Optional Standby Systems are those systems intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system. These systems are intended to supply on-site generated power to selected loads either automatically or manually.

Generators are by far the most common type of optional standby system but battery storage systems and even some PV systems can be used for optional standby power.

Quite often, the maximum amount of Available Fault Current (AFC) is only considered when it comes from the electric utility company. Many electricians forget to think about optional standby power sources such as generators as another source of AFC. NEC^® section 702.4(A) requires optional standby system equipment to be suitable for the maximum AFC at its line terminals.

Because of distance to the utility transformer, it’s quite possible to have a very low AFC at an electrical panel when it is supplied by the electrical utility. If the AFC coming from the utility company is 10k or less, circuit breakers with an ampere interrupting capacity (AIC) rating of 10k may have been installed. If a large optional standby generator is installed, care must be taken to ensure that any existing electrical panels backed up by the generator will have circuit breakers with an AIC rating sufficient for the AFC coming from the generator. Extremely large or parallel generators can produce well over 10,000 amps during a fault condition.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014-2017 Code Language:

702.4 Capacity and Rating.

(A) Available Short-Circuit Current. Optional standby system equipment shall be suitable for the maximum available short-circuit current at its terminals.