How would the costs change if you split the service, with a higher voltage service drop for the heaters and a 120/240V drop for your general lighting load? Is 575V available at the location; this would take the currents way down into the manageable range
Separate question, and probably not the domain of the sparky involved; why 100KW of electric heat??? We are probably talking $10K-$30K USD for the electric bill to run these heaters, unless they are run very infrequently. I would think that something fuel burning, or a heat pump system, would be more cost effective over time; though I've never thought in terms of heating large buildings.