Battle Caster( Complete Arcane, p. 75)
Building on your existing training allows you to avoid the chance of arcane spell failure when you wear armor heavier than normal.
Ability to ignore arcane spell failure chance from armor,
You are able to wear armor one category heavier than you can normally wear while still avoiding the chance of arcane spell failure. For example, if you have the ability to normally wear light armor without incurring a chance of spell failure, you can wear medium armor and continue to cast spells as normal. This ability does not extend to shields, nor does it apply to spells gained from spellcasting classes other than the class that provides the ability to cast arcane spells while in armor.