Home / Basic Electrical / American Wire Gauge | Chart | Sizes | Circular Mil Definition

American Wire Gauge | Chart | Sizes | Circular Mil Definition

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Many conductors are much less than an inch in diameter. To simplify area calculations for such conductors, the foot-pound system uses an area unit, called a circular mil, based on a mil, or one-thousandth of an inch (see Figure 1).

Definition: One circular mil (CM) is the area of a circle 1 mil in diameter.

A circle with diameter of one mil

Figure 1:  A circle with the diameter of one mil

When using circular mils, the formula for the area of a circle becomes

$\begin{matrix}   A={{d}^{2}} & {} & \left( 1 \right)  \\\end{matrix}$

Where A is the area of the circle in circular mils and d is the diameter in mils.

Wire Size Example

Find the cross-sectional area of a wire 0.0280″ in diameter.

Solution

$\begin{align}  & d=0.02800”=28\text{ }mills \\ & A={{d}^{2}}={{28}^{2}}=784\text{ }CM \\\end{align}$

Resistivity Definition

Resistivity can be defined in terms of a piece of wire 1 ft long with a cross- sectional area of 1 CM, as shown in Figure 2. The volume of this wire is 1 circular-mil foot, and the units of ρ become CM-Ω/ft. Table 1 lists the resistivities for some metals.

One circular-mil foot

Figure 2 One circular-mil foot

Material Resistivity
Silver 9.56
Copper 10.4
Gold 14.0
Aluminum 17.0
Tungsten 34.0
Brass 42.0
Iron 61.0
Nichrome 675.0

Table 1 Specific Resistivities (${}^{\Omega }/{}_{c.mil-ft}$ at 20 oC)

Resistance Calculation Example

Calculate the resistance of 50.0 miles of copper wire 0.3500 in diameter.

 Solution

$\begin{align}  & d=0.350”=35mils \\ & A={{d}^{2}}=122.5CM \\ & l=50miles\times \frac{5280ft}{1mile}=2.54\times {{10}^{5}}ft \\ & R=\rho \frac{l}{A}=\frac{10.36\times \left( 2.54\times {{10}^{5}} \right)}{122.5\times {{10}^{3}}}=22.4\Omega  \\\end{align}$

American Wire Gauge System

Definition: The system of notation for measuring the size of conductors or wires.

In North America, the most wire is produced in the standard American Wire Gauge (AWG) sizes listed in Table 2. The larger the gauge number, the smaller the wire size.

Wires larger than 4/0 (sometimes called 0000) are usually specified in thousands of circular mils (MCM) instead of having a gauge number.

The ratio of the diameter of adjacent American Wire Gauge sizes is about 1.1229, and the ratio of their areas is about 1.26.

The area ratio between wire sizes three AWG numbers apart is very close to 2. For example, #10 wire (10 380 CM) has about twice the area of #13 wire (5178 CM) and half the area of #7 wire (20 820 CM).

Since resistance is inversely proportional to area, the resistance per unit length also differs by a factor of about two for wires three AWG numbers apart. For example, #1 wire (0.1239 Ω/1000’) has half the resistance of #4 wire (0.2485 Ω/1000’) and twice the resistance of #3/0 wire (0.0618 Ω/1000’).

Wire Size Calculation Example

What size of aluminum wire has a resistance of 1.62 Ω for a length of 2500’?

Solution

Since,

\[R=\rho \frac{l}{A}\]

So,

\[A=\rho \frac{l}{R}=17\times \frac{2500}{1.62}=26.2MCM\]

From the Table 2, we see that this area matches AWG #6.

Gauge Number Diameter (Mils) Circular Mil Area Ohms Per 1000 Ft
00 365 133,000 0.0795
0 325 106,000 0.100
1 289 83,700 0.126
2 258 66,400 0.159
3 229 52,600 0.201
4 204 41,700 0.253
5 182 33,100 0.319
6 162 26,300 0.403
7 144 20,800 0.508
8 128 16,500 0.641
9 114 13,100 0.808
10 102 10,400 1.02
11 91 8,230 1.28
12 81 6,530 1.62
13 72 5,180 2.04
14 64 4,110 2.58
15 57 3,260 3.25
16 51 2,580 4.09
17 45 2,050 5.16
18 40 1,620 6.51
19 36 1,290 8.21
20 32 1,020 10.4
21 28.5 810 13.1
22 25.3 642 16.5
23 22.6 509 20.8
24 20.1 404 26.2
25 17.9 320 33
26 15.9 254 41.6
27 14.2 202 52.5
28 12.6 160 66.2
29 11.3 127 83.4
30 10.0 101 105
31 8.9 79.7 133
32 8.0 63.2 167
33 7.1 50.1 211
34 6.3 39.8 266
35 5.6 31.5 335
36 5.0 25.0 423

Table 2 American Wire Gauge (AWG) Wire Sizes Chart

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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.