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Zinsco was a manufacturer of circuit breakers and electrical distribution panels, founded by Emile Martin Zinsmeyer and his son Martin Emile Zinsmeyer in the early 1930s.


Zinsco was originally part of the Frank Adams label. Frank Adam Electric originally founded 1891 in St, Louis, Missouri. A young sales manager named Emile Zinsmeyer was sent to Los Angeles, California in 1928 to run the west coast division. In the mid 30’s when the great depression hit it hurt a lot of manufacturers. With the Frank Adams company based in Missouri, Zinsmeyer negotiated a deal to buy out their west coast division in 1938. The parent company “Frank Adams” continued on through the depression and kept producing that label through the 1950s. The Adams panels and breakers are now extremely rare.

Emile Zinsmeyer named his new company, the “Zinsmeyer Company”. It was located at 729 Turner street, Los Angeles. Zinsmeyer had two of his sons working in the business, Wilford who went by Bill, and Martin. In 1943 Martin bought the company from his father and renamed it “Zinsco Electric”. Almost immediately Zinsco switched their focus to the development of new panel and circuit breaker designs, and had patent applications dating back as early as 1946 for the Zinsco panels and breakers.

The first Zinsco panels and breakers were of copper bus-bars and copper breaker clips. Original equipment and breakers were also labeled “Magnatrip”. This patent was issued in 1950. The next series of breakers listed five total patents. These early breakers also had copper bus-clips, which matched the coper bus-bars in these panels.

By 1952 the country faced its second major copper shortage due to the Korean war. The first copper shortage during WWII promoted code changes, as the only allowed conductors at the time were copper, gold, and silver. Aluminum was both expensive to manufacture and wasn’t considered a usable conductor. However, with advancement in the processing and production of usable aluminum wire, a code change was promoted in 1942. And aluminum, as a conductor was added to the NEC and soon became available.

Most electrical fabricators initially used all copper busing. Some even remained copper through the 60’s, such as FPE, for their stab-lock panels and breakers, as well as Square-D. But most all others switched from copper busing to an aluminum alloy, shortly after aluminum was added to the code, for both bus-bars and breakers.

Zinsco stayed with copper in both their panels and breakers until the third major copper shortage in the early 1960s. However, when Zinsco changed to an aluminum bus, the aluminum selected was alloy 6061. This is an aluminum alloy first developed in 1935. It is a good conductor, and a hard aluminum. 6061 and 6063 are the two most popular aluminum alloys for welding today. In 1961, another aluminum alloy was developed: 6101. This alloy has a higher tensile strength and is the best conductor for electrical use. It is the aluminum used in the current aluminum busing of panels and load-centers.

The aluminum shortage of the 1960s was primarily caused by the US government, which had huge contracts out, many of which were for air-conditioning, requiring over a million new copper coils. This action depressed the market of available copper for other manufacturing needs. Also, promoting the first real demand for aluminum NM cable. A demand along with some new practices, which left a gap in safe wiring and needed code changes.

A key factor to needed change was the introduction of a new class of receptacles. Receptacles with the first quick-wire stabs. And these early quick-wire devices would accept both #12 and #14 wire: as minimum aluminum cable was #12 it was a fast installation. After 1996 quick-wire devices were mandated to only accept #14, restricting the use of larger loads such as kitchens, or the standard #12 aluminum. And, by the mid 70’ aluminum (NM) cable was no longer manufactured. But from 1964 to 1972 this new aluminum wire and these quick-wire receptacles were being speed-wired into over two million homes, particularly tract homes. This was the formula for disaster.

The real problem was the wire itself. Prior to 1972 the wire was inferior, and the aluminum had tremendous expansion and contraction between heating up on power consumption, and at rest. The outlets were also constructed of dissimilar metal, and the receptacles caused the aluminum to oxidize. Oxidization of aluminum creates aluminum-oxide which becomes an insulator rather than a conductor. This combination caused innumerous fires, outlet burn-outs, and nearly 100 deaths as published by the NFPA. Thus, aluminum wiring received a very bad reputation.

