No insurer authorized to do in this State the business of fire insurance shall expose itself to any loss on any one fire risk, whether located in this State or elsewhere, in an amount exceeding ten percent (10%) of its surplus to policyholders, except that in the case of risks adequately protected by automatic sprinklers or risks principally of noncombustible construction and occupancy such insurer may expose itself to any loss on any one risk in an amount not exceeding twenty-five percent (25%) of the sum of (i) its unearned premium reserve and (ii) its surplus to policyholders. Any risk or portion of any risk which shall have been reinsured shall be deducted in determining the limitation of risk prescribed in this section. (1945, c. 378.)