Resistance of copper wire
Calculating the resistance of copper wires
(If you're not interested in all the number theory, skip to the chart below)
- resistance = resistivity × length / area
Resistivity for Cu is 0.00000001678 ohm.m
Circular mils The easiest measure of a wire's cross section is its diameter. For many years this measure has been expressed as Circular Mils (thousanths of an inch). To use that in resistance calculations, we need to convert to area: Area
- Area = pi * r^2 (a mnemonic: circles are round, but pie are squared)
where pi is 3.14159265 and r is the radius or 1/2 of the diameter
Convert mils to inches
- inches = mils / 1000
Putting it all together
- area in square inches = ((circ mils / 1000) / 2) ^ 2 * pi
Since our constant for resistivity is expressed in Ohmmeters, we'll need to convert the area to meters also.
- meters² = inches² / 39.370079
For your convenience, the resistance of some standard wire sizes have been calculated and reproduced in the chart below. Remember that the resistance of copper increases with increasing temperature.
Standard wire resistance chart (copper at 77°F/25°C,
resistivity 0.00000001678 ohm.m)
AWG
diam (inch)
diam (mm)
Ohms/ft
Ohms/m
000
0.4096
10.40384
0.000060163
0.000197385
00
0.3648
9.26592
0.000075847
0.000248843
0
0.3249
8.25246
0.000095620
0.000313715
1
0.2893
7.34822
0.000120602
0.000395675
2
0.2576
6.54304
0.000152110
0.000499049
3
0.2294
5.82676
0.000191806
0.000629286
4
0.2043
5.18922
0.000241832
0.000793411
6
0.162
4.1148
0.000384609
0.001261842
8
0.1285
3.2639
0.000611285
0.002005528
10
0.1019
2.58826
0.000972079
0.003189236
12
0.0808
2.05232
0.001546063
0.005072386
14
0.0641
1.62814
0.002456597
0.008059701
16
0.0508
1.29032
0.003911313
0.012832391
18
0.0403
1.02362
0.006214982
0.020390361
20
0.032
0.8128
0.009857119
0.03233963
22
0.0254
0.64516
0.015645251
0.051329564
24
0.0201
0.51054
0.024983763
0.081967727
26
0.0159
0.40386
0.039925993
0.13099079
28
0.0126
0.32004
0.063578296
0.208590209
30
0.01
0.254
0.100936902
0.331157815
32
0.008
0.2032
0.157713909
0.517434086
34
0.0063
0.16002
0.254313182
0.834360835
36
0.005
0.127
0.403747608
1.324631261