Any combination of power armor parts can be equipped on a power armor frame, which is a furniture object that does not exist in the inventory. A power armor station can be used to modify power armor components on a frame.
The base frame provides 60 Damage Resistance, and Energy Resistance, but negates effects of other apparel and armor worn underneath. It also sets the base Strength to 11, which can result in negative effects to carrying capacity. Although the weight of worn apparel and armor is not ignored, any power armor plate segments attached to the frame do not affect carrying capacity.
There are only two empty power armor frames available in the base game (all other frames have power armor pieces attached). One is located at the Revere satellite array, in the northeast corner of array's dishes, in the dish array accessible by stairs. The other frame is inside the Atom Cats garage. All other power armor frames contain at least one piece of power armor; a full list of locations can be found at the power armor page.
When purchased from a vendor, the frame will spawn in the power armor station in the shop. The only exception is when purchasing from Atom Cats, which will appear outside garage.