So this is confusing me a lot.
I have a gaming computer in my office, connected to an UPS (battery backup). If I plug my space heater in and turn it on in my bedroom (same breaker), the outlet the UPS is plugged into stops working. It doesn’t trip the breaker; it just stops supplying power until I turn the space heater off.
This is also true for the other outlet in the same room as the computer.
Does anyone know the cause or how to fix this?
And this is dangerous. This could set your house on fire.
It happens all the time with backstab connections that a poor connection creates a virtual "fuse" that "burns out" above a certain current (often well within the rated ampacity of the circuit). This results in a dead branch. It would be less common, but certainly not impossible, for such an "accidental fuse" to be self-healing; making it an "accidental circuit breaker".
The problem is, such things cannot be counted on to be stable. This is likely to devolve into a more serious arcing fault, which can create hundreds of watts of heat in a near-pinpoint area of the receptacle. This can melt the receptacle, and even set the box or surroundings on fire. It's a big cause of house fires, and the reason AFCI breakers are Code for most circuits today. AFCI breakers listen for the electrical sound of arcing. (if you've ever jiggled a headphone or speaker wire and heard crinkle-crunch, that's the sound, and it manifests both electrically and aurally.)
A gaming PC. Those have "850 watt" power supplies - but that's on the DC side. You still have to count efficiency and power-factor, so if you look at the AC-side nameplate, that typically says 10 amps. But all the cool kids today are using 1300W supplies that AC-nameplate at 15 amps.
So you're putting that, and ~1A worth of UPS (figuring battery recharge current), and 12.5A worth of heater... on a 15A (typical for bedrooms) or 20A circuit.
All but the UPS need a 125% derate, so over 30A provisioned, all on a 15/20A circuit.
This is treating wall outlets like a cornucopia, and that doesn't work. I expect what's been happening is this "accidental circuit breaker" has been preventing you from overloading the circuit, and so your actual circuit breaker has not tripped. But you must fix the bad connecton, and when you do, the breaker will see full draw, and likely, you'll get trips.
Best tocall an electrician and have them fit a dedicated circuit (or two or three; the hard part is the fishing; tossing in an extra cable or two is no trouble) to support your hardware. Bedrooms are simply not wired for big PCs and heaters. And if you want heaters, rather than buying a rinky-dink $20 heater-fan every year, heck, in 2-1/2 years you've paid for a proper 2000W baseboard heater ($50) that'll last 20 years! So have the electrician install that on a 240V circuit.
As a breaker has nothing to do with voltage drop I would be looking for a bad connection. Back stabbed receptacles are a leading cause of circuit failure, weak connections have high resistance and create a voltage drop, the other possibility is a very long circuit, but in reality it would need to be ~100’ and fully loaded with small wire to be the issue. If these are not the issue.
I would suggest your UPS configuration is set at a high level for safety and possibly adjusting its cut out value may be needed if the true voltage is above 112vac (old school but 115v transformers are still out there). Adjusting the configuration may be appropriate.
I don’t find as many old school transformers today but my house was on one until the last big snow storm last year when they upgraded it. So this may be the root cause if all the connections are good.
