How do you meet NEC requirements for Overcurrent Protection, SCCR, and Selective Coordination?

Properly designing an electrical system's overcurrent protection is not just about sizing breakers and fuses for the load; it's a critical life safety and equipment protection requirement governed by the National Electrical Code (NEC). Two of the most important—and often misunderstood—concepts are Short-Circuit Current Rating (SCCR) and selective coordination.

Here are the core takeaways for design professionals:

• Short-Circuit Current Rating (SCCR): All electrical equipment must have an SCCR equal to or greater than the maximum available fault current at its point of installation. This is a "withstand" rating, ensuring the equipment doesn't fail catastrophically during a short circuit.
  • Governing Codes: NEC §110.9, §110.10, §110.24
  • Documentation: The available fault current must be calculated, marked on service equipment, and documented on the construction drawings, typically on the single-line diagram and equipment schedules.
• Selective Coordination: In critical facilities like hospitals, overcurrent protective devices (OCPDs) on the essential electrical system must be selectively coordinated. This means only the OCPD closest to a fault opens, isolating the problem without causing a wider outage.
  • Governing Codes: NEC §517.31(G), §700.32; NFPA 99 §6.4.2.2.5.2
  • Documentation: A selective coordination study, including Time-Current Curves (TCCs), must be performed and made available to the plan reviewer and Authority Having Jurisdiction (AHJ). The drawings must reference this study and specify the required OCPD settings.

RequirementWhat It MeansKey NEC / NFPA SectionsPrimary DocumentationSCCREquipment must safely withstand the available fault current.NEC §110.9, §110.10, §110.24Single-Line Diagram, Equipment Schedules, Field LabelSelective CoordinationOnly the OCPD nearest the fault opens, isolating the outage.NEC §517.31(G), §700.32, NFPA 99Selective Coordination Study with TCCs

Why This Topic Matters

Overcurrent protection and short-circuit ratings are foundational to safe and reliable electrical design. Neglecting these requirements can lead to catastrophic equipment failure, dangerous arc flash incidents, extended power outages in critical facilities, and significant permitting delays.

• Safety: A device with an inadequate SCCR can explode when subjected to a short circuit, creating a severe arc flash hazard and sending shrapnel through a room. NEC §110.10 is written specifically to prevent this.
• Reliability: In a hospital, a fault in a single patient room cannot trip the panelboard main breaker, which could de-energize life-support equipment in adjacent rooms. This is why NEC Article 517 and NFPA 99 mandate selective coordination for the essential electrical system.
• Code Compliance: Plan reviewers and electrical inspectors are highly focused on these requirements. Inadequate documentation of available fault current, SCCR, and selective coordination is one of the most common reasons for electrical plan rejection.

These concepts are deeply interconnected. To achieve selective coordination, you must first know the maximum available fault current at every point in the system. That same fault current value dictates the minimum required SCCR for your equipment. This analysis must be performed early in the design phase, as it directly influences the specification of every major component, from the main switchgear down to the smallest panelboard.

For a healthcare facility, what are the NEC Article 517 and NFPA 99 requirements for selective coordination of overcurrent protective devices on the essential electrical system, and how do I document this for the plan reviewer?

The National Electrical Code (NEC) and NFPA 99, Health Care Facilities Code, both mandate that the essential electrical system (EES) in a healthcare facility be selectively coordinated. This ensures that a fault is isolated by the nearest upstream device, preventing a localized issue from causing a widespread outage that could endanger patients.

The core requirement is found in NEC 2023 §517.31(G), which states:

Coordination. Overcurrent protective devices serving the essential electrical system shall be coordinated for the full range of overcurrents.

This is reinforced by NFPA 99 (2024) §6.4.2.2.5.2, which applies similar language. The EES includes the life safety, critical, and equipment branches that serve vital patient care and building functions.

Deeper Explanation and Nuances:

• "Full Range of Overcurrents": This is a critical phrase. It means coordination must be achieved not just for overloads (low-level, long-duration events) but for all fault currents up to the maximum available fault current at that point in the system. This often makes it impossible to achieve coordination with standard thermal-magnetic circuit breakers, whose instantaneous trip curves tend to overlap. Achieving full-range coordination typically requires:
  • Electronically-tripped circuit breakers with adjustable short-time and instantaneous settings.
  • Current-limiting fuses carefully selected to coordinate with each other and with downstream breakers.
• System-Wide Application: The requirement applies to all OCPDs in series on the EES, from the generator or alternate source down to the final branch-circuit device. For example, a fault on a 20A branch circuit serving a critical receptacle must not trip the 42-pole panelboard's 100A main breaker.
• Coordination with Normal Power: While the mandate applies to the EES, the feeder overcurrent device on the normal power side that supplies an Automatic Transfer Switch (ATS) must also be considered in the study to ensure proper coordination during a fault event before the system transfers to the EES.

