Q&A for How to Use Armored (BX) Electrical Cable

Article 518 of the National Electrical Code (NFPA 70, 2019: 514.A) requires all "assembly occupancies" to be wired using "metal raceways, flexible metal raceways, nonmetallic raceways encased in not less than 50 mm (2 in.) of concrete, Type MI, MC, or AC cable." Certain wiring outside of fire-rated construction may use non-metallic-sheathed cable, AC, or non-metallic rigid conduit. 518.4(B,C). Local jurisdictions may interpret such rules differently.

You can easily find spools of BX cable up to 1,000 feet long. The actual gauge for your project will depend upon the length and the operating load to be fed at the far end.

The National Electrical Code prohibits use of BX ( technically named Armored Cable/AC ) in wet locations. Use metal-clad (MC) with a metal sheath specifically listed as impervious to water, or underground feeder/branch circuit cable (UF), either by itself or in a conduit system approved for use in wet locations (if you desire additional protection from physical damage to the cable).

It is generally found in 12/2 gauge and 14/3 gauge. 12/2 is used for 20-amp circuits and 14/3 wire has 2 "hot" wires (black and red) and is rated for 15-amp circuits. 14/3 wires are generally used in 3-way switches.