Is the interconnecting cable between outdoor and indoor AC units classed as a control cable or power cable?

I suppose there is rather a precedent for the HVAC industry using SY ... even though many manufacturers labelling it as a control cable (although others have claimed at least part BS compliance).

should comply with BS or harmonised standards

OR ... IEC or the appropriate standard of another country (133.1.1) OR the designer confirms that it provides at least the same degree of safety (133.1.3) - so it's not quite a simple yes/no.

Of course there's nothing stopping a control/auxiliary circuit running at full mains voltage (e.g. see fig 55.1)

   - Andy.

Indeed, plenty of thermostats, fan controllers, lamp dimmers and so on are  a kind of "mains level" logic

Mike.

how is the cable size calculated where these type of control cables have very little data to enable calcs? in industrial/ commercial applications some indoor units fed from the Outdoor are large and require considerable amps to run multiple fans/ pumps?

The manufacturer has already done it.

• How? unless they specify the exact cable type/size, max length, environment etc etc which they dont know

They don't have to be exact - that can specify a large-ish cable (say 1.5mm2 - as that's often the min for many countries wiring regs anyway) which will likely be fine for several amps for anything that's not run in a perfect vacuum, say don't group it (it's usually run alongside the pipes anyway so they'll have an idea of temperature it'll be in when operating), a suitable maximum length (the refrigerant pipework will limit how far apart the units can be anyway) and probably specify a 30mA RCD so they don't have to worry about Zs.

  - Andy.

OK, so are the two portions of the heat pump supplied separately, or does one supply the other?

HVAC isn't my area of expertise, but AFAIK, you have a motor which drives a compressor, and a fan to draw air over both the compressor and the evaporator.

The control unit is presumably located within one or other box, but it could be separate. So let's assume that it is in the box containing the evaporator. The thermostat clicks in and closes the contactor, which energises the compressor, the temperature falls and the thermostat opens. And so on.

Granted the contactor could be situated in either box, but what do the manufacturer's instructions say?

OK, so are the two portions of the heat pump supplied separately, or does one supply the other?

HVAC isn't my area of expertise, but AFAIK, you have a motor which drives a compressor, and a fan to draw air over both the compressor and the evaporator.

The control unit is presumably located within one or other box, but it could be separate. So let's assume that it is in the box containing the evaporator. The thermostat clicks in and closes the contactor, which energises the compressor, the temperature falls and the thermostat opens. And so on.

Granted the contactor could be situated in either box, but what do the manufacturer's instructions say?