W. Watson said:
My son lives in San Francisco, and would like me to re-wire some of his
house. It's pretty old and he says it has knobs and tubes wiring. What
is that, and how difficult would it be to replace whatever is there with
current outlets, and maybe upgrade his entire electrical service?
Knob and tube wiring, if it has not been abused, is not automatically
dangerous. Splices are twisted and taped, but they are also soldered. I
have seen only one K&T splice fail (it was a ‘cold’ solder joint from
the start). If the wiring has been properly fused, current is no more
problem than it is for any other type of wiring. See also the last point
below.
There certainly are some virtues in replacing it, but entirely replacing
K&T wiring is *very* expensive.
As ehsjr said, one code acceptable method is using GFCI protection for
grounding type outlets (could just be a GFCI outlet). The outlet has to
be marked as Ed details.
A ‘ground’ wire may be added and a grounded outlet installed. For this
purpose, the ‘ground’ wire does not have to be routed with the other wires.
In the past the added ‘ground’ wire could be connected to a metal water
pipe. That is no longer permitted. They now have to be connected to
anywhere on the ‘grounding electrode system’, which includes the panel
ground bar, the heavy conductors to the grounding electrodes, the first
5 ft of water pipe inside the building if the water pipe is used as a
grounding electrode (the service connection to the pipe should then be
in the same 5 ft.).
Water pipe, at least 10 ft buried, has been required to be used as a
grounding electrode for a very long time. More recently, because water
service pipe can be replaced with copper, a “supplemental” electrode has
been required. A ground rod is the easiest to install. A municipal metal
water system will have a much lower resistance to earth than a rod. The
code does not require old systems to add a rod unless ‘additional’ work
is done. The code now usually requires an additional “concrete encased
electrode” for new construction.
‘Ground’ wires are not allowed to be attached only to isolated ground
rods. As Mark said, this is a major safety hazard. Assume your metal
case ‘grounded’ drill develops a short from hot to case. The ground
current flows to the ground rod. Further assume the ground rod is not
only the code acceptable 25 ohms (with 2 rods it can be any resistance)
by a very good 12 ohms. There is 10 amps added. Depending on the other
load on the circuit, the fuse/breaker will open in a little while to
never. In the meantime you are connected to the hot wire holding the drill.
A major function of the ‘ground’ wire is to cause a high current to flow
on a short to open the fuse/breaker. The path is ‘ground’ wire to
service panel, through the required neutral-ground bond, and through the
service neutral to the supply transformer. Earth is not permitted to be
a part of the path.
Some insurance companies in some areas do not insure houses with K&T wiring.
Phil Munro posted 2 links on the subject:
http://www.waptac.org/sp.asp?id=7190
is a report to the Illinois Department of Commerce and Community Affairs
on adding building insulation around existing K&T wiring. No record of
hazard was found in the large number of K&T installations that had
insulation added around them.
The second link appears to no longer be online - was the record of a
complaint to the Maine state Bureau of Insurance by a homeowner against
an insurance company. The insurance company denied renewal of a policy
based on K&T wiring. The insurance company was ordered to renew the
policy because the insurance company "provided no justification for its
position that knob and tube wiring per se automatically provides grounds
for nonrenewal".
