How large a power supply will I need?

Irrespective of how you get your power there needs to be enough of it. In this section it is assumed that you are using a phase converter from a domestic supply to power a small detached workshop, but the same points would apply equally in other locations, or if you had selected a motor drive or fitted single-phase motors etc.

Any piece of machinery has a normal operating full load current (FLC) at a given voltage and/or a full load power. So if say it is a 3-hp (2.2-kW) lathe it will draw 9.2 Amps at 240 Volts single phase when fully loaded and in steady state operation:

2200 W / 240 Vrms = I = 9.17 A single phase
2200 W / 415 Vrms = I = 5.3 A total at unity power factor and 100% efficiency
5.3 A/√3 = 3.06 Arms per phase which is what you should find on the motor tally plate

However it will draw 50 Amps or so from the single phase when it starts up. Exactly how much current it will draw and for how long depends on the inertia of the load - clearly a lathe with a clutch is an easier start than a clutchless lathe with a heavy large diameter workpiece (you are probably now beginning to appreciate some of the challenges the electrical engineer faces in designing a cost-effective phase converter).

So the house's supply will need to be at least 9.2 Amps. However even if the utility company has provided a 60 Amp supply this might not be enough if, at the same time as you start your heavily loaded lathe, the cooker, the washing machine, the tumble drier, and several electric heaters are all switched on. It is possible to get quite technical about fuseboard ('consumer unit') design and various sorts of load factors so as to achieve a given probability of not tripping out, but in practice you simply need to think about the likely domestic situation at the times you are going to use your machinery and make due allowance.