Conductor size
It should be commonsense knowledge that
liquids flow through largediameter pipes easier than they
do through smalldiameter pipes (if you would like a
practical illustration, try drinking a liquid through straws
of different diameters). The same general principle holds
for the flow of electrons through conductors: the broader
the crosssectional area (thickness) of the conductor, the
more room for electrons to flow, and consequently, the
easier it is for flow to occur (less resistance).
Electrical wire is usually round in
crosssection (although there are some unique exceptions to
this rule), and comes in two basic varieties: solid and
stranded. Solid copper wire is just as it sounds: a single,
solid strand of copper the whole length of the wire.
Stranded wire is composed of smaller strands of solid copper
wire twisted together to form a single, larger conductor.
The greatest benefit of stranded wire is its mechanical
flexibility, being able to withstand repeated bending and
twisting much better than solid copper (which tends to
fatigue and break after time).
Wire size can be measured in several ways.
We could speak of a wire's diameter, but since it's really
the crosssectional area that matters most regarding
the flow of electrons, we are better off designating wire
size in terms of area.
The wire crosssection picture shown above
is, of course, not drawn to scale. The diameter is shown as
being 0.1019 inches. Calculating the area of the
crosssection with the formula Area = πr^{2}, we get
an area of 0.008155 square inches:
These are fairly small numbers to work with,
so wire sizes are often expressed in measures of
thousandthsofaninch, or mils. For the illustrated
example, we would say that the diameter of the wire was
101.9 mils (0.1019 inch times 1000). We could also, if we
wanted, express the area of the wire in the unit of square
mils, calculating that value with the same circlearea
formula, Area = πr^{2}:
However, electricians and others frequently
concerned with wire size use another unit of area
measurement tailored specifically for wire's circular
crosssection. This special unit is called the circular
mil (sometimes abbreviated cmil). The sole
purpose for having this special unit of measurement is to
eliminate the need to invoke the factor π (3.1415927 . . .)
in the formula for calculating area, plus the need to figure
wire radius when you've been given diameter.
The formula for calculating the circularmil area of a
circular wire is very simple:
Because this is a unit of area
measurement, the mathematical power of 2 is still in effect
(doubling the width of a circle will always quadruple
its area, no matter what units are used, or if the width of
that circle is expressed in terms of radius or diameter). To
illustrate the difference between measurements in square
mils and measurements in circular mils, I will compare a
circle with a square, showing the area of each shape in both
unit measures:
And for another size of wire:
Obviously, the circle of a given diameter
has less crosssectional area than a square of width and
height equal to the circle's diameter: both units of area
measurement reflect that. However, it should be clear that
the unit of "square mil" is really tailored for the
convenient determination of a square's area, while "circular
mil" is tailored for the convenient determination of a
circle's area: the respective formula for each is simpler to
work with. It must be understood that both units are valid
for measuring the area of a shape, no matter what shape that
may be. The conversion between circular mils and square mils
is a simple ratio: there are π (3.1415927 . . .) square mils
to every 4 circular mils.
Another measure of crosssectional wire area
is the gauge. The gauge scale is based on whole
numbers rather than fractional or decimal inches. The larger
the gauge number, the skinnier the wire; the smaller the
gauge number, the fatter the wire. For those acquainted with
shotguns, this inverselyproportional measurement scale
should sound familiar.
The table at the end of this section equates
gauge with inch diameter, circular mils, and square inches
for solid wire. The larger sizes of wire reach an end of the
common gauge scale (which naturally tops out at a value of
1), and are represented by a series of zeros. "3/0" is
another way to represent "000," and is pronounced
"tripleought." Again, those acquainted with shotguns should
recognize the terminology, strange as it may sound. To make
matters even more confusing, there is more than one gauge
"standard" in use around the world. For electrical conductor
sizing, the American Wire Gauge (AWG), also known as
the Brown and Sharpe (B&S) gauge, is the measurement
system of choice. In Canada and Great Britain, the
British Standard Wire Gauge (SWG) is the legal
measurement system for electrical conductors. Other wire
gauge systems exist in the world for classifying wire
diameter, such as the Stubs steel wire gauge and the
Steel Music Wire Gauge (MWG), but these measurement
systems apply to nonelectrical wire use.
The American Wire Gauge (AWG) measurement
system, despite its oddities, was designed with a purpose:
for every three steps in the gauge scale, wire area (and
weight per unit length) approximately doubles. This is a
handy rule to remember when making rough wire size
estimations!
For very large wire sizes (fatter
than 4/0), the wire gauge system is typically abandoned for
crosssectional area measurement in thousands of circular
mils (MCM), borrowing the old Roman numeral "M" to denote a
multiple of "thousand" in front of "CM" for "circular mils."
The following table of wire sizes does not show any sizes
bigger than 4/0 gauge, because solid copper wire
becomes impractical to handle at those sizes. Stranded wire
construction is favored, instead.
WIRE TABLE FOR SOLID, ROUND COPPER CONDUCTORS
Size Diameter Crosssectional area Weight
AWG inches cir. mils sq. inches lb/1000 ft
===============================================================
4/0  0.4600  211,600  0.1662  640.5
3/0  0.4096  167,800  0.1318  507.9
2/0  0.3648  133,100  0.1045  402.8
1/0  0.3249  105,500  0.08289  319.5
1  0.2893  83,690  0.06573  253.5
2  0.2576  66,370  0.05213  200.9
3  0.2294  52,630  0.04134  159.3
4  0.2043  41,740  0.03278  126.4
5  0.1819  33,100  0.02600  100.2
6  0.1620  26,250  0.02062  79.46
7  0.1443  20,820  0.01635  63.02
8  0.1285  16,510  0.01297  49.97
9  0.1144  13,090  0.01028  39.63
10  0.1019  10,380  0.008155  31.43
11  0.09074  8,234  0.006467  24.92
12  0.08081  6,530  0.005129  19.77
13  0.07196  5,178  0.004067  15.68
14  0.06408  4,107  0.003225  12.43
15  0.05707  3,257  0.002558  9.858
16  0.05082  2,583  0.002028  7.818
17  0.04526  2,048  0.001609  6.200
18  0.04030  1,624  0.001276  4.917
19  0.03589  1,288  0.001012  3.899
20  0.03196  1,022  0.0008023  3.092
21  0.02846  810.1  0.0006363  2.452
22  0.02535  642.5  0.0005046  1.945
23  0.02257  509.5  0.0004001  1.542
24  0.02010  404.0  0.0003173  1.233
25  0.01790  320.4  0.0002517  0.9699
26  0.01594  254.1  0.0001996  0.7692
27  0.01420  201.5  0.0001583  0.6100
28  0.01264  159.8  0.0001255  0.4837
29  0.01126  126.7  0.00009954  0.3836
30  0.01003  100.5  0.00007894  0.3042
31  0.008928  79.70  0.00006260  0.2413
32  0.007950  63.21  0.00004964  0.1913
33  0.007080  50.13  0.00003937  0.1517
34  0.006305  39.75  0.00003122  0.1203
35  0.005615  31.52  0.00002476  0.09542
36  0.005000  25.00  0.00001963  0.07567
37  0.004453  19.83  0.00001557  0.06001
38  0.003965  15.72  0.00001235  0.04759
39  0.003531  12.47  0.000009793  0.03774
40  0.003145  9.888  0.000007766  0.02993
41  0.002800  7.842  0.000006159  0.02374
42  0.002494  6.219  0.000004884  0.01882
43  0.002221  4.932  0.000003873  0.01493
44  0.001978  3.911  0.000003072  0.01184
For some highcurrent applications,
conductor sizes beyond the practical size limit of round
wire are required. In these instances, thick bars of solid
metal called busbars are used as conductors. Busbars
are usually made of copper or aluminum, and are most often
uninsulated. They are physically supported away from
whatever framework or structure is holding them by insulator
standoff mounts. Although a square or rectangular
crosssection is very common for busbar shape, other shapes
are used as well. Crosssectional area for busbars is
typically rated in terms of circular mils (even for square
and rectangular bars!), most likely for the convenience of
being able to directly equate busbar size with round wire.

