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?

  • Space heaters can draw a lot of amperage. If it's a smart UPS, it may detect it can't draw enough current to operate properly and for safety it cuts it off.
    – Phaelax z
    Mar 6, 2020 at 13:52
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    A couple of possibilities come to mind. First, it's possible that you have a bad connection in this circuit. When it's lightly loaded things are OK but when you turn on the heater, which is likely 1500W (i.e. 12.5A) you load it and the voltage drops. Alternatively, you have too large a circuit breaker on this circuit. Either case is dangerous and should be fixed.
    – jwh20
    Mar 6, 2020 at 14:30
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    Get a multimeter. Check voltage at the receptacle where the UPS is plugged in (not the output from the UPS). See what you get. Turn on the computer. See how much it changes. Turn on the space heater. See how much it changes. It when everything is on it drops from ~ 120 to ~ 100 (or similar, YMMV), then the UPS is kicking on when it sees the voltage drop and, if the UPS batteries are worn out, it turns off altogether. Mar 6, 2020 at 14:54

2 Answers 2


Sounds like a bad connection to me.

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.)

Don't overload the circuit

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.

  • If it is a older system I disagree , there are a lot of 115/230v systems out there and even a light load will put most ups’s in an under voltage state unless the configuration is changed. I covered backstabs also as this is a likely cause.
    – Ed Beal
    Mar 6, 2020 at 22:36
  • Abusing comments here (I'll delete after you've read it). I was looking at homedepot.com (I know, not your favorite but I can order everything online and pick up an hour later - great normally, fantastic under current circumstances) for receptacles - kids at home, got my laptop plugged in to a surge protector to a 3-2-pin stupid adapter for one of them to video classes. Figured now is the time to finally upgrade my receptacles...looking at Q&A on a Leviton and I see a comment: Mar 18, 2020 at 16:32
  • ""Back stabs" are hokey and unreliable. That's where you stick the wire in and it grabs automatically. "Screw-to-clamp" back wiring is far, far more reliable, supports 2 wires per screw and supports 12 AWG wire (backstabs do not). Make sure you torque it to spec." I think "that's Harper!". Sure enough, I look to the side harper. Man, you are everywhere! Mar 18, 2020 at 16:32

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.

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    Yes, a breaker CAN be the cause of apparent voltage drop. Say you have a 12 AWG circuit with a 30A breaker and you pull 25A from it? You are overloading the wires and losing more voltage than you should yet the breaker will not trip since it's < 30A. Just like people used to upsize fuses to keep them from blowing, folks have been known to upsize breakers for the same reason. Not a good idea but people do stupid things all the time.
    – jwh20
    Mar 6, 2020 at 15:42
  • I test breakers every week and the only time a breaker will have a voltage drop is with a damaged hammer. Look at the fault curves a house breaker size will not be a voltage drop issue unless it is damaged.
    – Ed Beal
    Mar 6, 2020 at 15:50
  • You're not listening to me. The drop is in the wire that is in an overcurrent situation due to the breaker being over-capacity. I'm discussing the SITUATION not the specific breaker.
    – jwh20
    Mar 6, 2020 at 16:00
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    Pretty far fetched if you ask me... but I've only got 60 years experience and corrected many overloaded circuits and never had an 11% drop on any of them.+
    – JACK
    Mar 6, 2020 at 18:44
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    Why would anyone DV this? It's a good answer. Mar 6, 2020 at 23:55

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