Code Reference
ElectricalNEC 2023

Section 110.9/Interrupting Rating

NEC 110.9 requires all equipment intended to interrupt current at fault levels to have an interrupting rating sufficient for the available fault current at its terminals.

What this section requires

Equipment intended to interrupt current at fault levels must have an interrupting rating not less than the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels must have an interrupting rating at nominal circuit voltage not less than the current that must be interrupted.

Why this section exists

Circuit breakers and fuses are designed to safely interrupt current up to a specific maximum fault current level. If the available fault current at a piece of equipment exceeds its interrupting rating, a short circuit event can cause the device to fail to clear the fault. This can result in an arc flash, equipment explosion, or fire. Section 110.9 establishes the fundamental requirement that every interrupting device must be rated for the worst-case fault current it could encounter.

What plan reviewers look for

Plan reviewers verify that the interrupting rating (AIC) shown on panel schedules, one-line diagrams, and equipment specifications is equal to or greater than the available fault current at each device location. This check applies to every circuit breaker, fused switch, and fused disconnect in the system, not just the main service equipment. The reviewer traces the available fault current from the service entrance downstream through the distribution system.

Common violations

Standard 10kA breakers in high fault current locations
The most common violation is specifying standard 10,000 AIC panelboards in locations where the available fault current exceeds 10,000 amps. This frequently occurs in commercial buildings fed by large utility transformers, where fault current at the main service can exceed 25,000 amps.
AIC ratings not shown on drawings
The electrical drawings specify breaker sizes and panel ratings but do not include the AIC or interrupting rating for the equipment. Without this information, the plan reviewer cannot verify compliance.
Downstream equipment not evaluated
The available fault current is calculated and verified at the main service but downstream panels, disconnects, and motor starters are not evaluated. The available fault current decreases along the circuit due to conductor impedance, but it must still be verified at each device location.
Compliance tip
Show the AIC rating for every circuit breaker and fused disconnect on the one-line diagram. Perform a fault current study that calculates the available fault current at each major equipment location, not just the service entrance. Include the study results on the drawings or reference them in the general notes.

Related NEC requirements

Section 110.24 requires the available fault current to be documented at service equipment, providing the baseline number that feeds into the 110.9 evaluation. Section 110.10 requires the total system (protective devices, equipment, and conductors) to be coordinated so that fault conditions can be cleared without extensive damage. Together, 110.9, 110.10, and 110.24 form the NEC's fault current protection framework.

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Related sections

110.16Arc Flash Hazard WarningNEC 2023110.24Available Fault Current DocumentationNEC 2023240.4Protection of ConductorsNEC 2023