BX Wiring

BX wiring (also known as armored cable) is the 2nd generation of wiring used in homes. It first appeared in the 1903 National Electrical Code (NEC) book. It became popular in the 1920s, and it was the main wiring type by 1932. BX wiring was used until the late 1940s, when it was replaced by early Romex (non-metallic sheathed cable).

Issues with BX wiring:

  • It relies on the outer coverings to maintain a ground path. Each time a BX wiring circuit went in and out of a box, it used a special connector with a lock nut. This nut and connector had to be tight. Otherwise, the circuit would lose its ground path.

    Let’s say the circuit goes in and out of ten different boxes (including outlets and light fixtures) before it finishes. If some or all of these connectors are loose, the ground connection will consistently get worse as you go down the line. We see this problem a lot when we measure voltages. If there are 120 volts between hot and neutral, there are only 60 volts between hot and ground. This measurement is a sure sign that the connectors are loose throughout the circuit, which can be difficult to solve without replacing the wiring itself.

    This problem becomes a safety hazard if there is a short circuit to ground. The breaker/fuse may not trip now because the ground path is no good. I’ve heard older electricians telling stories about the metal outer covering becoming a resistor, which potentially causes it to turn red hot!

  • We cannot expand off it with new/modern wiring. Given its grounding issues, we cannot extend off existing BX wiring to add new wiring.
  • It uses rubber-with-cloth insulation over the copper wires. This kind of insulation is especially sensitive to heat.  Older light fixtures had NO insulation in them that would protect the wiring above from the heat of incandescent light bulbs. So when people would install larger 100-watt incandescent light bulbs in these fixtures, they would cook the wiring in the junction box above.

    We’ve seen MANY junction boxes with insulation that crumbles apart after the light is removed. Scary stuff!

  • BX wiring was installed with two-prong outlets, which won’t accept three-prong cords. (Duh!)

Combination of Wiring Types

BX wiring was often installed alongside knob-and-tube wiring and ungrounded Romex wire. In certain years (typically during the 1920s and early 1930s), electricians could choose from all three kinds of wiring. In some cases, electricians at that time stuck with one style of wiring throughout. But we more frequently see all three styles mixed together. You may be able to go into your basement and quickly identify the three types of wiring!

A slightly zoomed picture of BX wiring
Above is a great example of both BX wiring and knob-and-tube wiring working together. Notice the two black wires (knob and tube) leaving the same junction box as the BX wires (metal covered wires).

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