In the years since Edison "invented" electricity, several different
	wiring "styles" have come and gone.  When you buy an older home you
	may encounter some of this stuff.  This section describes the old 
	methods, and some of their idiosyncrasies.

	The oldest wiring system you're likely to encounter is called
	"knob and tube" (K&T).  It is made up of individual conductors with
	a cloth insulation.  The wires are run along side structural
	members (eg: joists or studs) using ceramic stand-offs (knobs).
	Wire is run through structural members using ceramic tubes.  Connections
	were made by twisting the wire together, soldering, and wrapping
	with tape.  Since the hot and neutral were run separately,
	the wiring tends to be rather confusing.  A neutral often runs
	down the centre of each room, with "taps" off to each fixture.
	The hot wire tended to run from one fixture to the next.  In some
	cases K&T isn't colour-coded, so the neutral is often the same
	colour as the hot wires.

	You'll see K&T in homes built as late as the 40's.

	Comments on K&T:

		- the people installing K&T were pretty paranoid about
		  electricity, so the workmanship tends to be pretty good.
		- The wire, insulation and insulators tend to stand up
		  very well.  Most K&T I've seen, for example, is in
		  quite good condition.
		- No grounding.  Grounding is usually difficult to install.
		- boxes are small.  Receptacle replacement (particularly with
		  GFCI) can be difficult.  No bushing on boxes either,
		  so wiring changes need special attention to box entry.
		- Sometimes the neutral isn't balanced very well between
		  separately hot circuits, so it is sometimes possible to
		  overload the neutral without exceeding the fusing on
		  any circuit.
		- In DC days it was common to fuse both sides, and no
		  harm was done.  In fact, it was probably a Good Thing.
		  The practise apparently carried over to K&T where
		  you may find fused neutrals.  This is a very bad
		  thing.
		- Building code does not usually permit insulation in
		  walls or ceilings that contains K&T.  Some jurisdictions
		  will allow it under some circumstances (eg: engineer's
		  certificate).
		- Connection to existing K&T from new circuits can be
		  tricky.  Consult your inspector.
		- Modern wiring practice requires considerably more
		  outlets to be installed than K&T systems did.
	
	Since K&T tends to be in pretty decent condition it generally
	isn't necessary to replace it simply because it's K&T.  What
	you should watch out for is renovations that have interfered
	with it and be cautious about circuit loading.  In many cases
	it's perfectly reasonable to leave existing K&T alone, and add
	new fixtures on new circuits using modern techniques.

	After K&T, they invented multi-conductor cable.  The first type
	you will see is roughly a cloth and varnish insulation.  It
	looks much like the romex cable of the last decade or two.
	This stuff was used in the 40's and 50's.  Again, no grounding
	conductor.  It was installed much like modern wiring.  Its
	major drawback is that this type of insulation embrittles.
	We've seen whole systems where the insulation would fracture
	and fall off at a touch.  BX cable of the same vintage has
	similar problems.  It is possible for the hot conductor to
	short out to the cable jacket.  Since the jacket is rusted, it
	no longer presents a low resistance return path for the current
	flow, but rather more acts like a resistance heater.  In
	extreme cases the cable jacket will become red hot without
	blowing the fuse or circuit breaker.  The best thing to do with
	old style BX is to replace it with modern cable whenever it's
	encountered and there's any hint of the sheath rusting.

	This stuff is very fragile, and becomes rather hazardous if the
	wires become bare.  This wiring should be left untouched as
	much as possible - whenever an opportunity arises, replace it.
	A simple receptacle or switch replacement can turn into a
	several hour long frustrating fight with electrical tape or
	heat-shrink tubing.

	After this wiring technique, the more modern romex was
	invented.  It's almost a asphalt impregnated cloth.  Often a
	bit sticky.  This stuff stands up reasonably well and doesn't
	present a hazard and is reasonably easy to work with.  It does
	not need to be replaced - it should be considered as safe as
	the "modern" stuff - thermoplastic insulation wire.  Just don't
	abuse it too much.

Some parts © 2009 Advameg, Inc.