Knob and tube wiring was a system of open electrical conductors supported on ceramic knobs. It also used ceramic knobs where wires cross each other or pass through framing. This type of wiring was in common use from the 1880’s to 1930’s, in North America. During the era when knob and tube was in common use, a typical home would have 30 or 60 amp electrical service, sufficient for general lighting and maybe a radio. These were the days of “iceboxes”, wood or gas stoves, coal heat, and no air conditioning. Wiring homes was a new trade, with very few rules.Do you want to learn more? Visit knob and tube.
The first home wiring rules appeared around the turn of the century, when Terrell Croft wrote the American Electricians Handbook. Crofts books defined early wiring practices. One of Croft’s rules was that knob and tube wiring should always be run through, never over the house framing. The problem with running wiring over framing is that it can get bumped, snagged, and damaged. Most electrical experts would agree that wiring run over the framing is a sign of amateur, cobbled-together work, and it never conformed to any electrical code.
The theoretical advantage of knob and tube wiring is that it dissipates heat into free air, and therefore has a higher amp city than cable systems with equivalent wire size. When originally installed in the 1900’s knob and tube wiring was less expensive than other wiring methods. Due to the installation cost, owners and electricians would opt for knob and tube versus conduit wiring and metal junction boxes. The conduit methods were known to be of better quality, but their cost was significantly higher than knob and tube wiring.
Modern wiring methods assume two or more load carrying conductors will lie against each other, for instance the standard non-metallic – 2 cable. Since the load carrying wires are in close proximity, when they heat up, the heating is shared across the wires, limiting the overall current load they can support. Since the load carrying wires in knob and tube wiring are widely spaced, the wires are capable of carrying higher loads without risk of fire.