Players have resistance to one or more spell schools. In battlegrounds, players may have shadow protection, the mark of the wild, paladin auras, or totems that increase player resistances, so having spell penetration is quite useful in PvP and should not be forgotten. However, this guide will focus on spell penetration from a PvE point of view.
The trash mobs in Molten Core will have high fire resistances or even immunity. Mages would spec Frost there, and so spell penetration would only be necessary for the bosses.
All bosses have resistances to all schools (fire, Shadow, Nature, etc.), different numbers depending on the school and the boss. Some bosses are more resistant to Shadow; some are more resistant to Nature, etc. This boss resistance means that a part of your damage will be reduced unless you find ways to reduce the enemy’s resistance.
Warlocks can cast Curse of the Elements and Curse of Shadow. When these 2 curses are on the mob (you will need 2 warlocks), Shadow, Arcane, Frost, and Fire resistance on the target is reduced by 75. It is not possible to reduce a player or monster’s resistance to less than 0.
Resistance comes after Hit.
When you cast on a mob, the first thing that happens is the Hit roll, which means you need Chance to Hit to overcome a level difference. This is before resistance is added to the picture.
As a caster, you will -always- have 1% chance to miss with your spells.
Vs. target of your own level: 4% chance to miss. 3% chance to Hit is needed for cap. (PVP)
Vs. target of your own level +1: 5% chance to miss. 4% chance to Hit is needed for the cap.
Vs. target of your own level +2: 6% chance to miss. 5% chance to Hit is needed for the cap.
Vs. target of your own level +3: 17% chance to miss. 16% chance to Hit is needed for cap.
Spell Penetration will only increase the effect of spells that have already successfully landed on the target. However, even spells that land has a chance to do nothing at all if the mob resistance is very high. That’s what spell penetration is for.
Miss is when you don’t have enough Chance to Hit.
Resist is when you don’t have enough Spell Penetration.
That being said, our combat log doesn’t make it easy for us to see what’s going on. For some reason, even when a spell misses, it will show as “resisted” in the combat log. Keep this in mind when you observe combat logs. But for the sake of understanding the mechanic, we’ll have to be aware that there is a difference.
How much Spell Penetration do you need?
You will need to bring the monsters’ resistance down to 0 if possible to deal with your full damage.
A simple rule of thumb is that each 10 spell penetration adds 2.5% DPS increase on average.
Raid Bosses everywhere have 145 resistance to Fire, Shadow, Frost, Arcane. 70 resistance to Nature, 15 resistance to Holy.
Note: Some may have slightly different resistances, and for example, Ragnaros has extremely high Fire resistance. But the numbers above are accurate in 80% of the cases.
After Curse of the Elements and Curse of Shadows have been applied, you will need 70 spell penetration from gear to reach the cap in the abomination wing. Mages need only 60 because they get 10 from the arcane talent: Arcane Subtlety.