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Based on the second table in this page for Copper No. 110 (>99.90% pure), a 1/16" x 1/2" copper bus bar can pass 103 A of current with a 30°C rise in an ambient temperature of 40°C. The area of this bar is 0.03125 sqin, or 20.16 sqmm. If we assume a roughly linear relationship for modest changes in area, we can infer approximately an ampacity per unit area of 5 A / sqmm (rounded down). Results for a range of areas are summarized in the table below:
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In order to provide suitable a safety factor to account for scaling non-linearities and a smaller temperature rise, we should
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size bus bars with a generous safety factor. We plot the ampacity data for bus bars of 1/16" and 1/8" thickness at various widths below.
We observe for each thickness the ampacity is best modelled using a power equation. We generate the following table of extrapolated points using the more conservative 1/8" model. We expect this to bound bus bars of smaller thickness (such as 1/32"). All areas are rounded up to the nearest tenth. Note that in the 60 A - 90 A region the area relationship is approximately linear, but this does not hold true for the entire dataset.
| Ampacity (A) | Projected Area (sq. mm) |
|---|---|
| 90 | 22.3 |
80 | 19.5 |
| 70 | 16.7 |
| 60 | 13.9 |
Recommendations
We recommend a hard lower bound on bus bar area of 17 sq. mm, corresponding to an ampacity of at around 70 A. This should give a moderate safety factor to ensure a temperature rise lower than 30°C. Where practical from an engineering tradeoff standpoint, larger areas should be selected to provide further confidence and margin during peak operating conditions.