How to Document for Plan Review:

Documentation is non-negotiable and must be clear and complete for the AHJ.

1. Selective Coordination Study: This is the primary deliverable. The study is performed by an electrical engineer using specialized software (e.g., SKM PowerTools, ETAP, EasyPower). It must contain:
   • A system single-line diagram.
   • Calculations of the available short-circuit current at each bus.
   • Time-Current Curves (TCCs) for each set of series-rated devices, demonstrating that no unacceptable overlap occurs in their trip characteristics.
   • A table specifying the precise model, rating, and required settings for every adjustable OCPD.
2. Drawings Note: The electrical drawings, typically on the single-line diagram or a general notes sheet, must include a statement confirming compliance. For example:
3. "THE ESSENTIAL ELECTRICAL SYSTEM SHALL BE SELECTIVELY COORDINATED FOR THE FULL RANGE OF OVERCURRENTS PER NEC 517.31(G) AND NFPA 99. A SELECTIVE COORDINATION STUDY HAS BEEN PERFORMED AND IS AVAILABLE FOR REVIEW. ALL ADJUSTABLE OVERCURRENT PROTECTIVE DEVICE SETTINGS SHALL BE AS SPECIFIED IN THE STUDY."
4. Availability: Per NEC 2023 §700.32(C), this documentation must be made available to the AHJ upon request. While you may not need to submit the entire binder with the initial permit set, the plan reviewer needs to know it exists and is complete.

I am designing the electrical service for a building with a high short-circuit current availability. What are the NEC 110.9 and 110.10 requirements for ensuring all equipment has an adequate short-circuit current rating (SCCR), and how must this be documented on the drawings for plan review?

NEC Articles 110.9 and 110.10 are the foundational safety requirements for protecting electrical equipment under fault conditions. NEC §110.9 requires devices that interrupt current (breakers, fuses) to have an interrupting rating (AIC) sufficient for the available fault current. NEC §110.10 requires all other components and assemblies to have an SCCR sufficient to withstand the fault current without extensive damage.

This means every piece of equipment in the path of a potential short circuit—from the service switchboard to panelboards, motor controllers, and busways—must be rated to handle the immense energy released during a fault.

Deeper Explanation and Nuances:

The compliance process involves three key steps: calculating the fault current, selecting properly rated equipment, and documenting it correctly.

• NEC §110.9 - Interrupting Rating: This section focuses on the OCPDs themselves. Their primary job is to open the circuit safely.
  • The rating is often called Ampere Interrupting Capacity (AIC).
  • The specified AIC must be equal to or greater than the calculated available fault current. For example, if the fault current at a panelboard is 18,000A, the main breaker must have an AIC of 22,000A or higher, not 10,000A.
• NEC §110.10 - Circuit Impedance, SCCR, and Protection: This is the broader "withstand" requirement. It ensures that the OCPD protecting a circuit will limit the let-through energy to a level that downstream equipment can handle.
  • For example, a panelboard has an SCCR. If the available fault current is 35,000A, the panelboard must have an SCCR of at least 35,000A.
  • This applies to all assemblies, including motor control centers (MCCs), variable frequency drives (VFDs), and industrial control panels (whose SCCR is often determined by UL 508A, Supplement SB).
• NEC §110.24 - Available Fault Current: This is the documentation and labeling mandate for services (excluding one- and two-family dwellings).
  • The maximum available fault current must be calculated.
  • This value, along with the date of the calculation, must be documented and made available to the AHJ.
  • A permanent label must be field-installed on the service equipment stating the maximum available fault current. If system modifications occur, the value must be recalculated and the label updated.

How to Document for Plan Review:

Clear documentation is the only way for a plan reviewer to verify compliance.

1. Single-Line Diagram: This is the most important document. It must clearly show:
   • The calculated available fault current (AFC) at the service entrance and at each major piece of distribution equipment.
   • The Interrupting Rating (AIC) of all fuses and circuit breakers.
   • The Short-Circuit Current Rating (SCCR) of all switchboards, panelboards, MCCs, and other assemblies.
2. Equipment Schedules: Panelboard, switchboard, and motor schedules should have dedicated columns for "AIC" or "SCCR" to list the required rating for each piece of equipment.
3. General Note: Include a note on the drawings referencing the labeling requirement. For example:
4. "THE CONTRACTOR SHALL FIELD-INSTALL A PERMANENT LABEL ON THE SERVICE EQUIPMENT STATING THE MAXIMUM AVAILABLE FAULT CURRENT OF [XX,XXX] AMPS AND THE DATE OF CALCULATION, IN ACCORDANCE WITH NEC 110.24."

