Just multiply the 3 phase current times the Sq. Rt. of 3, 1.732. So the 15HP 208V motor at 46.2A FLC 3 phase will draw 79.46A at 208V single phase.
However, the VFD must be sized larger than that. It's not just about the increased current, you also have a significant increase in DC bus ripple because you will be pulling current from only 4 diodes instead of 6, and the DC bus will drop all the way to 0V between ripples now. So the basic rule of thumb is to double the VFD amp rating compared to the motor amps, as long as the VFD is in a cool environment, i.e. 77 degrees F (25C). If you want to use the VFD at it's full temperature rating, ie 40C or 50C, the de-rate factor is usually 60-65%, so 2.5-2.8x the motor current. So let's break it down:
25C (77F) Cool dry environment for the VFD, minimum size = 93A, which for a 208V VFD will equate to a 30HP drive.
40C (104F) environment for the VFD, 60% de-rate, 2.5x FLC = 115A, which equates to a 40HP VFD
50C (122F) environment for the VFD (assuming VFD is rated for that), 65% de-rate, 2.85x FLC = 131A which equates to a 50HP drive (141A rating) **
MOCP on a 30HP or 40HP VFD is 350A, but you could get by with a 200A
MOCP on the 50HP drive is a 375A breaker, but you could get by with a 250A.
Now, you also must apply 430.122 to this, which says that the CONDUCTORS feeding to the VFD must be sized at 125% of the VFD max amp rating. So your conductors must be rated for 116A, 144A, or 176A respectively on the above sizings. That's one reason why you can't go too small with the breaker even though the motor load is much lower, because the conductors must be so much larger, meaning it's unlikely tou could get conductors rated for 176A, for example a 125AF breaker will not hold 2/0 wire.
It's not quite as simple as some VFD salesmen want to make it out to be.
** I need to clarify, this is if you do NOT know if the VFD is already designed to accept a single phase input, some are. For example AB drives, in the PF70/700, are already designed so that you can use a specific chart in the manual for sizing them for a single phase input where they have tested values lower than what I just showed above. If that's the case, then go with what the mfr. says.
