Every ductless outdoor unit needs its own supply, and what that supply has to be is printed on the unit rather than chosen by rule of thumb.
What the nameplate specifies
MCA — minimum circuit ampacity. The smallest conductor size the circuit may use.
MOP or MOCP — maximum overcurrent protection. The largest breaker permitted.
These two figures are what size the circuit, and they are not the same as the running current. The breaker is sized from MOP, not from the amps the unit draws in normal operation, because it is protecting the conductors rather than limiting the load.
Using a larger breaker than MOP is a code violation and a fire risk. Using a smaller conductor than MCA is the same.
Dedicated circuits
Each outdoor unit gets its own circuit. Nothing else on it.
Indoor heads on most residential systems are powered from the outdoor unit through the interconnecting cable, so the heads do not need separate supplies — but some configurations power the heads separately, and which arrangement applies is a manufacturer detail rather than a general rule.
Voltage
Small single-zone systems are commonly 115 V; larger single-zone and most multi-zone systems are 208/230 V.
A 115 V unit is easier to supply in an existing house and is limited in capacity. Choosing one to avoid running a new circuit is a reasonable trade for a small room and a poor one for a main living space.
The disconnect
A means of disconnection is required within sight of the outdoor unit, so a technician working on it can be certain it cannot be energised.
It is a small weatherproof box on the wall beside the unit, and it is one of the items most often missing on a cheap installation. An inspector will look for it.
Interconnecting cable
The cable between the outdoor unit and each head carries power and communication, and manufacturers specify the conductor count and size.
It runs in the same bundle as the refrigerant lines and the condensate, and it is a common source of intermittent faults when the wrong cable is used or when a joint is made in the run. There should be no splices between the units.
Panel capacity
This is where whole-house ductless projects meet an unexpected cost.
Replacing a gas furnace with ductless heating moves the entire heating load onto the electrical service. Several outdoor units, plus any electric resistance backup, plus the existing house load, can exceed what an older panel or service can supply.
An electrical load calculation answers it, and it belongs at quote stage rather than on the day. Where an upgrade is needed it is a significant cost, and it is sometimes eligible for the same incentive programmes as the heat pump itself.
Grounding and bonding
Standard requirements apply, and the outdoor unit's ground is part of what protects the control electronics.
Inverter equipment is sensitive to poor grounding, and intermittent communication faults between the head and the condenser are sometimes traced back to it.
Surge protection
Inverter boards are the expensive part of a ductless system and they are vulnerable to supply transients.
A surge protective device at the panel is inexpensive relative to a control board, and in areas with frequent lightning or an unstable supply it is worth fitting. Some manufacturers' warranties are more favourable where one is installed.
What to confirm on a quote
The MCA and MOP for each proposed outdoor unit, the breaker and conductor size to be used, the disconnect, whether the existing panel has spare capacity, and who is providing the electrical work.
If the HVAC quote says "electrical by others", that is a second quote to obtain before comparing anything.