Provsional Sum in Option B

We are the PM administering an NEC3 ECC Option B contract.

The BoQ contains provisional sums, with the sums allowed for having being determined by the Employer for the purpose of the tender (so that it remain a constant in each tendered price - appreciate this isn’t text book and their cleaner and less ambiguous ways of managing this).

One of the sums is for electrical connection costs charged by the DNO for the installation of a new electrical supply to a number of new feeder pillars, with these works also shown in the Works Information (again, aware it didn’t necessarily need to be included as a provisional sum but this is how the Employer elected to bill the connection element). With that, and to manage the inclusion of the sum would it be correct for us to issue an instruction, the quotation for which being the defined cost of the connection offset against the sum amount?

A sum has also been allowed for drainage surveys which are shown in the Works Information, but with the length of gully connection / highway sewer to be surveyed unknown, hence the reason the Employer elected to bill as a provisional sum. Similarly, to manage the sum should we issue an instruction and the quotation being the defined cost of the survey works offset against the sum?

Advice on the above would be greatly appreciated - and again, aware this is why provisional sums should not be used / encouraged as inevitably lead to ambiguity.