For those in the Electrical trade
I am also an end user. What you're wanting to do is dangerous and will either get you killed or leave your expensive servers a pile of slag. If you really have the time, money, and knowledge to run a server bank with "many gpus", then get a real electrician to enable you to even install it correctly.
Honestly your question makes me question your preparedness. I'd consider more research if I were you.
You're equating 120+120 = 240 and while that works in math, it does not work in alternating current voltage.
Except that's exactly how it works. Your house has two live feeds coming in that are 180 degrees out pf phase with each other. When combined, they create a 240V total potential. Your stove, clothes dryer, and water heater all use 240V circuits. A 240V breaker is actually two breakers adjacent to each other (because each alternating 'blade' in your panel is separate phases) with their switches linked together, so when one trips it forces the other off as well.
To OP: you can get a 240 breaker from Home Depot or wherever, but you will also need to replace your receptacle with one that can handle the increased power as well as 240V. You will also need to upgrade to 12/2 wiring on that circuit most likely. Also also... unless you have a 1500W or greater PSU, it doesn't make a difference if you have 120 or 240. Also also also, you can run multiple PSUs with multiple 120V standard household circuits, just without a joined breaker they won't all trip together.
Ok fair. Two 120V rails in series would supply 240V but I think the dangerous part comes from the 'also' and that's where an electrician would come in and turn this from 'just a breaker' to rewiring your house.
you will also need to replace your receptacle
True.
You will also need to upgrade to 12/2
True.
I just don't think it's safe to say 'yes just add a 240V breaker and plug it in' which appears the OP may be at their with level of understanding.
Agreed, it would be not a great idea, but also pretty useless unless OP is actually building a rackmount server which has power supplies that use 240V, at which point they would need 10/2 and an L6 receptacle, probably an upgraded panel... and really good home insurance.
Consumer grade PC power supplies that are rated for use here top out at 1600W, which is juuust enough to run on a dedicated 120V 15A circuit (though I'd feel more comfortable with 20A). Just dumping 240V into it doesn't automatically double it's power output, it would just use half the current from each phase
I would say no, it is not a good idea.
If you have a 15A or more x 120V circuit (not 100 or 110V) and copper, not outdated aluminum or heaven forbid knob&tube wiring in your home/facility (which typically can only support 10A max anyway), you've checked and you have no other high powered electronics that would be on the 1 circuit (TV, more than 4 monitors, hair dryer, vacuum, heater or AC unit, microwave, cooking appliances, fridge), you should be fine having a sustained 1500W load on one circuit so you won't need to resort to any screwing around.
If you have suspicions that a hackjob was done with the internal wiring any point or you have an old home grandfathered into old code, don't consider this idea without having consulted an electrician who has inspected your home first.
You can't just string two 120VAC together and call it 240VAC. In North America how it's usually done is that the utility has two out of phase at their transformers that go to your home, so that -120V to 120V gives you 240V and you have separate neutral and ground wires. I do NOT advise you to DIY your own using two in-phase circuits it will not work and will be dangerous.
I'm sure you can find some device that can give you 120V x more amps using two separate home circuits, but if you go this route, please don't get the cheapest thing off ebay, Amazon or Aliexpress if you don't want to cause a fire, and ensure that the cable to your intermediate and end devices are thick enough to support the maximum theoretical load.