City staff say 350 Windsor city hall square will reopen Wednesday.
The building, along with 400 city hall square, were shut down Tuesday because of what was originally labelled as a blown underground transformer.
The source of the fire is now being listed as a cable malfunction in an underground conduit.
It broke out just before 7 a.m. Tuesday, and the city's communications department said no other damage was done and no one was injured.
The 400 building will remain closed on Wednesday — meaning services like thee provincial offences court, Service Ontario and Service Canada won't be accessible in-person.
Municipal staff who normally work in the building will work from home.
An expected time frame on the second building reopening has not been provided.