And FYI, trying to plug them into different outlets in the same room will usually not help. Bedrooms are usually wired with a single 15A line.

Long extension cords are also a bad idea.

I highly recommend trying to lower the power usage, even if it means lower hash rates. You can also get higher efficiency power supplies which will decrease the overall heat generated and power used.

The best thing to do is to hire a professional electrician to put in 30A at 240V (or 208V if you have 3 phase). Higher voltages are slightly more efficient. A 30A 240V breaker can provide ~5700W (derated to 80%) safely. This is usually enough for 2-4 rigs, depending on their power usage.