Wire types and junction boxes: Romex, conduit, BX, MC compared

One-line orientation

Wire-protection type is chosen by building type, exposure, and code — and the BX-vs-MC distinction (who carries the ground?) is the single most tested detail in this topic.

Key points

• Non-Metallic Sheathed Cable (Romex / NM):
  • Plastic jacket over insulated conductors; flexible, easy to install.
  • Typical for dwelling/light-frame concealed dry runs; not for exposed, wet, or high-damage conditions. NEC also permits NM in some Type III/IV/V structures, so “residential only” is too absolute.
  • Contains its own bare copper ground wire.
• Steel Electrical Conduit:
  • Rigid or flexible metal pipe housing individual conductors.
  • Strong physical protection; common in commercial, industrial, and exposed locations.
  • Disadvantage: expensive and time-consuming to install; conduit itself can serve as the ground path in some configurations.
• Flexible Armored Cable — BX vs MC:
  • BX (Armored Cable / Type AC):
    • Interlocked metal casing serves as the equipment-grounding path, but NEC requires an internal aluminum bonding strip running the length of the armor to make that path code-compliant. The strip is inside the armor — Type AC is not a simple “bare casing = ground” assembly.
  • MC (Metal-Clad Cable):
    • Commonly contains a separate internal ground wire. Some listed MC cable armor may also serve as an equipment-grounding path, so confirm the listed cable type.
  • In NEC terms, treat this as two categories: Type AC (the legacy “BX” name) and Type MC. Do not count “BX, MC, and AC” as three separate cable types.
  • Both Type AC/BX and MC are easier to route in tight spaces than rigid conduit.
• Common installation practice:
  • Steel conduit protects the main wiring run between panel and junction box.
  • Flexible armored cable (BX or MC) connects from the junction box to the outlet or fixture, providing flexibility at the termination point.
• Junction Box:
  • A protective metal or plastic enclosure that houses wire splices and connections.
  • Ensures splices are safe, secure, and inspectable.
  • Types: surface-mounted or flush-mounted (recessed in wall).
  • NEC requirement: junction boxes must remain permanently accessible — never cover with finish materials without an access cover.

Confusions / comparison

Wire type	Outer protection	Ground method	Typical use
Romex (NM)	Plastic jacket	Bare wire inside	Dwelling/light-frame concealed dry runs
Steel conduit	Rigid/flexible metal pipe	Conduit or separate wire	Commercial, exposed, industrial
BX (Type AC)	Interlocked metal casing	Armor + internal bonding strip = ground	Tight residential/commercial spaces
MC (Metal-Clad)	Interlocked metal casing	Commonly a separate wire; some listed armor also qualifies	Flexible protected wiring

Related

→ Electrical distribution (existing card): wire gauge/ampacity, hot/neutral/ground functions · Switches (this module): how switches are wired into junction boxes · Code: NEC conductor protection requirements.