What is a sub panel?

A sub panel is a secondary breaker box fed from your main panel through a feeder breaker. It does not add capacity to the service; it adds circuit spaces and puts them closer to the loads.

The classic reasons: a detached garage or shop, a basement finish, an addition, or a main panel that is simply out of slots.

Electrically it differs from the main in one key way: neutrals and grounds stay separate, on their own bars, with the bonding screw removed. Bonding happens once, at the service, and never again downstream.

Distance from the main is unlimited by code; voltage drop on the feeder is what sets the practical limit for far buildings.

Size the feeder and check the numbers with the sub panel load calculator.