I am not an electrician, but an end user.
I am planning to build a very powerful server for running LLMs. It will have many GPUs and can realistically hit a 1500 watt sustained load. The PSU in my computer can handle 240v but I do not have access to a 240v circuit.
My question is whether it is a good idea to somehow balance the load between 2 or 3 120v circuits. If so, what are some methods to safely do this?
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If you have a 15A or more x 120V circuit (not 100 or 110V) and copper, not outdated aluminum or heaven forbid knob&tube wiring in your home/facility (which typically can only support 10A max anyway), you’ve checked and you have no other high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.