Smoke was reported at about 9:30 a.m. in an industrial area at North First Street and Montague Expressway.
PG&E spokeswoman Katie Romans said the failure occurred in the elbow of a cable in the vault, and initially resulted in power loss to nearly 70 customers, including Altera and Hynix Semiconductor, as well as the Santa Clara County Superior Court.
No injuries were reported.
The cause of the failure is under investigation, but Romans said the cable likely failed because of age.
San Jose firefighters also responded.
Crews hope to have power restored to the remaining two businesses by noon.