Wire Gauge
The common standard for the diameter
(gauge) of round drawn wire is the American Wire Gauge
(AWG).
As strands of wire are made, they are
drawn through progressively smaller dies. This is true
of all wire. In fact, the AWG sizing system suggests
this drawing procedure. For example, a size 22 AWG wire,
smaller than 20 AWG, is drawn, theoretically, through
22 progressively smaller dies. Larger wire is drawn
through fewer dies; hence, the lower-number "gauge."
See Table 1.
 But
there’s some background to these numbers —
which may help lend some “rhyme & reason”
to how they relate… and in fact will provide a
means of relating one gauge to another.
Factor 1 — Every
three gauge numbers (#20 to #23, for example) represents
a division (or multiplication) of the cross-section
and resistance by a factor of 2. Or, referring to the
table, which lists only even-numbered gauges, AWG #20
vs. #26 would yield a factor of 4. To illustrate, #20AWG
copper wire has a cross section of 1,000 circular mils
(CM) and resistance/1000 ft of 10 ohms. #26 AWG, which
is smaller, will have a cross section of 250 CM and
resistance of 40 ohms. (All values are nominal.)
Factor 2 — Every
10 gauge numbers (#20 to #30AWG, for example) represents
a 10-fold increase or decrease in cross section and
resistance. Example: #30AWG wire is 100 CM (1/10 that
of #20AWG) and 100 ohms per 1,000 feet (10 times that
of #20AWG).
Factor 3 — As
a basis for all these numbers, #10AWG copper is 1 ohm
per 1,000 feet.
Having knowledge of these factors can
help to simply calculate (or at least estimate) these
wire parameters.
Stranded vs. Solid
Well, they are clearly different
in appearance, though their purpose is the same. It
stands to reason stranded construction would be more
flexible. So unless you actually want stiffness —
to push a wire through an opening, for example —
wouldn't stranded appear to be the better choice?
Then, too, there's strength in numbers:
rope, for example, is made of many parallel fibers —
individually weak, but together quite strong. If one
fiber breaks, there are many left to carry the load.
House wiring is generally solid; wiring
for machine tools, automobiles, and aircraft is almost
all stranded — for flexibility and redundancy
in the face of vibration.
The application dictates the choice of conductor type.
At high frequencies — above, say, 1,000 MHz —
conductivity relies more on the surface of the conductor
than its core. This is the "skin effect,"
and the reason silver plating becomes important. This
also applies in very high current situations —
beyond that experienced in the typical aircraft situation,
but occurring in major power distribution grids, for
example.
The center conductors of some land-based
high-power RF antenna feeds, where size and flexibility
are not issues, may actually be a hollow tube —
giving further evidence to the relative unimportance
of the interior of the wire as a conductor in such applications.
With adequate support by the insulation
— as with coaxial cable — a solid conductor
will survive the vibration and yet carry an RF signal
more efficiently than its stranded counterpart.
This is not meant to imply that all
good RF cables should have solid conductors; for the
sake of flexibility, some coaxes often have stranded,
silver-plated center conductors and work very well.
As always, trade-offs are omnipresent.
A side issue: Why do you suppose that
the number of strands is almost always an odd —
usually prime — number? The answer is below…
 Table
2 is a chart of some stranding configurations,
and some of their factors. This is hardly all-inclusive,
but illustrates the idea.
The construction of stranded wires
almost always involves a prime number of strands. [A
prime number is defined as one which is divisible only
by itself and by 1.] Among larger numbers of strands
(more than, say, 250), this may stray from "primeness,"
but remains an odd number. And in wires having a very
great number of strands, (above maybe 1000), there are
instances of even-numbered strand counts. These departures
from the norm, however, are few: the norm is truly a
prime number.
Why?
A solid (1-strand) conductor is the
heart of a wire. Stranded wires, then, are surrounded
by additional strands, and if all the strands are of
the same gauge, six of them fit, ideally, around the
center strand. Total: 7. Add another layer (12 will
lay best, in minimum space) around those, and it becomes
19.
And so on...
Stranding in larger numbers often entails
using bundles (“odd-ly”, or “prime-ly”
stranded) as if they were individual wires — so
that a given high-number stranded construction may become
a prime number of prime-numbered "mini-" bundles.
Confusing? Why not? It's the legacy of a very old business
— rope-making.
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