Differance Between Single Core & Muilticore Cable

Because of the distance and changes in terminology are you asking by core difference in solid single strand or multi-stranded conductor? Or bundled wires in the same jacket? Either way whats the difference the customer has given his disapproval. If you are looking for information to argue his disapproval then you won't get much work that way. Customer though they may not be right must be appeased to think so. Do it his way. If this is a change in the designed as put down on the contract then get it in writing. You should also be looking at local codes if you want to argue the change.

Just for information, what is the primary voltage?

primary voltage of transformer is 13.8V 60Hz

I believe that you are asking why some customers will allow multiple parallel conductors per phase while others won't. Ignoring cost differences which are easy to calculate, the engineering reason is based on experience and is more subtle. After all 4 smaller conductors each carrying 500 amps deliver the same 2000 amps as one appropriately sized conductor on a per phase basis. Unfortunately the current doesn't always divide so evenly, especially on very long runs, and is very dependent on having perfectly matched lengths for each of the 4 conductors as well as terminations that are also exactly the same resistance. Why? Because if there is any significant difference in the path (one conductor longer than the others or a termination that is poorly made) then the current will divide unequally and sooner or later the more heavily loaded conductors will become overloaded, overheated and fail. Your customer has experienced this and is concerned with the reliability of his installation.

Before commencing work did the Consultant give specifications for equipment?. Single core cable is easier to handle(bending/terminating/pulling/laying),especially if bending radius is small flexible single core is advisable. Normally in specifications consultant gives,full description of cable. What nationality is he?.