You're asking about a fascinating detail of early electrical installation! Let's break down whether metal clips were required for knob and tube wiring entering metal junction boxes in the 1920s, and add some context.

The Short Answer: Highly Likely, but Documentation is Key

It's highly likely that some form of protection was required, likely involving metal clips, but proving it definitively with primary source documents from the 1920s is challenging. The key considerations are: Purpose of the Clips

The primary function of those clips was to prevent abrasion of the insulation on the wires as they entered the metal box. Metal boxes have sharp edges that could easily cut through the rubber or cloth insulation of the knob and tube wiring. A short circuit to the box would energize it and become a dangerous shock hazard.

The NEC's Stance

The National Electrical Code (NEC) did exist in the 1920s. The NEC was not law, it was a voluntary standard set forth by NFPA (National Fire Protection Association) that was adopted (or modified) by local jurisdictions. However, in the 1920s, the NEC was not as universally adopted or as consistently enforced as it is today. It's reasonable to assume that the NEC of that era addressed the need to protect conductors entering metal enclosures, but finding the specific language would require delving into the archives.

Local Codes

Even if the NEC existed, local codes and ordinances were often the ultimate authority. Some cities or states might have had stricter or different requirements than the NEC. Therefore, whether clips were required could have varied geographically.

Available Materials

Remember, we're talking about a time of evolving technology. The specific types of clips or connectors available in the 1920s were limited compared to today. It's likely that simpler metal clips, bushings, or insulating tape were the most common solutions for protecting the wiring.

Enforcement

Even if a rule existed, enforcement could be spotty. The rigor of electrical inspections varied greatly.

What We Can Infer & Some Supporting Information

While finding exact wording from the 1920s is hard, we can infer a high likelihood of a requirement based on:

Basic Electrical Safety Principles

The core principle of preventing insulation damage at box entries is timeless. It was just as important in the 1920s as it is now.

The NEC's Evolution

Later editions of the NEC (even going back to the mid-20th century) do explicitly require methods to protect conductors where they enter enclosures. This suggests that the concern was present earlier, even if the exact wording was less precise.

Industry Best Practices

Even without a strict legal mandate, responsible electricians likely employed methods to prevent insulation damage at box entries as a matter of professional practice and to avoid call-backs due to shorts and failures.

Anecdotal Evidence

The fact that you observed these clips as a common practice suggests they were driven by either a code requirement, industry best practices, or a combination of both.

Purpose Built Products

There were purpose built clips and bushings available for knob and tube. Example is a "Gem Connector". These were metal stampings with a smooth, rolled edge to prevent abrasion to the wires.

Acceptable Methods

It was generally acceptable to wrap the wire with friction tape to protect it from abrasion. Some early boxes were fitted with porcelain bushings.

Where to Look for More Definite Answers



NFPA Archives

The National Fire Protection Association (NFPA) is the custodian of the NEC. Their archives might hold copies of the NEC from the 1920s.

Local Historical Societies/Libraries

Researching the building codes or ordinances of specific cities or states from that era could reveal local requirements.

Trade Journals

Publications like "Electrical World" or "Electrical Merchandising" from the 1920s might discuss best practices or code updates.

Books on Electrical Wiring

Some historical texts on electrical wiring practices might mention these details.

In Summary:

While a definitive, unambiguous statement from the 1920s NEC or a specific local code might be elusive, the following is probable: It was very likely that some method of protecting conductors entering metal junction boxes was required (either by code or by best practice) to prevent abrasion. Metal clips were a common and practical solution at the time. Local codes could have varied.Good luck with your code history research! It's a fascinating area. Flag for review