Especially in PvP, it is difficult to find a well-balanced amount of spell penetration. Against targets with no resistance, it has no effect (and as such is wasted). However, against targets with resistances, spell penetration is a powerful and, in terms of item values, cheap stat. Ideally, a caster should have exactly as much spell penetration as necessary to lower the target's resistance to zero.
In PvP, good spell penetration values are 40 (to counter unimproved Mark of the Wild, 75 if the druid spent two points in Improved Mark of the Wild) and 70 (to counter various single-school resistance buffs, such as the Paladin's resistance auras); these are usually enough. 20 will often do for 'occasional' PvP.

No hay "cap" de spell penetration, en pvp basicamente significa tener tanto penetration como resistencias tengan tus enemigos generalmente.

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