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Gents, with respect I think you are missing the point of my question. I am not asking for help calculating the cable size etc etc. Forget any of the figures I have written in my question they are irrelevant.
Lets re-word:
I have been asked to install a sub-board which has a capacity of 40A but the design current is 22A. The feed to the board is trench dug so any future upgrade would be expensive. Would you suggest sizing the cable now to the capacity of the board in case of future upgrade or size it for the design current?
Ask the person who specified the 40A board?
I'm still at a bit of a loss as to the point of this thread, you're not making much sense from an electrical design point of view.
Who specified the 40A capacity and why? How did you establish the design current?