There are four main ways to determine the capacity of your house’s circuit breaker panel. This panel is usually referred to as the breaker panel or electrical panel. Each method offers a different way to learn its capacity.
1. Service Entrance Cable Ampacity
Service entrance cable ampacity is a function of size and ambient temperature. The bigger or thicker the wire, or the colder the outside air, the higher the ampacity will be. Look at the cable label. It will give you voltage and power ratings. Divide the power rating by the voltage rating to determine the number of amps the cable can carry. The rated voltage for most service cables is 600 volts.
Most households are fed by 240 volts, split into two cables at 120 volts apiece. 120 volts is approximately 1/5 of 600 volts.
To determine the 120-volt ampacity of the service cable, multiply the 600-volt ampacity by 5. A cable capable of handling 2400 watts at 600 volts can deliver 4 amperes at 600 volts or 20 amperes at 120 volts.
2. Service Meter Ampacity
Service meters are rated for total power dissipation. To determine this rating, read the sticker on the side of the meter. This tag will also specify the maximum voltage and power the service meter is capable of handling. Service meter bases are typically rated the same as the meter. To determine the service meter base rating, contact a qualified inspector or electrician.
3. Main Input Breaker Rating
Though this is not an accurate measure of what the panel is able to actually handle, or what the service cables are capable of supplying, this is an accurate representation of the maximum that is currently able to be supplied by the electrical service panel. This measurement is found by simply reading the label on the breaker handle.
4. Panel and Buss Ratings
More important than the other three ratings, the panel and buss ratings will be your best guide for the total amount of power or current that your circuit breakers panel can safely handle. There will be a tag on the inside, usually on the door, of the breaker panel that lists maximum voltage, current and power dissipation capability for the panel and busses. This reading can’t be modified or expanded the way the other ratings can by simple substitution of a higher rated component.