# Re: [TCML] Re: Breakdown voltages of toroids

```Jim Lux wrote:
```
```For non-weird shapes with reasonably smooth curves, you can use the smallest radius of curvature to determine breakdown voltage. 30kV/cm is a handy approximation.
```
Not so simple.... My table mentioned in the post shows this clearly. The 20x1 cm toroid breaks down with 63.9 kV, not 15 kV, that would be the expected value considering 0.5 cm of radius. I have the theory and a table here:
```http://www.coe.ufrj.br/~acmq/tesla/capcalc.pdf
```
```20    cm sphere:    300 kV  11.1 pF
20x10 cm toroid:    226 kV  9.69 pF
20x9  cm toroid:    216 kV  9.50 pF
20x8  cm toroid:    205 kV  9.31 pF
20x7  cm toroid:    193 kV  9.09 pF
20x6  cm toroid:    180 kV  8.86 pF
20x5  cm toroid:    164 kV  8.60 pF
20x4  cm toroid:    147 kV  8.30 pF
20x3  cm toroid:    126 kV  7.92 pF
20x2  cm toroid:    99.9 kV 7.41 pF
20x1  cm toroid:    63.9 kV 6.62 pF
```
```Antonio Carlos M. de Queiroz

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