The voltage of a direct lightning strike is virtually unlimited. In a lightning strike to medium voltage AC distribution lines (over 30 kV), the voltage is air-gap limited to around 100 kV at transformers.
Recommended protection capacities in 230 VAC line are (according to IEEE C62.41):
- Category C: open distribution lines: 0 kV/10 ... 20 kA
- Category B: building entry point: 6 kV/3 kA
- Category A: distribution outlets: 6 kV/1 kA
An indirect strike within 100 m (330 ft) of cables or buildings can induce surges up to 5 kV and 1.25 kA in AC cables.
Telephone and instrumentation cables have higher series impedance, which typically limits the current to over 200 A.
- Category C: (high) 10 kA (design value, acceptable for many small surges)
- Category C: (low) 2.5 kA