Current Carrying Capacity and Temperature Rise
The amount of current a copper circuit can safely carry depends on how much heat rise a project can withstand, since heat rise and current flow are related. When current flows along a trace, there is an I2R (power loss) that results in localized heating. The trace cools by conduction (into neighboring materials) and convection (into the environment). Therefore, to find the maximum current a trace can safely carry, we must find a way to estimate the heat rise associated with the applied current. An ideal situation would be to reach a stable operating temperature where the rate of heating equals the rate of cooling. An IPC formula can be used to model this event.
| Cu weight(oz/ft2) | Thickness(inch) | Track Width(inch) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 0.0625 | 0.1250 | 0.2500 | 0.5000 | 1.0000 | 2.0000 | 4.0000 | 8.0000 | 16.0000 | ||
| 1 | 0.0014 | 4.6 | 7.6 | 12.5 | 20.7 | 34.2 | 56.6 | 93.6 | 154.7 | 255.6 |
| 2 | 0.0028 | 7.6 | 12.5 | 20.7 | 34.2 | 56.6 | 93.6 | 154.7 | 255.6 | 422.5 |
| 4 | 0.0056 | 12.5 | 20.7 | 34.2 | 56.6 | 93.6 | 154.7 | 255.6 | 422.5 | 698.4 |
| 6 | 0.0084 | 16.8 | 27.8 | 46.0 | 76.0 | 125.5 | 207.5 | 343.0 | 566.9 | 937.1 |
| 8 | 0.0112 | 20.7 | 34.2 | 56.6 | 93.6 | 154.7 | 255.6 | 422.5 | 698.4 | 1154.4 |
| 10 | 0.0140 | 24.4 | 40.3 | 66.5 | 110.0 | 181.8 | 300.5 | 496.7 | 821.1 | 1357.1 |
| 12 | 0.0168 | 27.8 | 46.0 | 76.0 | 125.5 | 207.5 | 343.0 | 566.9 | 937.1 | 1548.9 |
| 14 | 0.0196 | 31.1 | 51.4 | 84.9 | 140.4 | 232.0 | 383.6 | 634.0 | 1047.9 | 1732.1 |
| 16 | 0.0224 | 34.2 | 56.6 | 93.6 | 154.7 | 255.6 | 422.5 | 698.4 | 1154.4 | 1908.1 |
| 18 | 0.0252 | 37.3 | 61.7 | 101.9 | 168.4 | 278.4 | 460.2 | 760.7 | 1257.3 | 2078.2 |
| 20 | 0.0280 | 40.3 | 66.5 | 110.0 | 181.8 | 300.5 | 496.7 | 821.1 | 1357.1 | 2243.2 |
| 24 | 0.0336 | 46.0 | 76.0 | 125.5 | 207.5 | 343.0 | 343.0 | 937.1 | 1548.9 | 2560.2 |
| 28 | 0.0392 | 51.4 | 84.9 | 140.4 | 232.0 | 383.6 | 634.0 | 1047.9 | 1732.1 | 2863.0 |
| 32 | 0.0448 | 56.6 | 93.6 | 154.7 | 255.6 | 422.5 | 698.4 | 1154.4 | 1908.1 | 3154.0 |
| 36 | 0.0504 | 61.7 | 101.9 | 168.4 | 278.4 | 460.2 | 760.7 | 1257.3 | 2078.2 | 3435.1 |
| 40 | 0.0560 | 66.5 | 110.0 | 181.8 | 300.5 | 496.7 | 821.1 | 1357.1 | 2243.2 | 3707.8 |
| 45 | 0.0630 | 72.5 | 119.8 | 198.0 | 327.3 | 541.0 | 894.3 | 1478.1 | 2443.2 | 4038.3 |
| 50 | 0.0700 | 78.2 | 129.3 | 213.7 | 353.3 | 584.0 | 965.2 | 1595.5 | 2637.1 | 4358.9 |
| 55 | 0.0770 | 83.8 | 138.6 | 229.0 | 378.6 | 625.7 | 1034.3 | 1709.6 | 2825.8 | 4670.8 |
| 60 | 0.0840 | / | 147.6 | 244.0 | 403.2 | 666.5 | 1101.7 | 1820.9 | 3009.8 | 4974.9 |
| 70 | 0.0980 | / | 165.0 | 272.8 | 450.9 | 745.3 | 1231.9 | 2036.2 | 3365.7 | 5563.1 |
| 80 | 0.1120 | / | 181.8 | 300.5 | 496.7 | 821.1 | 1357.1 | 2243.2 | 3707.8 | 6128.6 |
| 90 | 0.1260 | / | 198.0 | 327.3 | 541.0 | 894.3 | 1478.1 | 2443.2 | 4038.3 | 6675.0 |
| 100 | 0.1400 | / | 213.7 | 353.3 | 584.0 | 965.2 | 1595.5 | 2637.1 | 4358.9 | 7204.8 |
| 120 | 0.1680 | / | / | 403.2 | 666.5 | 1101.7 | 1820.9 | 3009.8 | 4974.9 | 8223.0 |
| 140 | 0.1960 | / | / | 450.9 | 745.3 | 1231.9 | 2036.2 | 3365.7 | 5563.1 | 9195.3 |
| 160 | 0.2240 | / | / | 496.7 | 821.1 | 1357.1 | 2243.2 | 3707.8 | 6128.6 | 10130.0 |
| 180 | 0.2520 | / | / | 541.0 | 894.3 | 1478.1 | 2443.2 | 4038.3 | 6675.0 | 11033.1 |
| 200 | 0.2800 | / | / | 584.0 | 965.2 | 1595.5 | 2637.1 | 4358.9 | 7204.8 | 11908.9 |
IPC-2221A, calculation for current capacity of an external track [1]:
I = .048 * DT(.44) * (W * Th)(.725)
Where I is current (amps), DT is temperature rise (°C), W is width of the trace (mil) and Th is thickness of the trace (mil). Internal traces should be derated by 50% (estimate) for the same degree of heating. Using the IPC formula, we generated Figure 3 (see table at end of text), showing the current carrying capacity of several traces of differing cross-sectional areas with a 20°C temperature rise.
What constitutes an acceptable amount of heat rise will differ from project to project. Most circuit board dielectric materials can withstand temperatures of 100°C above ambient, although this amount of temperature change would be unacceptable in most situations.