This is the problem. You cannot rely on the circuit interruption capabilities of whatever devices happen to be installed on that circuit at a given point in time. If these were permanently installed, you might have an argument that it is passable.

The situation that is dangerous here is if a power source were to be connected to one of these inlets and it did not have a sufficiently-sized breaker and the other side has a dead short, you are looking at a fire inside your walls.

I just installed a 50 amp generator inlet on my main panel, and even though my generator has a matching 50 amp circuit breaker, I installed a separate matching 2-pole 50a breaker on the inlet circuit. This protects the electrical system from even the most screwed up of generators being connected to the panel.

Surely that could be said for anything though? There is nothing stopping me from getting an adapter to plug a space heater with a cheap 16awg cord with an adapter into a 30a outlet

Here there is a breaker, and if the UPS is disconnected, there is no power. Of course someone could go and plug this into a 30a outlet, however that's something that I wouldn't do, so its not really my issue

In the case of a cheap 16awg cord, yes you have created a dangerous situation, but it is not one that is considered part of a larger structure in terms of code. This is the electrical equivalent of spraying gasoline around at the gas station and acting surprised when it turns out poorly for everyone.

> however that's something that I wouldn't do, so its not really my issue

Electrical code is designed so that even an incompetent operator cannot produce a dangerous situation inside a building's electrical system. OSHA, USCSB, et. al. would be the first to tell you that relying on a specific individual to interlock a dangerous situation can result in extremely adverse outcomes.

>There is nothing stopping me from getting an adapter to plug a space heater with a cheap 16awg cord with an adapter into a 30a outlet

That would be against code.

There's nothing stopping you from running bare wire through your walls.

There's nothing stopping you from not bothering to ground any outlets.

There's nothing stopping you from using 14AWG romex on a 50 amp breaker.

Etc., Etc.

All of that is against code, and I've personally seen all of that in the wild at people's homes, offices, etc.

If you wanted to do things the #rightway, you'd most likely need a subpanel for the UPS to feed.

A breaker panel isn't just used for overcurrent protection, though—it's also a standard point to disconnect power when needed. The possibility of house wiring being energized even when all power to the house is cut is dangerous for anyone not familiar with the setup. This could be lethal if work needs to be done on it and OP isn't around to explain how it's wired up.

True, but signs that there is more than on disconnect are common in industrial settings. They are rare in house situations, but putting a sign on the main breaker box is probably enough to meet code. Though it would surprise the inspector.

This is common in home systems where you have a second power source like solar. Mine has markings like this: https://www.jadelearning.com/blog/pv-system-marking-and-labe...