Cob: electrical

[Shannon C. Dealy]

On Fri, 12 Sep 2003, Dorothy Bothne wrote:

> I think he was worried about uneven heat dissipation, hot spots or
> areas where the heat would be held around the wires.  (And he does have
> the credentials - over 30 years designing electrical and lighting
> systems for commercial and residential buildings.)  The conduits open to
> the j boxes, and the jboxes are open to the room, so the air can move
> through the conduits and vent to the room. This would remove any heat
> build up much quicker than conduction through the cob wall.
[snip]

Sorry, but this isn't right.  Possibly your friend thinks that cob is a
better insulator than it is.  The cob will definitely remove the heat
faster than the air in the conduit will, air is a lousy thermal conductor,
only by comparison to things which are more insulative does it provide
good thermal transfer.  Hot spots occur under one of two conditions that
immediately come to mind:

    1 - wire passes through something more insulative than the rest of the
        wire runs through, such as a hole in a wood stud (the
        combination of the wood and reduced air flow around the wire in
        a tight hole creates a better insulator than the free air between
        studs).

    2 - A local defect in the wire which increases it's resistance in that
        spot, this can be caused by a manufacturing defect, a kink in the
        wire, or a poor quality splice.

You would have to hook a fan up to the conduit to force air through to
even begin to approach the rate at which cob would remove the heat, and
even in a conventional residential building, far more heat is transfered
out through the walls of the conduit than is carried by natural air flow
out to the boxes.  Conduit is not intended as a remedy for thermal
problems, it is a protective casing to prevent damage to and possibly
short-circuiting of the wires, or to make routing of wires more
convenient.  The code specifies conduit sizing requirements based on the
amount of heat generated from current passing through the wire in the
conduit, but these requirements are a reaction to the fact that the
conduit acts as an insulator by trapping air around the wires rather than
allowing free air flow, in other words, the use of conduit creates a
thermal problem and this is why the code has minimum sizing requirements
in it to compensate.

For reference, the thermal resistance or "R" value of some
materials (1" thickness):

   Most softwoods -  1.25
   Air            -  1.33
   Stucco         -  0.20
   Plaster        -  0.12
   concrete       -  0.08

as you can see, stucco and plaster (which are thermally probably the
closest materials to cob in my reference book) are between six and
ten times more conductive of heat than air is, and even if cob is a better
insulator than either of these materials, there is no way it could come
close to the R value of dead air.  There are other factors to consider as
well, but this is accurate enough for the relative considerations we are
interested in.

Having said all that, this doesn't mean you shouldn't use conduit in cob
walls, it can make it easier to make changes, though personally, I've
pulled enough wire through walls to last me a life time and have
found that I always have to make changes, so I have opted for all exterior
wiring, going into the wall only when I need to go directly through (there
is a six foot section of 3" conduit going through the stem wall of my
power building, and then through the cob wall to the interior of my house.
If you think conduit is to ugly to use on your interior walls (I would
agree), careful use of trim, embedding conduit just below the surface
plaster, and routing wires in ceilings and between the floor joists
(assuming you have an easy way into these areas - which I do) are all
options, the point of my approach is to keep it all as accessible
as possible without having to punch a hole in a cob wall when you need to
make a change, and preferably minimze the use of conduit as well.

One final note, if you are going to have to have your work inspected, SOME
electrical inspectors will take the approach that if they don't know or
understand the material/environment you are installing wiring in, then
you have to put it in conduit, after all, it's much easier to just
require conduit for everything you don't understand than to think about
it, and regardless of what the person is building or doing, if the wire is
in properly sized conduit, it meets code.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
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