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Exchange Structural Framing and Wiring


Most images of telephone exchange rooms depict consecutive rows of large switches and relay racks, mounted side-by-side on iron frames. Tying all this together is miles of connecting wiring. This section will describe some aspects of the structural framing and wiring for building an exchange.  Of course, it's only a snapshot of the effort. One metric to be discussed may surprise you, “miles per foot”.  The amount of iron and wire used to make an exchange come to life is astonishing.


Bay after bay


Fig 1 shows ~14 bays (Bell System naming), side-by-side, of #1 Crossbar equipment– relays, crossbar switches, cabinets, etc, with most wiring hidden behind the bays.  Multiply this by 20 or so for a typical central office and you can appreciate the magnitude of the structure required to mount the components. Eleven foot tall, four foot wide iron bays were often used. Rolling aisle ladders were common for accessing the individual pieces for troubleshooting and repairs. See Figs 2-4 for other examples.

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bays racks in an exchange office switching equipment

Fig 1, Typical bays in a #1 Crossbar system, 1938 [Xbar]

typical row of step-by-step switches in an exchange

Fig 2, Typical equipment row of a Step-by-Step exchange 1937 [Step]

rear wiring of step-by-step telephone exchange

Fig 3. Framing and wiring for SxS switch banks (rear side)
       -- Source [JKL]

front side step by step telephone exchange wiring example

    Fig 4, Framing and wiring terminals associated with typical SxS exchange
               -- Source [JKL]

Welders, take your stations


How much effort was needed to assemble a panel exchange in the 1930’s?


A 7-digit Panel installation took about two years with many installers (Fig 5). About 200 men would do the iron work first. Then about six months later 100 installers would do the as­sembly and wiring. So about 40% of the total installation effort was only for iron work! The first large-scale exchanges were a hardy mix of elec­tro-mechanical components, wires, and iron frames.


Installing a SxS or Crossbar system was equally challenging with an abundance of frames and wiring. Many frames came from the factory prewired but there was much interconnecting wiring needed on site.                                                 

Western Electric installers on a panel telephone exchange

Fig 5

Wiring at one mile per foot


This subtitle is not a misprint. Imagine a single 4 foot wide panel-exchange bay using a mile of harness wire for every foot of its height!


As mentioned, individual exchange frames are wired at the factory and interconnected on site. Fig 6 shows a section of a particular panel frame’s wiring circa 1925. In total, eleven miles of wire, cut into 6,854 pieces, are needed to interconnect the coils, contacts, over four hundred relays, and other elements for just this one 11’ tall panel frame [Hance].

image of one mile per foot of rack height for telephone exchange

                                         Fig 6, Wiring harness beauty [Hance]

 While most panel exchange frames have a similar construction, this example from 1927 likely represents one of the more complex wiring configurations.  Assuming an average panel bay uses say 5 miles of wire, with 70 frames in an exchange, 350 miles of wire would be needed.

​Fig 7 shows the wiring harness (attached to a layout board) used to wire the apparatus in Fig 6.

Eleven miles of wiring harness for one bay telephone exchange panel type

Fig 7. Eleven miles of wiring harness for one bay [Hance] 

More mountains of wire


To get a grand total we need to include the wiring between the frames and the wiring connecting the subscriber’s incoming lines (from the building’s cable vault) to the core switching frames. Fig 8 shows a main distribution frame (MDF). These are terminals for connecting the switching equipment (inside plant) to subscriber lines on poles (outside plant). 


Adding it all together, there may be a few thousand miles of inside wiring to support 10K subscribers. Some large metro exchange buildings may house ten separate 10K line exchanges. So, one can only imagine the plexus of wiring for such exchange complexes.

Fig 8, Main Distribution Frame at C&P Telephone Co., 1905

Fig 8, Main Distribution Frame at C&P Telephone Co., 1905

Connective wiring of crossbar switches  and wire spring relays

Fig 9, Connective wiring of crossbar switches (top) and wire spring relays
The waxy cable lacing twine helps keep the wire routing organized. 

Bottom line, exchanges are complex beasts. Together the framing, relays, switches, wires, and other elements form part of a large tapestry that is truly an engineering marvel.



Hance, P.D., “Eleven miles of wire”, Bell Laboratories Record, August 1927

JKL, Figs 3, 4.  Thanks to Remco Enthoven, JKL Museum of Telephony, 2023

Panel, “The Last Panel Sys­tem”- WE (Western Electric Mag­azine), First Quarter 1983 issue

Step, THE STEP-BY-STEP DIAL TELEPHONESYSTEM, 1937, Western Electric Co. publication 

Xbar, THE NO. 1 CROSSBAR DIAL TELEPHONE SYSTEM, 1938 Western Electric Co. publication

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