(From NWN Forums)
Terra's Guide to Armor Class and Stacking
Disclaimer: The purpose of this topic is to help anyone that does not understand the different types of Armor Class bonuses and/or simply wishes to have more information on how the various bonuses work together. If you find this topic helpful, I would ask you to post a link to it if you see someone else with questions that would be answered by it. Even if it only helps one person though, it was well worth writing.
If you've played Neverwinter Nights for a while, you've probably learned that there are several types of Armor Class bonuses and that some will work together (Stack) and some will not. Knowing what stacks and what doesn't can often mean the difference between useful equipment and useless equipment.
To begin, I have divided up the types of Armor Class into 6 categories: Natural, Armor, Shield, Deflection, Dodge, and Other. The first 5 categories will be familiar to anyone who has looked at the types of Armor bonuses the various items give. The sixth (Other) category is collection of several Armor Class bonuses that don't fit into any of the other categories. Below you'll find a list of all the types of Armor Class bonuses found in Neverwinter Nights however first we should talk about Stacking.
As mentioned above, Stacking is a term used to describe when two types of Armor Class bonuses will both add to your character's Armor Class or whether only the higher bonus of the two will apply.
All bonuses stack with each other (ie Armor will stack with Shield and Deflection will stack with Natural...etc). In other words, if you have an Armor with a +4 Armor bonus and a ring with +5 Deflection bonus, you will have +9 Armor Class since they stack.
Armor, Shield, Deflection, and Natural Armor bonuses will not stack bonuses of the same type. This means that if you have a Ring which gives +4 Deflection Armor class and a cloak which gives +5 Deflection Armor class, you will only get the +5 Deflection from the cloak since it's the higher bonus.
Dodge bonuses will stack up to a maximum of 20 points*. This means that if you have a pair of boots giving you +15 Dodge bonus, Haste (+4 Dodge), and Shadow Evade giving +4 Dodge, you will only have 20 points of Dodge.
* Note: It should be noted that the maximum bonus of any type (Other not included) is 20 points however this usually only matters for Dodge since its the only type of AC that stacks with itself.
Onto the list!
Sources of Natural Armor Class
Item: Amulet AC Bonus (+1-20)
Feat: Epic Mage Armor (+5)
Spell: Mage Armor (+1)
Spell: Barkskin (+3-5)
Spell: Shadow Shield (+5)
Spell: Tenser's Transformation (+4)
Sources of Armor Armor Class
Item: Armor AC Bonus (+1-20)
Item: Bracer AC Bonus (+1-20)
Feat: Epic Mage Armor (+5)
Spell: Mage Armor (+1)
Spell: Magic Vestment (+1-5)
Sources of Shield Armor Class
Item: Shield AC Bonus (+1-20)
Spell: Magic Vestment (+1-5)
Sources of Deflection Armor Class
Item: Ring AC Bonus (+1-20)
Item: Belt AC Bonus (+1-20)
Item: Helm AC Bonus (+1-20)
Item: Cloak AC Bonus (+1-20)
Item: Weapon AC Bonus (+1-20)
Feat: Epic Mage Armor (+5)
Spell: Shield (+4)
Spell: Mage Armor (+1)
Spell: Shield of Faith (+2-5)
Spell: Protection from Evil (+2 versus selected alignment)
Spell: Aura Versus Alignment (+4 versus selected alignment)
Spell: Magic Circle of Protection (+2 versus selected alignment)
Sources of Dodge Armor Class
Item: Boots AC Bonus (+1-20)
Feat: Bard Song (+2-7)
Feat: Shadow Evade (+1-4)
Feat: Divine Shield (+1-20)
Spell: Haste (+4)
Spell: Mage Armor (+1)
Spell: Epic Mage Armor (+5)
Spell: Undeath's Eternal Foe (+4)
Other Sources of Armor Class
Char: Character Base AC (10)
Item: Armor Base (+1-8)
Item: Shield Base (+1-3)
Feat: Dodge (+1)
Feat: Bone Skin (+2-16)
Feat: Small Stature (+1)
Feat: Monk Wisdom AC Bonus (+1-20)
Feat: Monk 5 Level AC Bonus (+1-8)
Feat: Draconic Armor (+1-4)
Feat: Expertise/Improved Expertise (+5/+10)
Skill: Tumble (+1-8)
The first five sources don't need too much explanation however the Other category could bear a little clarification. The Other category holds several bonuses that belong to no other category and could actually be categories in themselves since they all stack with each other however for simplicity I have put them all together.
The first source is your character's base AC of 10. This is your natural base. The next two items (Armor and Shield base) are the standard Armor Class bonuses from wearing armor and carrying a shield.
The Dodge feat, oddly enough, doesn't provide Dodge Armor Class but it is instead counted as its own separate Armor Class. Next we have Bone Skin which is a Pale Master feat that gets bigger with more Pale Master levels. The Small Stature bonus which comes from being a Halfling grants a +1 bonus to Armor Class.
The Monk Wisdom AC bonus is just what it sounds like, it's the Armor Class bonus a Monk gets from her Wisdom modifier. The Monk 5 level bonus represents a +1 bonus to Armor Class a Monk gets every 5 levels. Draconic Armor is the Armor Class bonus a Red Dragon Disciple gets.
Expertise and Improved Expertise are feats which give the character a bonus to Armor Class but reduce Attack Bonus. Finally, the Tumble bonus is such that a character gains +1 to Armor Class for every five ranks in the Tumble skill.
Addendum
I believe this list to be both complete and accurate, however additions and/or corrections would be plenty appreciated. I would ask that anyone posting with additional information please test it first to make sure it is indeed correct. The less misinformation the better.
Finally, not to sound harsh but please do not post anything about the PRC in this thread. Those who would be helped most by this topic are those still attempting to understand the mechanics of Neverwinter Nights itself and including PRC information would likely only serve to cause unnecessary confusion. Please and thank you.
Edit: Bolding fix
Edit 2: Changed a couple words, added Weapon AC Bonus.
Edit 3: Added Undeath's Eternal Foe
~Terra~
