referred us to a British Standard that does not permit this

Which BS is that?

   - Andy.

Is the installation method the reason that they are 240mm CSA? If so you might be able to reduce the CSA near the ATSs

Hmm, that means the Customer is very clued up, or is not very clued up, but thinks it is 'wrong'. If the Manufacturer agrees it is OK to do what you propose, then I wouldnt think anyone could complain.

In the worst case, possibly put a busbar chamber below, have the 3 (12) cables terminated in that, then run up a single cable to the switch.

Though I think the Customer is being slightly too sensitive about your proposal, maybe show them the paperwork confirming all is good, and see what they say, and why they think it isnt acceptable.

Hmmm… 

What torque setting are you proposing, on what basis? Will there be spacers or other washers?

There *are* standards available for making off compression lugs etc including specifying use of washers and torque settings (though I’m at home so don’t have one to hand) and I suspect that this may be where the customer is going. I’d be interested to see which one is being called on.

Given the size, I suspect significant current even if installation method or volt drop is having an impact.

I presume you’re not literally thinking of three palms stacked face down in series on the bar, rotated to fit and slotting the bolt through the lot!

I would be asking the manufacturer for extended terminals or as Alan suggests an external BBC onto which cables can be terminated before a more manageable connection is made.