Transformer core

Hi all,

I have a large transformer core that I want to wind for my inductive 
ballast - 13mH.  It must handle 3kVA without saturating, so could 
some of the transformer experts tell me if my core is large enough, 
and minimum number of turns required?

Overall dimensions are 7.5" x 6.25" x 2.5"
cross sectional area 1.25" x 2.5" on perimeter sides and 2.5" x 
2.5" in center leg of the El  shape.

This is from a 1kVA Topaz isolation transformer, but the size of the 
core seems like it can take much more power.

Matt Behrend