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It varies from carrier to carrier and probably differs on occupancy, but I've seen somewhere from 15% to up to 40%.

Sprinkler discount of 15% to 40% is a good guess. There are so many variable factors. ISO will manually rate every Sprinklered building; Some of the factors: wet or dry, water pressure if wet, Building partially sprinklered or full, construction ,occupancy, physical condition, types of fire protection equipment, maintenance and testing of the system, protection class. etc. After ISO developes loss cost rates then individual companies will add their loss cost rates to produce a final rate.

I've found that protection class is the biggest factor.