrying conductors. For example, a 16 AWG ground wire was permitted for 14 and 12 gauge copper NM, and 14 AWG ground for 10 gauge copper NM.
In 1969, new requirements no longer permitted an undersized grounding conductor for 14, 12 and 10 AWG NM cable.