Why not 120V DC?
Mainly thinking 48V as that is already a standard in the marine and automotive world. AFAIK 120V DC is not a commonly used standard ? (Maybe in aviation?)
Most modern aircraft have a 400V 360-800Hz three-phase grid, as this allows thinner (read:lighter) conductors. There are some high voltage DC rails on some plains in the range 270VDC or ±270VDC.
On the usage side for instrumentation, avionics etc. it is usually 28 VDC.
50V is the limit for "safe extra low voltage" rules. Considered to be touch safe.
It would be great to work on my home's electrical system without the risk of straight-up killing myself. Especially considering most of the things I plug into my wall (lighting, TVs, tablets, phones, etc.) don't need to draw anything like 120/240V. I've debated installing low volt circuits for things like lighting and such, but I think the hassle won't make it worth it.
1. It is not considered safe voltage, so you have all the downsides of the current AC grid with the downsides of high DC voltages
2. Nearly no equipment uses 120VDC by itself, so you would need specialized DC-DC converters that can deal with the 120VDC input to go down to usual voltages like 48VD, 24VDC or 12VDC
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DC also has issues with arcing, so switches get complicated and expensive.