How do electricians calculate the maximum allowed on circuit breakers if the current is measured in AC?
The breakers are chosen for fire safety and the standard practice is to have a current rating near the power entry on an imprinted label to define the RMS current for safety to calculate the branch current requirements.

The electrician simply adds all these ratings to ensure the wire and breaker ratings  “meet or exceed” the local standards and use the sum total by a safe margin to prevent nuisance trips from high inrush motor surges.

So breaker ratings do not include inrush current, which for charging up caps or starting motors  can be ~10x nominal , when starting at peak Vac, unless reduced by design.

These vary with input voltage range and within the range so a worst case rating is used on the label, not nominal.

The device must have its own protection if it a low power device.