Busking laws vary from city to city, and enforcement varies from cop to cop.
Some towns ban busking while other towns encourage it.
San Francisco's Fisherman's Wharf has a well-planned set of rules with detailed specifications
in regard to
scheduling, performance spots, volume level, amp placement, etc..
Non-amplified music is usually unregulated unless it creates an obvious disturbance.
In places where no specific busking laws exist amplified music is governed by local noise ordinances.
As a general rule continuous loud music is offensive and will cause complaints.
Inconsiderate noise is a nuisance and risks a total ban on busking by the authorities.
Buskers should limit volume levels and duration of performances.
Download this PDF example.