I certainly run my 3HP VFD converted lathe on a 20A 220V circuit. Several tools that I have came factory with VFD + 3 phase motor come with a 6-20p plug, and call for a 20A circuit. On a scientific basis, 3HP = 2.2KW. At 220V single phase, that requires about 10.25 amps (theoretical 100% efficient motor). 65% efficiency for VFD+motor would put you at 16A, right at the 80% for a 20A circuit. That would be a pretty poor efficiency.
A motor may draw more that rated current temporarily (startup for example). The VFD will normally be spec'd to supply that current even though the motor nameplate doesn't shows that. To me this is a bit of a rating/load game, what number constitutes the "load", in one case (a motor) we'd use the nameplate number which is NOT the peak, in another we're using something that is basically the VFD peak load. I will agree with
@mksj, that straightforward reading of the NEC calls for wiring installation to meet 125% of that spec. I would also argue that a VFD is not an appliance, or tool. It is a component. When you replace a manufacturers controls with a VFD, you are becoming a designer and voiding the UL approval of that tool. A component's rated load is not the overall system's rated load.
As a
personal judgement call in my own shop, I feel using 125% of the VFD's maximum is over doing the safety margins. I'd have to submit that to an approval body such as UL to really get it approved. As home hobbyist, not happening. Also, In part the NEC is mandating what a new wiring design is expected to accommodate. We all know in our homes we don't quit plugging things into a wall circuit until the breaker pops, 100% load not 80%.
I interpret this as a design criteria to give people a bit of growth room, not strictly a safety issue. As long as the wall wiring gauge meets or exceed the breaker rating (typically 12ga for a 20A circuit), I would use this logic and feel quite comfortable and will continue to use 3HP VFD's on a 20A circuit.
Four questions: 1) Does it work? 2) Is it safe? 3) Is it per code? 4) Will it pass inspection?
Ideally the last two are the same, but there is some room for argument. Code is trying not only for safe, but does it meet reasonable expected use, so 2 & 3 are not always the same.