T-45d power armor

The T-4x series of Power Armor models was the first version of Power Armor to be successfully deployed in battle. These suits were lacking the mobility of later versions (such as the T-51b Power Armor), limiting the wearer's agility. The first version to hit the frontlines, the T-45d Infantry Armor, ran on Small Energy Cells, burning through them at an alarming rate. In response, later versions were manufactured with built-in MicroFusion Packs. The T-4x Power Armor was originally built by the the United States Military has become used by many factions, including the Brotherhood of Steel and the Outcasts. Each one of these groups has their own variant of Power Armors.

History
In 2067, the T-45d first suit was deployed in Alaska during the Anchorage Reclamation. This Power Armor was incredibly effective against Chinese tanks and infantry. Its ability to carry heavy ordinance became key in various localized conflicts, and it had the power to destroy entire towns without endangering the wearer. In 2076, the more advanced T-51b was introduced.

Nearly two centuries later, around a hundred T-4x suits with MicroFusion packs were discovered by the Capital Wasteland Brotherhood of Steel in the ruins of Pentagon and were extensively used, with modifications and makeshift repairs.

Design
While it is similar in general shape to the more advanced T-51b Power Armor, it is significantly more primitive, using riveted steel plates instead of composite materials for construction and various subsystems of the armour exposed on the outside rather than sealed within.

In addition, a specialized body suit is required to be worn in order to use the armour. Compared to the simple body suit of the T-51b model, designed to simply reduce irritation, the underarmor is far more complex.

The design was used as basis for the MP-47/A Prototype Medic Power Armor is visually identical to the T-4x series.

Appearances
A late model in this series (possibly the T-47, based on the designation of the Prototype Medic Power Armor) is the most prevalent Power Armor model in Fallout 3.

The history of The T-45d was mentioned in the Fallout Bible timeline, while its designation was first mentioned in the Van Buren tech demo by Corporal Armstrong who wears the armor. The Museum of Technology in Fallout 3 has a log of proposed exhibits for 2078, which includes a suit of T-45d power armor.

For some reason when wearing this suit you cannot use the large lamp attached to the helmet instead of your Pip-Boy light.

Fallout 3 locations
Most of the time you will have to take badly damaged Brotherhood Power Armor suits from corpses. However, by the time you acquire the training necessary to wear power armor, you should have had the opportunity to scavenge several badly damaged suits and combine them into a single suit in a decent state of repair.

Regular Power Armors can be acquired from merchants. You also receive a complete suit of Power Armor in perfect 100% condition from Sarah Lyons at the beginning of the game's final quest, Take it Back!.

Outcast Power Armors can be acquired from the corpses of fallen Outcasts and from Fort Independence. There is a unique version of this armor - Linden's Outcast Power Armor. However, be warned that having Outcast Power Armor in your inventory (equipped or not) when speaking to Protector Casdin will cause him to take the armor. You don't get any reward for Outcast Power Armor given to or taken by Casdin.

Lyons' Pride Power Armor can be acquired from the corpses of fallen Lyons' Pride members.

Army Power Armor can only be acquired on the PC version using console commands.

IDs
In Fallout 3, there are several variants of this armor model, with slight variation in appearance (different color and insignia), but the same statistics. Each of these variants has a different Base ID.

Trivia

 * In Fallout 3, this armor is just called "Power Armor".
 * According to the Fallout 3 artbook, the power armor was designed directly in 3D, skipping the concept art phase.
 * You can repair Power Armor with the T-51b Power Armor, but you can't do the reverse. It might be a bug or it might be the fact that the T-51b Power Armor is more advanced and uses more advanced parts.
 * Wearing power armor normally requires the player to aquire Power Armor Training by talking to Paladin Gunny in the Citadel. However, the skill can be unlocked via the console in the PC version by typing 'setpccanusepowerarmor 1'

Quotes

 * "Though that design was intended for the "real" F3 power armor (T-51b), we wanted to have an alternate texture for the armor Armstrong describes, T-45d, which ran off small energy cells. It would only have appeared in the tutorial as a "back in the day" detail." - J.E. Sawyer
 * "I [...] have fifty three hours of field experience in the T-45d Power Armor, citizen. Most of those were during the Anchorage Reclamation.[...]This T-45d eats up small energy cells like Chans at a rice buffet!" - Corporal Armstrong