Shaman · Elemental Combat · tier 5 · from level 30

Lightning Overload

+9.8 Lightning Bolt

3/3 · level 60 · per cast · and 1 more

Lightning Overload 123

Rank 3/3

Gives your Lightning Bolt and Chain Lightning spells a 10% chance to cast a second, similar spell on the same target at no additional cost that causes half damage and no threat.

Lightning Bolt +5.0% +9.8 Chain Lightning +5.0% +6.2

Averages, counting crits and misses, for a level 60 Shaman with a typical weapon and no other gear or buffs, against a target your level. Worked out from the game's own data, the same way as the Foresight addon.