Section 27-941. Cross connections

(a) Standpipe systems that include more
  than one riser shall have all risers cross-connected at, or  below,  the
  street  entrance  floor  level,  except  as  otherwise  provided in this
  section.
    (b) Standpipe systems in  buildings  required  by  the  provisions  of
  section  27-943  of  this  article to have one or more zones shall be so
  designed and installed that the risers supplied from each zone  will  be
  cross-connected  below, or in, the story of the lowest hose outlets from
  the water source in each  zone.  Horizontal  intermediate  check  valves
  shall  be  installed  in  the run of each riser continuing into a higher
  zone in such manner as to permit all upper zones of the system to be fed
  through one riser from the zone below and to prevent any lower  zone  of
  the system from being supplied from a zone above.
    (c)  Risers  supplied  by  an  upper  level  cross connection shall be
  provided with  manual  control  valves  or  remote  control  valves,  so
  arranged  that  risers supplied by the upper level cross connections may
  independently be shut off from the tank supplies.
    (d) Cross connections shall be at least as large as the largest  riser
  supplied  by  the cross connection. However, when supplying two, but not
  more than four four-inch risers, the cross connection shall not be  less
  than five inches. The cross connection shall not be less than six inches
  for all other riser combinations.
    (e)  Where  there is no cellar, cross connections may be hung from the
  ceiling of the lowest story.
    (f) Each siamese connection shall be connected to  a  riser  or  to  a
  cross  connection  connecting  other siamese hose connections or risers.
  The pipe from the siamese connection to the riser  or  cross  connection
  shall  be  five  inch  i.p.s.,  except  that  a  four inch pipe shall be
  sufficient when such pipe supplies a single four inch riser system.  The
  pipe from the siamese connection shall be run as directly as practicable
  to the riser or cross connection.