By providing this information upfront, you demonstrate a thorough design and prevent delays during plan review and inspection.

Additional Supporting Sections

Common Mistakes and Misinterpretations

• Confusing SCCR and AIC: Remember, AIC is for devices that interrupt a fault (breakers/fuses), while SCCR is for equipment that must withstand it (panels/switchboards). They are not interchangeable.
• Defaulting to Minimum Ratings: Assuming a standard 10,000A AIC/SCCR for all panels is a common error. With modern utility systems, available fault currents often exceed this, especially in commercial buildings near a substation.
• Ignoring Control Panels: Forgetting that industrial control panels (e.g., for HVAC or pump systems) also have an SCCR is a frequent oversight. Per UL 508A, the panel's overall SCCR is typically limited by the lowest-rated component inside.
• Failing to Update Calculations: If the utility company upgrades their transformer or the building service is modified, the available fault current will change. The NEC 110.24 calculation and label must be updated.

Jurisdictional Variations

While the NEC provides the model language, state and local amendments can add requirements.

• California: The California Electrical Code (CEC) adopts the NEC with few amendments in this area, but local jurisdictions like the City of Los Angeles may have specific submission requirements for fault current calculations.
• New York City: The NYC Electrical Code often has specific administrative requirements for documenting and submitting electrical plans, including fault current and coordination data.
• Florida: The Florida Building Code adopts the NEC, but healthcare projects are also strictly regulated by the Agency for Health Care Administration (AHCA), which has its own rigorous plan review process for EES coordination.Always verify requirements with the local Authority Having Jurisdiction (AHJ).

Coordination Considerations (Design → Construction)

Properly executing these requirements requires coordination across the project lifecycle:

1. Design: The Engineer of Record (EOR) performs the calculations and specifies the required ratings and settings on the drawings.
2. Bidding: The contractor must use the specified ratings when bidding the project. Proposing equipment with a lower SCCR as a "value engineering" option is unacceptable unless a new calculation proves it is safe.
3. Submittals: The EOR must meticulously review equipment submittals to ensure the products the contractor intends to purchase (e.g., Square D, Eaton, Siemens) meet the specified AIC/SCCR ratings and have the necessary adjustable trip functions for coordination.
4. Installation & Commissioning: The installing contractor must set any adjustable breakers according to the coordination study. A commissioning agent or inspector may verify these settings in the field before the system is energized.

Frequently Asked Questions (FAQ)

What is the difference between SCCR and AIC?

‍Ampere Interrupting Capacity (AIC) is an interrupting rating that applies to devices like circuit breakers and fuses that are designed to open and clear a fault. Short-Circuit Current Rating (SCCR) is a withstand rating that applies to components and assemblies like panelboards and motor starters, indicating their ability to remain intact without exploding when subjected to a fault.

Does the NEC 110.24 requirement to label fault current apply to houses?

‍No. NEC 110.24(A) specifically exempts service equipment for one- and two-family dwellings.

Can I use a series-rated system to meet SCCR requirements?

‍Yes. Per NEC 240.86, a series-rated combination, where an upstream OCPD helps a downstream device with a lower AIC achieve a higher system rating, is permitted. However, the specific combination must be tested and listed by the manufacturer.

Is selective coordination required for the normal power system in a hospital?

‍Generally, no. The NEC 517 mandate is specific to the Essential Electrical System (EES). However, other NEC articles may require coordination for specific normal power loads, such as for multiple elevators on the same feeder (NEC 620.62) or for legally required standby systems (NEC 701.32).

Who is responsible for calculating the available fault current?

‍The design engineer is responsible for performing the calculation. The process starts by obtaining the available fault current data from the electric utility company at the secondary of their transformer.

What happens if the utility company changes its transformer after the building is built?

‍Per NEC 110.24(B), if a modification to the electrical system occurs that affects the available fault current, the calculation must be re-done, and the label on the service equipment must be updated.

How is the SCCR of a custom control panel determined?

‍The overall SCCR of a listed industrial control panel is determined according to the methods in UL 508A, Supplement SB. It is typically limited by the SCCR of the lowest-rated component in the power circuit (e.g., a power distribution block, drive, or breaker) unless specific current-limiting devices are used.

Where on the construction drawings should I show SCCR and fault current values?

‍The best practice is to show the calculated available fault current and the required SCCR/AIC for all major equipment directly on the main electrical single-line diagram. These ratings should also be included in the corresponding equipment schedules.

‍