I haven’t built a lot of turbines 4200 mm diameter (14 footers for the North American market). I use 16 magnets the same as the 3600 (12 foot) turbines but put them on a larger disk (450 mm/18 inches) compared with 400mm/16 inches. This allows me to put in a lot more copper to compensate for the lower rpm of the larger 4200 rotor blades.
However Jay Hudnall of Tripalium and Tieole in France has told me that he uses 20 magnets on the larger rotors and gets a higher power output. With the rising price of neodymium magnets this might not be very attractive to people in low wind areas, but it does offer better efficiency and higher peak output in stronger winds.
For those who wish to charge 48 volt batteries, the 20 magnets recipe is to use 54 turns of 2 mm diameter wire in 15 coils. For grid tie, using a Windy Boy 2500 inverter use 260 turns of 0.9mm diameter wire. The 20 magnets allow you to comfortably reach a peak output of 2500 watts instead of the 1700 watts that I would normally recommend. This inverter also operates at a higher voltage (up to 600 volts) whereas the 1700 only goes up to 400V DC.
Higher voltage and higher peak power will not win you much extra in the low winds when power is scarce, or the medium winds where most of the energy comes, from but it does improve the efficiency and productivity in higher winds for those blessed with really good sites.
Comet ME in the middle east are using these 20 magnet rotors in their latest project at Tubah.