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We have a proposed installation of a remote generator, supplying a VFD and then to a pump motor.

I believe the generator will need an earth electrode and MEC and have looked to 4509.1 for information. With having no mains I'm struggling to understand what we size the MEC to. My only logic seems to size it to the outgoing conductors but can't find information to back this up.

Anyone able to shed some light?

As a side question, not to add confusion the VFD is housed in a common cabinet with other items supplied elsewhere. This also seemed a little odd but as far as I'm aware this should be fine with labelling and such and can not find any segregation requirements in this regard.

4.4.2.2 say use AS/NZS3000 for main earthing lead requirements.
The cables between the generator and the installation would be the mains for the purpose of working out the earthing requirements

WRT mains, yes the cables from genset will be consumers mains; using the definitions in "4509"
ESR 4 (2) requires us to use definitions from Act / ESRs in preference to definitions in the standard the term is used in.
But sometimes this simply does not work - and this is one of those times.

WRT items supplied from different sources; these are really 2 different installations.
The segregation requirements are in 3.9.8.2.

But the definition in 4509 indicates the conductors supplying the main switchboard. Where as supplying a single load I don't have a main switchboard.

That's your choice.
Personally even for a single load I'd be following 4.5 and setting up an installation MSB.

I assume you intend using one option of 4.4 (standalone power system directly feeding a single load).
Your genset either has an earthed neutral output [4.4.2 & Fig 4.1 (a)];
or it provides a separated supply [4.4.3 & Fig 4.1(b)].
Only one of these options includes an earth electrode;
and since there's only one final subcircuit, the live conductors of that will determine the size of the associated earthing conductors.
The max current that can flow in the conductor to the electrode is is the max output of the genset, decreased by the resistance of ground between the point of earth fault and the electrode.

Max output is a limiting factor for provision of fault protection, as unless the source is capable of supplying a high fault current you won't achieve fault protection by automatic disconnection of supply using an over-current device. Hence the RCD.
This is often an issue with gensets & inverters.

Using the separated supply option, fault protection is by separated supply.

Thanks Alec, great points to consider