Nothing impacts a project’s bottom line like late-stage setbacks and rework. This case study delves into the repercussions of neglecting to make an accurate analysis of available fault current in electrical power systems and emphasizes the need for proactive measures to ensure compliance and enhance electrical safety.

A midsized electrical contractor near the final stages of a project performed punch-list items for wiring a 45,000-square-foot office building and an adjoining 15,000-square-foot manufacturing facility. At the preliminary final electrical inspection, the inspector raised concerns about the rooftop heating and air conditioning units (RTUs).

Hitting the books

The 2023 National Electrical Code Section 440.10(B) requires this type of equipment to be marked with short-circuit current rating (SCCR), as well as the available fault current present at the equipment and the date the available fault current was calculated. The RTUs were marked with SCCR (5 kA), but not with the calculated available fault current at the equipment. Without this value, there is no way for the AHJ to determine if the 5-kA short-circuit current rating is adequate for the connected power supply.

The inspector brought these concerns to the foreman, who was not familiar with these Code requirements. The state this project was in recently adopted a newer edition of the NEC; the previous edition did not include these RTU marking requirements. 

In discussions with the project manager and designing engineer, the foreman determined a power system study was performed that included the necessary available fault current information. A close review of the power systems study and the SCCR of the HVAC equipment revealed the available fault current at each RTU exceeded the equipment’s 5-kA rating, in some cases by as much as 4,000A. Such an issue could certainly jeopardize the certificate of occupancy.

A joint effort

The electrical contractor first sought assistance from the mechanical contractor that ordered and installed the RTUs. The mechanical contractor specified units with the 5-kA SCCR since they were not privy to the information on the power system study, and these units were more cost-effective than models with higher SCCRs. Replacing the current RTUs with higher-SCCR units would be time-consuming and expensive.

Next, the EC reached out to a trusted industry resource, an overcurrent protection device manufacturer. The manufacturer’s solution involved the installation of current-­limiting, low-peak fuses within a fused disconnect ahead of each RTU. This innovative approach was backed by the product listing and manufacturer testing data, offering a solution specific to this installation.

The contractor presented the solution to the AHJ with manufacturer-supported documentation, received an OK and the changes were made. After completion, the AHJ returned to verify the modifications and documentation (including the updated power system study). Satisfied, the AHJ gave final approval, enabling the issuance of the certificate of occupancy.

While not an overall failure, this project provided valuable learning opportunities.

Power system study: The power system study is often far down on the list of project priorities. This issue could be avoided by conducting a thorough power system study in the early stages of a project. This study arms designers with invaluable information that can be used to select and purchase equipment with a proper rating.

Communication: Armed with the available fault current information from the power system study, collaborate with the mechanical contractor to select HVAC equipment. This will aid in compliance with Section 440.10 by ensuring the HVAC equipment has appropriate SCCRs. This exact issue is so common that manufacturers design products, software and training programs on it.

Training: At the crux of this compliance issue is training. All staff involved in power system wiring should—at the very least—be trained and knowledgeable in:

• The importance of SCCRs and available fault current in the safe operation of an electrical system.
• Power system studies and how to find the available fault current information therein.
• NEC requirements for available fault current and SCCRs of equipment and labeling. The requirements for equipment ratings and labeling have drastically expanded over the last few Code revisions.
• Industry resources available such as manufacturers, industry partners and trade associations with staff dedicated to helping contractors in need of solutions.

Documentation: The power system study should be made available to the project foreman and AHJ for review as needed.

This case underscores the importance of taking a proactive project management approach. Even simple measures go a long way in mitigating costly compliance issues and enhancing electrical safety. 

By adopting these steps, ECs can navigate the evolving landscape of compliance requirements, mitigating potential safety concerns and safeguarding both property and people.

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