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Background

Imagine you were an 19th century engineer and were given thetask to sort up among the different sizes wires your employerused. The simplest way would be to use an aritmetic scale: 1, 2,3, 4, 5, 6, 7, 8, 9, 10 and so on. This would give you an awful lotof sizes. Worse, 2 is 100% larger than 1, but 9 is only 12,5%

larger than 8. Here the mathematicians come to the rescue: Usea geometric series. In a geometric series, the sizes increases with

a fixed increment. The simplest geometric series is 1, 2, 4, 8, 16,32, 64...

Although neat and simple, this series is a little bit too coarse forwire size. Two different series were developed, one in the UnitedStates in the 1850's and one in France in the 1870's. The former

is known as Brown and Sharp gauge, now called American WireGauge or AWG for short, the latter is a Reynard series in sq.mm.

American Wire Gauge

The diameter of corresponding to an AWG size is calculated bythis expression:

D = (92 ^ ((36-AWG)/39))*0.005 inch

The higher the number, the smaller the size. Each size is about25% larger than the previous. This mean moving three sizesdoubles the cross sectional area and moving ten sizes, e.g. from20 to 10 AWG, increases the area about tenfold. Only every othersizes is used in reality. The increase in area between these isabout 60%: I.e. 18 AWG is about 60% larger than 20 AWG.

Size 0 is often written as 1/0 and the size -1 is written as 2/0,pronounced two-aught. The scale ends with 4/0 AWG

Circular Mils

A circular mil is the area of a circle with the diameter of 1/1000".In practice this number is about a thousand times to small to beusable for wire sizes. Therefore, sizes are usually given in

thousands of circular mils, denoted kcmil or previously MCM. Onekcmil 0.5067 mm2, which means that for practical purposes the 1mm2 = 2 kcmil can be used as approximation. (The error is only1.3%)

Kcmil sizes are used instead of AWG for sizes larger than 4/0

AWG. The smallest standard size is 250 kcmil, the largest 2000kcmil. The sizes follow no obvious logic.

Metric Wire Sizes

The French military engineer Charles Reynard came up with a

neater formula than Brown: 10^(n/10) where n=1, 2, 3 and soon. Just like the AWG, each sizes is 25% large than the previous.The neat part is that moving ten steps increases the areaexcactly tenfold. Normally, only every other size is used. Thismeans you can write the formula as 10^(n/5). The resultingnumbers are then:

10^(0/5) = 110^(1/5) = 1.584810^(2/5) = 2.511910^(3/5) = 3.981110^(4/5) = 6.309610^(5/5) = 10

In practice these numbers are always rounded. However, for

AWG <=> Metric

AWG mm² Metric

0.50

# 20 0.519

0.75

# 18 0.823

1.0

# 16 1.310

1.5

# 14 2.080

2.5

# 12 3.310

4.0

# 10 5.261

6.0

# 8 8.367

10

# 6 13.30

16

# 4 21.15

25

# 3 26.67

# 2 33.62

35

# 1 42.41

50

# 0 53.49

# 2/0 67.4370

# 3/0 85.01

95

# 4/0 107.2

120

kcmil <=> Metric

kcmil mm² Metric

250 127

150300 152

350 177

185

400 203

240

500 253

300

600 304

700 355

750 380

400

800 405

900 456

500

1000 507

630

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© Global-Electron.com | Updated 22 August 2004 | Disclaimer

some reason only the sizes from 1.0 mm2 to 25 mm2 follow thislogic. Standard sizes up to 1000 mm2 are used, but sizes 35-95mm2 follow a different series. (See the table below) The metricwire sizes in the electrical industry are always in mm2, never inmm dia. The size of other types of wire, e.g. fence wire, is oftengiven in mm dia.

Japanese sizes

Japan and Korea use a separate system. It appears to have been

based on the American Wire Gauge, but the sizes are in sq. mm,rounded and with fewer steps.

Comment

The ampacity of wires depend on a number of factors andconverting between metric and AWG sizes is a bit more involvedthan it seems. Ampacities for wire sizes from 18 AWG - 1000kcmil and 1.0 - 500 mm² can be found here

1250 633

1500 760

800

1750 887

1000

2000 1013

Japanese sizes

mm² mm²

0.75 100

1.25 150

2.0 200

3.5 250

5.5 325

8.0 400

14 500

22 600

38 800

60 1000

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