Sizing sub panels to the need of your anticipated load can be tricky. You’ll need to consider the amount of available power load you’ll need and what the main service has to offer. For instance, If you have a 200-amp main service, you’ll have no problem adding a 100-amp sub panel to feed a shed, garage, barn, etc. or a 60-amp sub panel to power lighting and general-use outlets in another section of your home. But if you only have a 60-amp service, to begin with and want to add a 60-amp sub panel, you’ll have to upgrade your main panel first to allow such a distribution addition.
When adding a sub panel, our electricians suggest adding at least a 12-slot circuit breaker panel.
This should provide ample room for lighting and general circuits. However, if you plan to add many 240-volt appliances like central air conditioning, baseboard heaters, water heaters, ovens, ranges, or 240-volt window air conditioners, then a circuit breaker panel with more opening may be required, as well as a circuit breaker panel with more openings and a larger main breaker rating.