The Collaborative International Dictionary of English v.0.48:

mains \mains\ (m[=a]nz), n. [Scot. See Manse.]
   The farm attached to a mansion house; a manse. [Scot. or
   Brit. dial.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

mains \mains\ (m[=a]nz), n. (Electricity)
   The source of electrical power in a building; the wiring
   system of a building.
   [PJC]