Main Service disconnect Switch Rated For
Amperes Interrupting Capacity (AIC)
State and local codes require the main disconnect switch and fuse, or the circuit breaker, of the service equipment to be rated at the available
short-circuit current value.
For new, single-family, residential customers (including mobile homes and duplexes) that are supplied by an individual service drop or lateral
and rated at 320 amperes or less, PG&E will design its facilities so that the short circuit duty at the service termination will not exceed 10,000
amperes symmetrical. For other electrical services, such as 400-anpere services, multi-meter residences and non-residential installations, it is
not feasible in most cases for PG&E to design its facilities to limit the short-circuit duty to 10,000 amperes. For these installations, and on
request, PG&E will provide the maximum available short-circuit current based on the service equipment capacity. If the customer increases the
service equipment capacity, the maximum available short-circuit current may be higher.
Series-Connected System
How Series-Connected Circuit Breakers save Component Cost
The National Electrical Code states in article 110.9:
110.0 Interrupting Rating. Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the system voltage
and the current which is available at the line terminals of the equipment.
Equipment intended to break current at other than fault levels shall have an interrupting rating at system voltage sufficient for the current that
must be interrupted.
There are two ways to satisfy this Code requirement. Either method offers the necessary protection but method two is a less costly alternative to
method one.
Method one is to select circuit breakers with individual interrupting ratings equal to or greater than the available fault current at the service
entrance.
Method two is to select circuit breakers with a series combination rating equal to or greater than the available fault current at the service
entrance.
Single Family Homes
In single family homes, the available fault current normally does not exceed 10,000 amperes. In this situation a load center with a main and
branch breakers with interrupting ratings of 10kA, RMS symmetrical will satisfy this Code requirement.
In some instances the available fault current may exceed 10,000 amperes but rarely will it exceed 22,000 amperes. In these instances a load
center with a main circuit breaker with an interrupting rating of 22kA RMS symmetrical would be required. The branch breakers in method one
would be required to be fully rated 22kA breakers while in method two the branch breakers could be 10kA breakers.
Apartments, Condominiums, Commercial
In larger installations the available fault current will normally be above 10,000 amperes so series combination ratings become much more
important. In these installations it is not uncommon to see three breaker series combinations. An example would be 65kA available with an 800
amp service disconnect (MD6 rated 65kA) ahead of a 125 amp feeder disconnect (QPH rated 22kA) ahead of branch breakers in the load center
(QP rated 10kA).
Series ratings are a part of the rating and marking of the equipment in which the downstream breakers are installed. Section 240.86 of the NEC
requires this marking to be on the equipment and the only acceptable combinations in a given device are those combinations marked on the
device.