Aluminum is lighter...

Copper dissipates heat better.  Takes less volume than Aluminum.

Aluminum is typically cheaper on standard efficiency transformers.  I think copper is cheaper on very high efficiency transformers, though (not positive on this).

You may need to consider the windings separately.  I think most manufacturers' standards are copper primaries and aluminum secondaries.  I might have that backwards, and have no information why they do that.

I used to have a strong preference for copper, based mostly just on tradition and rumor.  Now I have a slight preference for copper, for the same reason, but aluminum doesn't bother me.

I'd suggest calling a few manufacturers.

Maybe:  http://www.eren.doe.gov/buildings/codes_standards/applbrf/pdfs/screening_analysis2.pdf
Seeming shamlessly biased:
http://www.cooperpower.com/products/transformer/points/default.asp
http://www.copper.org/cable/outlook-cable.html#utilities
  

Something else comes to mind, the expected loading of the transformer may come into play with this.  That is, the optimum winding material (based on efficiency) might change depending if your transformer is usually 35% loaded vs. 80% loaded.

The first cost for aluminum windings will be less than for copper.  

If the transformer is for a critical application, you need to consider the issue of transformer repair.  If a transformer with aluminum windings has an internal winding fault, the aluminum will leave an incredible splatter mess inside the tank.  Copper will usually not be nearly so bad, making it more likely that the transformer can be repaired.  

If you are an electric utility with 25 of these transformers in your bone yard, this may not be a factor, but if this transformer powers your manufacturing line, it could be important.  

You may also want to base your evaluation on the life-cycle cost of the transformer losses.  This is done by computing a present worth value to the future core and load losses of your transformer.  This cost is then added to the purchase cost for purposes of evaluation.  You will have to make assumptions regarding load factor and power costs.  This helps to put everyone on an equal footing in terms of efficiency.  

Escamillo:
The Al is lighter than Cu but needs more Fe for the same
power, so the weight saving may be less -- same about cost.

<nbucska@pcperipherals.com>

nbucska, I think you have that backwards.  Al volume for the Iron is greater than the volume for Cu.  For the same current rating, Al has to be one gage size larger than Cu.  So, the Fe has to be bigger to accomodate the larger space occupied by the Al.

From my research copper adds 15% to the cost. However it holds up over the long run better than aluminum with no de-lamination worries, lower coefficient of expansion (creep) and higher tensil strength. Also this will be a liquid cooled unit as opposed to a air cooled unit. This is being installed into the research section of a university. We expect to run it until it drops dead so long life spans are a must. Keep those comments coming.
Thanks.

Lewish:
Exactly. Al is lighter but has higher specific resistance therefore needs more space for the same power, requiring more iron to accomodate the biger coil, so the weight and cost  saving in the coil is at least partially negated by the need of more iron.

Sorry, for being too short und unclear..

<nbucska@pcperipherals.com>

As everyone has previously stated aluminum tends to be lighter and slightly less expensive than copper.  It really depends on the ultimate design characteristics that are required.  There are many custom design transformer houses that may be able to help you decide.  You can try Norlake Manufacturing in Elyria, OH.  www.norlakemfg.com

Thanks to every one who responded. Your comments & links were very helpful.

There are many other factors that may be more important to transformer selection than the copper vs. aluminum argument.  In my experience there is usually not enough performance gain to use copper over aluminum in large transformers.  Other factors such as the type of fill (oil vs. silicon), insulation levels, heat rise ratings, Basic-Impulse-Level, and other construction issues that need to be considered.

I prefer copper winding for several reasons, most of them already mentioned.

I'd like to add one topic regarding the repair:

It's much easier to get copper conductors for repair because copper is used more widely.

The only technical reason for using aluminum is for some types of cast coil transformers where the aluminium matches the coefficient of thermal expansion of the resin better than copper.