A rule requiring fire sprinkler systems be put in new apartment buildings has been deemed unenforceable by the state. This comes just six months after the Department of Safety and Professional Services decided to keep the rule. It said new apartment buildings with three to 20 units have to have fire sprinkler systems installed. But state law says buildings with 20 or more units must have sprinklers. The three to 20 unit requirement was a state rule. Last month, the Wisconsin Builders Association challenged the rule, saying DSPS can’t enforce a rule that’s more restrictive than state law, based on 2011 Wisconsin Act 21.