Hey stuperduck, hey all,
But this would be for single-phase AC. Spindles run with three-phase AC whose formula is
P = U * I * √3
and the current (I) here would be the current per phase, and you have three of them, each shifted by 120°. Producing such a three-phase electricity is the purpose of the VFD. Also the VFD that is in front of the spindle has its own efficiency (or better said, lack of), which means the power draw would be even higher.
Anyway, at the input side of the VFD, there is usually single-phase electricity, and the formula for single-phase AC is
P = U * I
This gives:
120 V * 15 A = 1800 W
Conversion:
1800 W / 0.745 kW = 2.4 hp = power at which a 15 A circuit breaker will pop.
This also explains that for the 100 V class, Hitachi offers only VFDs for max. 0.75 kW spindles. This is because they don’t dare to endanger you when connecting it to a usual domestic U.S. supply circuit, like the cheap chinese spindle sellers at am*zon, Alib*ba, Ali*xpress or eb*y do, who simply omit the VFDs input current draw rating to let you believe you can run such a spindle like a 3 hp @110 V on a U.S. domestic supply circuit safely.
To know what supply circuit and circuit breaker rating you need, it has to be on the VFD nameplate and in the VFD manual. So I would not buy a VFD where this rating is willingly omitted. If you buy a serious VFD, like a Hitachi, Omron, Invertek, then you will always find the input current draw of the VFD, as shown on the two nameplates above. The cheap no-name chinese VFDs omit that, may they have a Hu*ny*ng name on it or not. I would never buy such a VFD.