Powering 3 DC devices from one outlet

Do not tie all 5V connections together. Only GND is required to be connected since it is the common reference for all voltages; each device will receive power separately from their corresponding source. The 5V wall wart should be connected to the 5V pin of the Arduino and every device other than the USB-UART bridge.

You should be able to connect the wall wart first and then the bridge, but if you want to be extra careful you can use a 74LVC2G240 powered by the bridge's 5V output; LVC devices support I_off, which basically means that all inputs and outputs are high-Z when no power is supplied.

Ground loops aren't usually a problem with 2-prong wall warts, but if you use a 3-prong wall wart then you should verify that its GND does not connect to earth ground; this will eliminate the possibility of a ground loop through the bridge. Star distribution will handle the rest.

I am a little over-budget on possible output power…

No, you are way over budget on power.

• You've added something up wrong. Your four power supplies sum to 1700W, not 1900W. They are completely inadequate for the aggregate load of your motors.

• You cannot assume that it is "unlikely that all motors will run max power at the same time". Electric motors draw the most power under stall torque, which will occur when the motor first starts turning from a dead stop.

  In the case of PSU1, this means that it will probably overload the power supply every time motor 1 starts up. You need a larger power supply for that motor.

  In the case of PSU2/3, the only way this configuration would be safe would be if you had interlocks in place to make it impossible for both motors to be powered at the same time. 288W x 2 is well in excess of the capacity of those power supplies.

• Power supplies are not 100% efficient. The wattage ratings you are quoting are for the output of the power supplies; the input will require more power. Read the specifications on your power supplies for details.

• With regard to the extension cord… read its specifications. Any reputable cord will have its power ratings marked on it. Do not chain extension cords under this type of load.

• With regard to the outlet, you absolutely cannot draw more power from the outlet than it's rated for. At best, you will trip a circuit breaker. At worst, you may cause an electrical fire.