Butch, for a typical 12-lead zig-zag rewire you will want to limit capacity - (de-rate) the head by 33.3% from its 3-phase rating. You will have two pairs of windings working in tandem and the third pair will be working by itself. The head will be unbalanced drastically but can run this way without trouble as long as you stay within (below) the temps for your insulation class. You will still have 120/240 capability from this configuration.
dieselgman
Actually wattage or volt amps will be reduced to 57.7% of rated nameplate which is 17.3 Kw and will require approx 26 shaft HP at full load. Line current will remain the same as does the line to neutral voltage at the load terminals when zig-zagged.
Should have 240 on either line to neutral and 480V from line to line.
The simplest way to reduce voltage would be to under field the alternator to obtain 120/240. Capacity will be reduced to approx 8.5 Kw.
You assume a genhead wired for 480volts and not zig-zag configuration for your 6 sets of windings ... Only high-voltage wye will yield 480volts.
Most of the common North American 3-phase 12-lead heads are pretty flexible and offer the whole range of output voltages in both single and 3-phase configurations from 120v, 139v, 208v, 240v, 277v, 416v, 480v
Some of the specially wound heads go up to 600v.
Zig-Zag wiring on one of these particular 3-phase 4-pole heads (with 12 output leads) yields ONLY 120/240 single-phase. You basically lose one of your 3-phases in the re-wire. Line to Line will NOT be 480vac but rather 240vac.
dieselgman