usability – Why are power adapters (wall warts) designed to block adjacent power outlets?

First, this obstacle does not affect all adapters. Transformer-based adapters take up space because they contain a large transformer. Switching supplies can be placed in a much smaller space. For example, look at the current crop of cell phone chargers.

Secondly, even the transformer adapters are small enough that they can be plugged into a two-socket outlet with another device or adapter. Where they become an obstacle are the power bars. This is due to the poor design of a power bar, which stacks vertically, without gaps between them, without anticipating the fact that users have many adapters.

The solution is to design a power bar whose outputs are oriented at 90 degrees and surrounded by space.

Look at this power bar. It has an exit with an extra space around it that can accommodate a wall wart, and the exits are at 90 degrees:

dusty power bar

The illustrated transformer wall wart is over ten years old and compact. Two of them could fit into a two-socket, and this is deliberately designed. In other words, the engineers took steps to ensure the exact opposite of what you suggest by placing the claws near the edge of the box.

Of course, you can not place N of these warts in a conventional N-socket power bar. This is simply not possible due to the size of the transformer.

The photo also shows a modern mobile phone power adapter, which is not much bigger than a passive plug. Its power handling is about the same as that of the other adapter.

Now, here's another shot, which contrasts two transformer-based adapters. The big model is the Yamaha unit of the late 1980s. It must be like that because it's sturdy. This thing works pretty hot, and still works after 25 years of use.

The little wall-wart is also based on a transformer (I think) rather than switching, and shows you that these things can go down to the waist. However, it is only expected for 200 mA at 9V. A switching power supply of this size can provide much more power.

As big as it is, Yamaha's can always be plugged into a two-socket wall outlet, while this jack can accommodate another device. Again, it is designed to minimize obstacles, while providing the space needed for the transformer and ventilation around it. Note that the little wart and the big one, both have roughly the same clearance between the teeth and the nearest edge.

Back of a music stand

Here are two of these power supplies easily plugged into the same socket, with a free space of half an inch!

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So, basically, your assumption that engineers deliberately designed adapters to prevent other devices from being plugged in is false; Just think of your adapters when shopping for a power bar.