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panelboards, switchgear, switchboards, the system with narrow or no margin increase available short-circuit currents
and service entrance equipment, they may be quite expensive. We can’t seen in the power distribution system.
will be there for a very long time and say that the NEC® didn’t warn us. Changing impedance contributors in
will face changes that will place more or the system, such as transformers, can
less demands on the power distribution have significant impact to the available
system. We arrive at the size of such short-circuit current. We use calculated
equipment as services, conductors, SHORT-CIRCUIT short-circuit currents when we select
and transformers based upon our load CALCULATIONS such equipment as circuit breakers,
calculations found in Article 220. A quick fuses, and other types of electrical
glance at Section 220.12, titled “Lighting A short-circuit calculation considers distribution equipment. Short-circuit
Load for Specified Occupancies,” shows the impedances of a system as well current ratings (SCCR) and interrupting
us an informational note that says, “The as all fault contributors. Popular key ratings are two very important parameters
unit values are based on minimum load contributors to short-circuit current that come to mind that are directly
conditions and 100 percent power factor include the utility feed, motors, and compared with the available short-
and may not provide sufficient capacity generators on site. Popular reducers of circuit current at the point of application.
for the installation contemplated.” short-circuit current include conductors, Equipment selected with SCCR and
Once we have selected conductors and transformers, inductors, and busway. interrupting ratings that are very close
transformers and electrical equipment Changes in the power distribution system to the calculated available short-circuit
and have them installed, expanding can impact the available short-circuit current do not provide for addition of
capacity when we have designed motors or replaced transformers.
current. For example, adding motors can

www.ieci.org | May/June 2019 | Insights Magazine 59