The subcontractor was working for Balfour Beatty on the South West Water sewer upgrade project.
He drilled through a power cable during roadworks in the late afternoon in the city centre. Witnesses described hearing a loud bang like a car crash.
He is understood to have suffered serious burns to his arms and face and has been transferred from the local Truro hospital to the Plymouth district hospital.
Balfour Beatty is replacing sewer pipes in Frances Street, Little Castle Street and Kenwyn Street, where the incident occurred, as part of a £3.5m programme.
A South West Water spokesman said: “We can confirm an incident took place on Monday on our sewer upgrade construction site in Truro resulting in a contractor working for Balfour Beatty being taken to hospital.
“Balfour Beatty is undertaking an investigation into the incident. It would be inappropriate to comment further at this time.”