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If I'm a Armorer Artificer with my power armor in Infiltrator Mode (the armor’s weight is negligible, and it's formfitting and wearable under clothing) and infused with the Enhanced Defense infusion (+1 or +2 to AC depending upon level), can I then don a +3 set of plate armor and if I do what's my AC?
Would other infusions from the under-armor work normally?
Armor changes how AC is calculated. If you have multiple ways to calculate AC, you choose which one you want to use. If you have Unarmored Defence, Mage Armor and Barkskin, you only benefit from one of those.
So, even if you manage to don both armors, you will have to choose which one to actually use.
The problem arises because of how magical armour words it bonus:
You have a +1 bonus to AC while wearing this armor.
So, wearing multiple armours with such a bonus should let them stack. However, wearing multiple items of the same type is generally not possible (depending on DM's common sense; emphasis added to quotation):
Multiple Items of the Same Kind
Use common sense to determine whether more than one of a given kind of magic item can be worn. A character can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of head wear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or be able to layer two cloaks.— Dungeon Master's Guide, p. 141
Whether the alterations to your power armour is sufficient to make it wearable underneath other armour in such a fashion that you can benefit from both is explicitly left to your DM's common sense.
I wouldn't be surprised if they used their common sense to say the magical improvement to the armour makes the AC when using that armour for AC calculation X higher, and that the wording simply assumes you won't be wearing multiple suits (because that's silly).
So, to summarize, ask your DM because you are dealing with the interaction between playtest material and magic items (which often need a little DM adjudication).
if you were somehow able to wear two suits of armor at the same time, only 1 calculation for your AC can be used, so I would assume you pick the higher one I would also like to point out that under the power armor ability
The armor continues to be power armor until you doff it, you don another suit of armor, or you die.
So donning a suit of armor over top of your power armor is going to cause your power armor to not function.
To clarify on your bounty. The magical bonus granted by magic armor is included as part of the armors ac calculation.
For example breastplate is 15+dex(max 2)
A +3 breastplate is 18+dex(max 2)
+3 plate armor is 21
DMG pg. 140-141
Using a magic item’s properties might mean wearing or wielding it. A magic item meant to be worn must be donned in the intended fashion: boots go on the feet, gloves on the hands, hats and helmets on the head, and rings on the finger. Magic armor must be donned, a Shield strapped to the arm, a cloak fastened about the shoulders. A weapon must be held.
and
Use Common sense to determine whether more than one of a given kind of magic item can be worn. A character can’t normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or to layer two cloaks.
Armor isn't intended to be layered therefor stacking magic armors doesn't provide magical benifit.
The armor continues to be power armor until you doff it, you don another suit of armor, or you die.
If you put a suit of armor on over the infiltrator armor, it no long is infiltrator armor, as it is no longer power armor, so it could no longer be worn under another suit of armor.
