The power coming down your line is AC power – Alternating Current. That means the voltage, and therefore current flow, in a 240V AC line oscillates between just under 340V and just over -340V, with a “root mean square” voltage of (unsurprisingly) 240V. The voltage is in a sine wave – it goes up, down, passes through zero, negative, and then back.
This means for part of the time, the power delivered is … zero. Well, you’ve laid all that expensive, heavy cable and for much of the time, it’s just sittin’ there chillin. That’s wasteful.
Multiphase power adds a second or third active wire to your supply, and that oscillation goes up and down the same way that the first “phase” does, but it hits zero voltage at a different point. The alternating electrical voltages are out of phase with each other.
Power transfer is based on electron flow, so with multiple phases, you’re in the zone of providing power more frequently (in fact, all the time). If you want even more physics, try this.
Three-phase power is effectively voltage and power invariant – the sum of the squares of the voltage of each of the three sine waves providing power is constant. It is therefore the most efficient way (in terms of literal pounds of wire used) to provide constant-potential power.
In the United States, it is apparently vanishingly rare to provide multiphase power to a residence, while for commercial/industrial usage, it’s much more common. Much of the high-power processing equipment I used to work with back when I was in the semiconductor hardware industry was three phase, high voltage power.
The multiphase systems (two-phase in the UK) can deliver and accept more power to your car than single phase. The Volvo’s internal charger can accept more power from the multiphase systems: 6.4 kW instead of the 3.6kW of the single-phase systems. But I don’t think you’re in the UK.
OK, next bit:
Power is current times voltage. So 120Volts x 10.5 Amps (which seems to be the max allowed capacity of the OEM-provided 120V cable with that adapter) is 120V x 10.5A = 1,260 Watts, or 1.26 kW. I bought a third-party (Lectron) charger that has a cable rated for 15A, pretty much the max available from a 20A circuit breaker … because under continuous load you don’t push a breaker past 80% capacity (20A x 80% = 16A) and EV/PHEV charging is a constant high-current application. My OEM cable takes 15 hours to fill an empty battery, my third-party 12 hours. The car has a maximum of 3.6kW, but it doesn’t drive max charge rate all the time. It optimizes based on what’s best for the battery. Thus, even with my 15A third-party charge, the 15kW-hr of available battery (80% of the max pack size of 18.8kW-hrs) still takes about 12 hours to charge rather than the 8 hours 20 minutes it “should” take if it could charge full rate all the time.
So…that means in the USA your charging speed is car-limited to 3.6kW. The OEM charger has two adapters, one that’s 120V and maybe 8-12A and the other that’s 240V and will actually pull the 16A.
If you take the 3,600 Watts (3.6kW) for which your car is rated and divide by 240V ( Power in Watts = Voltage x Amperage) then you get a current required for that power delivery of 3600W / 240V = 15A. So for the Volvo T8 in the USA with the 3.6kilowatt charger limitation, it simply cannot accept more than 240V and 15A … and since you don’t run a circuit for continuous load at more than 80% rated capacity, that means for a “I’m only using this for my PHEV” charger, you will gain nothing by providing more than a 240V and 20A circuit.
If you will have a friend or another car with a higher charger capacity (say, you pick up a new Volvo EV), you might need to go higher. If you hit the lottery and snag a Porsche, I believe at least one of their models will accept up to 19.2kW to recharge the battery (240V at a staggering 80A constant load, requiring a dedicated 100A circuit breaker to pull it off). That’s a very spendy install for a Volvo PHEV, no matter how cool it is. 