Updating old house wiring

Not all knob and tube installations utilized cleats.Ceramic bushings protected each wire entering a metal device box, when such an enclosure was used.Loom, a woven flexible insulating sleeve, was slipped over insulated wire to provide additional protection whenever a wire passed over or under another wire, when a wire entered a metal device enclosure, and in other situations prescribed by code.

From the fusebox, a single supply wire would go to the first switch.

From there, it would divide into two switching wires.

Only in the last case were metal boxes always used to enclose the wiring and device.

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.

In modern circuits, these two wires are grouped together, and for each 240 V 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.

The foregoing is an example of the so-called Carter system wire layout.

In the case of modern North American split-phase power, the supply wires from two opposite single-phase circuit breakers are used to supply 240 V AC for high power devices.

Knob and tube wiring was eventually displaced from interior wiring systems because of the high cost of installation compared with use of power cables, which combined both power conductors of a circuit in one run (and which later included grounding conductors).

At present, new knob and tube installations are permitted in the US only in a few very specific situations listed in the National Electrical Code, such as certain industrial and agricultural environments.

Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them.