For the NEMA 34 stepper motor with a maximum current of 6 amps, 18 AWG wire would technically be sufficient. According to the American Wire Gauge (AWG) standard, 18 AWG wire can handle up to around 10 amps of current in chassis wiring and around 16 amps in power transmission. If you are considering running a very long length of cable from the driver to the stepper motor, you may want to consider using 16 AWG.
As for the spindle, this wire gauge is well-suited. There are three phase shifts, each at 120 degrees and each conductor is carrying only 1/3 of the power. At 220v, 2200 watts / 220 V = 10 amps. 1/3 of 10 amps is 3.333 amps, so each conductor will be have a load of 3.33 amps.
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 Is this enough rating for a Nema 34 motor with max 6 amps? Also what about a 220V 2200Watt Spindle?