Remember the old StarTrek episodes where the computer would go klicky-klack for a couple of seconds and then intone "Insufficient Data for Meaningful Response"?
Copper versus Aluminium is not the issue. The issue is initial cost versus efficiency and operating cost. You need to get the specifications on the loss in the transformer at different load levels, and you need to calculate the loading that you expect on the transformer. You combine the expected loading and the loss data to determine the losses to be expected over the operating time of the transformer. You should also include the cost of air-conditioning if the transformer is in a cooled space.
The Copper Development Association has lots of resources for doing these calculations, but they are a shill for the copper industry, and will have you believe that a Copper wound transformer is inherently more efficient than an Aluminium wound transformer. This is not necessarily the case, and it is really necessary to get the specs from the transformer manufacturer.
Very often it pays to buy a more efficient transformer, even with a considerable price premium. Those losses keep ticking 24x7x365, so reducing the losses means reduced electrical power consumption.
I have been in the industry for almost 30yrs I have seen copper wound general purpose 30 and 45KVA three phase 480-208Y/120 transformers installed on fewer than 30 jobs. These jobs were usually of the "zero intelligence" type design.
Like Jon said, the design for energy efficiency is much more important, and complex, than simply the winding material.
winnie and Jbd, thanks for the replies. Assuming I had one of each type of xfmr with equal loads from the same manufacturer, apart from price, which would be more efficient? Would you suggest approaching the manufacturer with this scenario and see which they recommend based on their experience and calculations? Either way they will make the sale whether cu or al, so I don't see bias being a problem. Other factors to consider is that we don't know the actual loads until tenants take over the leased space, which, in some cases will be long after we are gone.