701.4 Legally Required Standby Systems. Capacity and Rating.

Extremely large or parallel generators can produce well over 10,000 amps during a fault condition.
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Extremely large or parallel generators can produce well over 10,000 amps during a fault condition.

Legally Required Standby Systems are those systems required and so classed as legally required standby by municipal, state, federal, or other codes or by any governmental agency having jurisdiction. These systems are intended to automatically supply power to selected loads (other than those classed as emergency systems) in the event of failure of the normal source.

Permanently installed generators are by far the most common type of legally required standby system. NFPA 110 considers a legally required standby system to be an emergency power supply system (EPSS) and classifies an EPSS as either Level 1 (critical to life and safety), or Level 2 (less critical).

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 other standby power sources such as generators as another source of AFC. NEC® section 701.4(A) requires legally required 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 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).

2020-2023 Code Language:

701.4 Capacity and Rating.

(A) Rating Legally Required standby system equipment shall be suitable for the available fault current at its terminals.

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701.4 Legally Required Standby Systems. Capacity and Rating.

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

Which of the following is true for a switchboard supplied from a legally required standby generator where the AFC from the generator to the switchboard is 36,000 amps?

A: Overcurrent devices in the switchboard must have an AIC rating of at least 36kA.
B: It must be listed for emergency use.
C: Only automatic transfer switches are permitted to supply the switchboard.
D: It shall be suitable for the minimum AIC at its terminals.
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