As a rule, I usually don't get involved in single to three phase discussions. There are so many machine specific variables involved that it isn't worth my time. I personally don't use any 3 phase and if I did each installation would be specific in how it was handled. I don't like "electronic gizmos" and have other methods of dealing with the conversion.
Even a Master Machinist can often be a novice to the electrical field, beyond an "on-off" switch operation. Flip the switch "Up" and the light comes on, flip it "Down" it goes dark. Unless you are in England and some European countries, where it is the opposite. With that given, let's take a motor. Discounting the inductive portion of the circuit and relying only on the nameplate load. When I was aboard an old (1945ish) ship, in the 1970ish era, power factor was considered good at 86% at the generator switchboard. So we take that as a "rule of thumb" where machines are concerned. The difference shows up on your power bill but is otherwise not much of a concern.
The conversion factor of 1.73 is the relationship between single phase and three phase. So stated above, What was not stated was how and where it applies. Take a 3 phase motor, rated at a theoretical 10 amps at 240 volts name plate load. That is for 3 phase power. For single phase power, as converted, the load becomes 17.3 amps. Derating that figure to meet the "code" of 80% normal load on a circuit breaker, the result falls to a 25 amp circuit. My knowledge of the code is antiquated, it may be different now. In any case, you could "get away" with a 20 amp circuit, maybe. But a 30 amp circuit would be a safer choice from all the miscellaneous losses that are not given.
It is rare to see 120 volt feed for a VFD but it is an option in some cases. Calculations for 120 volt feed must be doubled. Inversely, a 480 volt motor being fed from a 240 volt circuit must also be doubled. Such as the 10 amps at 480 volts becomes 20 amps at 240 which then becomes 35 amps at the input to the VFD. Which becomes a 50 amp circuit at 240 volts. Or 100 amps at 120 volts. Let's not go there. That's one hell of a load imbalance. . .
Taken to the next step, a 30 amp circuit requires AWG 10 wire, which allows a 30 amp breaker.This again is the "code" and is a factor that covers heating of the wire. A lighter wire may carry the load but is subject to getting hot. Very hot so it cannot be touched. Best follow the code there. Followed through to the end, a "theoretical" 10 amp load requires a 30 amp circuit on 240 volts. Minimum. . . I used the theoretical load as an example because it makes the arithmetic simpler. To use for any other load, the calculation must be adjusted up or down persuant to the machine load.
There isn't room here, or reader's time, to cover every possible combination. For what it's worth, I was "mastered" in Florida in the '70s. That was a long time back and I have worked many jobs since that had little or no bearing on licensing. And quite honestly, the license had little bearing on reality even then. Before these powerful "gizmos" came into being. Calculating a machine load is a lengthy process. Having a pencil and paper handy, enumerating each conversion, will aviod a broken heart when things don't work out.
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