In 1972 aluminum was changed from an inferior product and into a modified allow. Modern aluminum wire is (AA800 series alloy) which has a higher tensile strength, so it stays tighter and has reduced expansion and contraction. This change was also followed in the late 90’s by COLAR rated or; (copper/aluminum rated) receptacles as a requirement in aluminum device wiring. Since that time aluminum for anything smaller than #8 has virtually disappeared.

When the aluminum shortage of the 60’s moved Zinsco from their copper busing to an aluminum bus, around 1963 they also introduced the R-38 twin breaker. This was the only twin circuit breaker that also made contact on both bus-bars for 240 volts in a single breaker space. This made the Zinsco brand hugely popular with contractors and millions of Zinsco service panels and load-centers were installed through the 60’s and 70’s.

With multiple breaker manufacturers in competition, and the increase in the typical number of circuits, many employed the “split-bus” panel to save cost. The split-bus was another popular scheme not only with Zinsco, but Bulldog, FPE, and a few others. The concept with split bus was a use of the six-handle-rule, or article 230 of the NEC. Typically, the first eight pole on a split-bus panel would have the major 240-volt feeds, such as oven, cooktop, water-heater; and a 2-pole 50 which fed a separate bus in the same panel. Thus, no need for a more expensive main breaker.

These days most installations have a single main and bus configuration of 12/24 or 20/40. But, some Like GE, and Square-D use lock-out perches. These panels could be 32/40; in that there are 32 full size spaces, but the top eight spaces are compatible with twin breakers for the forty poles maximum.

In 1973 Martin Zinsmeyer sold his “Zinsco Electric” Co. to GTE Sylvania. During that ownership, the product line remained the same, with new labeling and branding, while dropping the “Magnatrip” label. In 1978 the line changed names again, and was re-labeled Challenger.

By 1981 GTE Sylvania divested itself of the electrical distribution business, and began selling off its product lines and manufacturing facilities. The Challenger line, mostly manufactured at the time in Jackson, Mississippi was sold to a former officer of GTE, who used the Challenger name as the name of his new company, Challenger Electrical Equipment Corp.

The new Challenger Brand immediately ceased production of any new Zinsco frame panels and load-centers, but still produced the re-branded Zinsco circuit breaker line. Challenger concentrated on a new line of panel equipment using the same bus configuration proven with Murray, Crouse-Hinds, and others. But, from 1982 until 1994, the 200-amp. Service panels used a Zinsco frame main breaker. Challenger flourished through the 80’s, and was eventually sold to Westinghouse in a multi asset deal, in order for Westinghouse to sell its remaining electrical manufacturing facilities to “Eaton Corporation” in 1994. Somewhere in this deal was the fact that both Challenger and T&B had manufacturing facilities in Mississippi and T&B received the Zinsco circuit breaker molds. Thomas & Betts then continued to fabricate and sell the breakers under the Thomas & Betts label, (T&B). But they also tripled the price at that time, from about $7.00 for the popular R-38 twin breaker to $19.00 … Eventually closing the Zinsco circuit breaker production altogether in 2005. Today Connecticut Electric continues making aluminum Zinsco bus replacement kits, and replacement breakers.

Safety Concerns[edit]

Zinsco electrical equipment is considered obsolete, due to a design flaw in which the circuit breaker's connection to the bus bar becomes loose, causing arcing and subsequent overheating. Long term exposure to this heat can cause the breaker to fuse to the bus bar, making it impossible to remove. Even worse, it can cause the breaker's contacts to fuse together, thus preventing the breaker from tripping even in an overcurrent situation, thereby causing a potential fire hazard.[1]

Replacement Parts[edit]

Aftermarket replacements for the Zinsco breakers are available; however, it may be more cost effective simply to replace the entire panel with a more modern and safer design from another manufacturer (such as Eaton, GE, Siemens, or Square-D), depending on the number of breakers to be replaced. If the bus bar shows signs of corrosion, or if any of the breakers show signs of overheating, the panel should be replaced entirely. Many electricians advocate replacement of the panel in any case, due to its historically poor reliability.

Manufactures of Zinsco Style Breakers:


3 How to Identify Zinsco Circuit Breakers with Pictures