REVIEW:

Electrons flow through largediameter
wires easier than smalldiameter wires, due to the greater
crosssectional area they have in which to move.

Rather than measure small wire sizes in
inches, the unit of "mil" (1/1000 of an inch) is often
employed.

The crosssectional area of a wire can be
expressed in terms of square units (square inches or
square mils), circular mils, or "gauge" scale.

Calculating squareunit wire area for a
circular wire involves the circle area formula:


Calculating circularmil wire area for a
circular wire is much simpler, due to the fact that the
unit of "circular mil" was sized just for this purpose: to
eliminate the "pi" and the d/2 (radius) factors in the
formula.


There are π (3.1416) square mils for every
4 circular mils.

The gauge system of wire sizing is
based on whole numbers, larger numbers representing
smallerarea wires and visaversa. Wires thicker than 1
gauge are represented by zeros: 0, 00, 000, and 0000
(spoken "singleought," "doubleought," "tripleought,"
and "quadrupleought."

Very large wire sizes are rated in
thousands of circular mils (MCM's), typical for busbars
and wire sizes beyond 4/0.

Busbars are solid bars of copper or
aluminum used in highcurrent circuit construction.
Connections made to busbars are usually welded or bolted,
and the busbars are often bare (uninsulated), supported
away from metal frames through the use of insulating
standoffs.
