Knob and Tube Wiring - Unusual Wiring Layouts

Unusual Wiring Layouts

In many older K&T installations, the supply and return wires were routed separately from each other, rather than being located parallel to and near each other. This direct routing method has the advantage of reduced cost by allowing use of the shortest possible lengths of wire, but the major disadvantage that a detailed building wiring diagram is needed for other electricians to understand multiple interwoven circuits, especially if the wiring is not fully visible throughout its length. By contrast, modern electrical codes now require that all residential wiring connections be made only inside protective enclosures, such as junction boxes, and that all connections must remain accessible for inspection, troubleshooting, repair, or modification.

For example, consider the installation of a light, controlled by two switches in different locations. In modern practice, the supply and return wires both arrive at the first switch, where the supply is connected to one of the two switching wires (hot/red); the return wire is carried inside the same multi-conductor cable to the distant switch second switch. From there, a supply wire carries power to the light, and a return wire goes back to location of the second switch, and then retraces the path back to the fusebox.

For older K&T installations, the supply and return wires were not necessarily installed as a pair, and did not necessarily have to be near each other at all. From the fusebox, a single supply wire would go to the first switch. From there, it would divide into two switching wires. At the second, remote switch the two wires would come together to be a single supply wire again. Finally this supply wire would be routed from the second switch to the light fixture, and then a single return wire would take the shortest path through the building back to the fusebox. The foregoing is an example of the so-called Carter system wire layout, which is described in more detail in a separate article.

In the case of modern North American split-phase power, the supply wires from two opposite single-phase circuit breakers are used to supply 240VAC for high power devices. In modern circuits, these two wires are grouped together, and for each 240V device a dedicated circuit is installed, usually with the two circuit breakers ganged together to operate simultaneously.

But with old K&T, a past electrician might have reasoned as follows: "I have a 20 amp circuit on one phase over here, and another 20 amp circuit on an opposite phase over there. Rather than run an entirely new set of wires for this one device, I will just tap off the supply wires of those two circuits and bring them together for this one device." This method of wiring does work and it reduces the amount of wire used, but it makes electrical problems very hard to find and fix, with (possibly undocumented) wires going all over the place within the walls and ceilings, to whatever was the most expedient supply or return wire at the time of installation.

Under the US electrical code, Carter system wiring layouts have now been banned, even for permissible new installations of K&T wiring. However, electricians must be aware of this older system, which may still be present in existing electrical installations.