Four Four inch from utility Transformer to CT Cabinet then to MCC. (XHHW).

three 600MCM & one 350 MCM each Conduit .

600 at 75C 420 amps x 4 = 1680 amp.

Very low Neutral load mostly 480 volt loads.

Using 310.15 (5) so no derating.

Engineer wants to use six six inch with four 400 mcm in each conduit.

I think conduit way over sized!

Maybe he got hollered at for spec'ing to small a conduit before.
How long and how many bends?
Sometime what "fits" is not what you want to pull.
Your ampacity looks OK to me

transformer to ct cabinet 35 ft .
ct cabinet to mcc max 10 ft.
you figure.????

It might be interesting to ask the engineer what his thinking is on this project. He may have just decided nobody ever got in trouble going too big ... as long as it isn't his money.

Maybe he will be paid a percentage of the job.

Space for future expansion? Cost pennies now versus dollars later

Transformer 1000 KVA ( 1208 Amps ) Switch Gears 1600 amps .

Utility Co. Never sizes as per our calculations. I called them on it.

There reply no problem.

That happens all the time.
Utilities always go much further into their overload ratings than we do.
I remember seeing (3) 2,500A Switchboards fed from a vault that (when I looked inside) only had a single transformer in it.

That is also pretty common in residential. They put a 50 KVA pole pig on 3 "all electric" homes with 200a panels. I understand most of the time they are still loafing along but there are times I am surprised that they are not smoking.
Take a cool Christmas eve, oven going, 2 or 3 burners going on the cook top every light in the house on plus outdoor lighting, hot water and 15kw heat strips going in the air handlers ... times 3

Hi guys
Been AWOL for a bit.
Our POCO does not allow any reduced neutrals; no way; all conductors must be full size.
They also 'may' have an issue with 6x6" conduits; next time I talk with my POCO inspector, I'll run it by.

I can't say I've seen anything larger than 4", except for HV