The ducts were indeed installed for the overhead wires to enter into the building, until a few years ago there was a concrete floor at first floor level.
Originally the engine house would have housed a transformer and the building (Which may be the boiler house modified) would have housed the switchgear.
The history of electricity in Camborne/Redruth (And indeed West Cornwall) is heavily tied into the mines. The whole idea of electricity in the area started as a joint venture between Dolcoath and South Crofty Mines, with the inital generating station being where McDonalds is now, which then expanded to become the tram depot - the tram system was designed principally to transport ore from East Pool to Tuckingmill.
Public electricity supply came much later, with Cornwall being one of the first areas in the country to have a rural distribution network - I have a Network map from 1914 at work and its quite impressive, with all the major substations being located at mine and industrial sites from Geevor to Perranporth. As the network grew to supply houses, the distribution 😠 were the existing substations, resulting in areas of electricity network still focusing in on those sites over 100 years later.