Industrial Wiring Standards: The Forgotten Codes

Industrial Wiring Standards: The Forgotten Codes


As an electrician, I often come across old industrial buildings that need rewiring or upgrading to meet modern electrical codes and standards. However, in many cases, these old factories and warehouses were originally wired to codes that have long since been forgotten or superseded. In this article, I'll provide an overview of some of the old industrial wiring standards that you may encounter if you work on renovating or rewiring historical industrial buildings. Understanding these outdated codes can help you safely deal with old wiring systems and know what needs to be brought up to date.

Knob and Tube Wiring

One of the earliest forms of electrical wiring found in old industrial buildings is knob and tube wiring. This system uses ceramic knobs attached to joists or studs, with wiring running between them through open air, not inside conduits or armored cable. It was the standard for industrial wiring in the early 20th century.

Key Features

While knob and tube can still be functional if in good condition, it lacks safety features of modern wiring like grounding and circuit breakers. I do not recommend reusing old knob and tube wiring in industrial settings due to fire and shock risks. All accessible wiring should be replaced.

Old NM Cable

Non-metallic sheathed cable (NM), with two or more insulated conductors wrapped in paper or fiber, was also used historically for industrial wiring. Early versions (from 1930s-1950s) lacked a ground wire, making them unsuitable for modern use.

Key Features

As with knob and tube, old NM cable without a ground should be completely removed and replaced. Even NM cable from the 1960s-1970s with a ground wire may need replacement due to insulation breakdown.

Old BX Cable

Armored cable (BX) with an aluminum or steel flexible covering over the wires was invented in the 1880s. Older versions used a fabric wrapping under the metal cover which deteriorates over time.

Key Features

I recommend replacing any old BX cable found in industrial settings. Even if the insulation looks intact, the underlying fabric wrap may be deteriorated and pose a hazard.

Solid Conductor Wiring

Another old wiring method used solid copper conductors rather than stranded wires. This type of wiring is stiff and can fracture easily when moved or vibrated—a serious risk in an industrial environment with moving machinery.

Key Features

My recommendation would be to replace any solid conductor wiring exposed during renovations due to the safety hazards posed.


When upgrading old industrial facilities, be aware of these potentially hazardous historical wiring methods. Always have a qualified electrician inspect any suspect wiring. While reusing old systems may save on costs, in industrial settings it often makes sense to completely replace outdated wiring with modern cabling and adhere to current electrical codes. This investment in safety will prevent expensive issues down the road. By understanding common old wiring types, you can make informed decisions about renovations.