Plate armor question

Question

I was playing around with the Character Builder and noticed an oddity with my level 7 half-orc Knight:

1. My character with no armor has a net AC of 16 [10 +3 (half level) +2 (heavy shield) +1 (dex)].
2. I equipped +3 Dwarven Plate Armor. 16 +8 (plate) +3 (enhancement) = 27. But my AC shows as 28.
3. I equipped +3 Dwarven Layered Plate Armor. 16 +9 (layered plate) +3 (enhancement) = 28. But my AC shows as 27.

I can only think of two possible reasons for this:

1. Character builder is bugged (would not be surprising at all).
2. There is some innate +1 bonus to plate armor I am missing, and then a -2 for not being proficient with the Layered Plate.

Does anyone see a legitimate reason for the AC values above?

Answer 1

Just figured this out. It looks like the rules for magic armor changed with HotFL.

The masterwork bonus is automatically applied no matter what type of armor you are using. You don't need Layered Plate or Rimefang Plate to get those masterwork bonuses, they are automatically rolled in to the native magic item.

The masterwork armors still exist if you want to change things up (like sacrificing some AC to gain in your NADs).

For example:

    Normal Plate: +8  base AC +0 enhancement = +8
+1 Dwarven Plate: +8  base AC +1 enhancement = +9
+2 Dwarven Plate: +9  base AC +2 enhancement = +11
+3 Dwarven Plate: +10 base AC +3 enhancement = +13
+4 Dwarven Plate: +11 base AC +4 enhancement = +15
+5 Dwarven Plate: +12 base AC +5 enhancement = +17
+6 Dwarven Plate: +14 base AC +6 enhancement = +20

Thus, in my example, the Dwarven Plate Armor +3 grants me 10 (base) + 3 (half level) + 2 (shield) + 10 (Armor) + 3 (enhancement) = 28 AC.

The Layered Plate is explicitly given a static +9 AC. Thus I am getting 10 (base) + 3 (half level) + 2 (shield) + 9 (Armor) + 3 (enhancement) = 27 AC.

Thanks to @waxEagle's comment for leading me to this answer!

Answer 2

What happened here is that unarmored characters and characters wearing light armor (cloth, leather and hide) use their Dex skill to determine AC. However, wearers of Mail, Scale and Plate use only the armor modifier.

 Formula for AC

      Light armor AC = Armor Bonus + 1/2 Level + Dex Mod + 10 + Enh + shield bonus + masterwork bonus
      Heavy Armor AC = Armor Bonus + 1/2 Level + 10 + Enh + shield bonus + masterwork bonus

Hope this helps.