Leaders and storm drains connected to a combined sewer shall be trapped. Individual storm water traps shall be installed on the storm water drain branch serving each conductor, or a single trap shall be installed in the main storm drain just before its connection with the combined building sewer or the public sewer. One trap may serve more than one downspout, and any such trap shall be on the downstream side of the connection to any sanitary sewer or any combined sewer or drain, and shall be set where not subject to frost.