Short circuit current rating (SCCR) requirements can be a serious liability for your company, your customer, and your supplier relations. Just knowing that the Code requires proper SCCR will not be enough to prevent liability. In order to evaluate your risks, you must review the NEC rules on labeling and understand the basic premise behind them.
The 2005 NEC introduced new SCCR labeling requirements that apply to industrial control panels (Photo 1) and machinery, HVAC panels, and almost every control panel with power flowing through it at 600V or less. Code Secs. 409.110(3), 440.4(B), and 670.3(A)(4) reference the labeling requirements in which “SCCR” replaced a similar older term, “withstand rating.”
Equipment installation, restricted by 110.10, requires the labeled SCCR to be equal to or greater than the maximum available short circuit current at the equipment terminals. If this Code requirement is not followed, the equipment can be at significant risk for fires and arc flash hazards. Although SCCR is similar to the interrupting rating requirements in 110.9, it's an additional panel requirement.
The common procedure for calculating the assembled panel SCCR can be called the weakest link method. Individual panel components have an SCCR or interrupting rating. The component with the lowest SCCR in the panel limits the overall panel assembly rating. This SCCR rating has to be displayed on the nameplate and is usually calculated by the manufacturer or assembler of the control panel.
UL panel shops will automatically provide SCCR labeling per the new Code requirements. However, because of product costs, many will default to a 5kA SCCR, unless the engineer specifies differently. After installation, any corrections or modifications to the panel would require changes that could be quite costly.
This Code change demands a high level of coordination. Typical Code requirements can be implemented by changing contractor work methods or a contractor selecting a device with a new appropriate UL listing. Implementation of the SCCR change involves several entities — the consulting engineer, panel manufacturer, contractor, and facility engineer all must work together. If one fails in his or her duties, the electrical inspector should catch it, preventing an unsafe installation from being energized. However, the following scenario demonstrates why this is not always the case.
The consulting engineer made up plans for a new industrial facility. He wrote a requirement for the installation to meet the 2008 NEC into the bid documents. However, he didn't specify the SCCR requirements of the HVAC electrical panels. When the contractor submitted shop drawings without SCCRs specified, the engineer accepted them.
The large manufacturer of the HVAC panels shipped UL-listed panels to the job site without the SCCR included on the nameplates. The manufacturer said it was listed to UL Standard 1995 for HVAC equipment — this standard doesn't require the SCCR to be marked on the nameplate. With no stated SCCR requirements in the bid documents, the manufacturer felt strongly that it had met all of the standards. However, other parties disagreed.
The electrical contractor installed the rooftop units (Photo 2), initially referencing Sec. 90.7 and saying that if the equipment was listed and showed no signs of tampering, he should be able to use it. Then, the electrical inspector noticed the nameplate SCCR was missing. He delayed the equipment from being energized and also called the minimum required value into question, reminding the other parties that the SCCR had to be at least as great as the available short circuit current at the panel supply terminals per the requirements of 110.10 (Photo 3 on page C20).
The facility engineer discussed the inspector's concern with the consulting engineer. Together, they agreed that the available fault current could be up to 35,000A at the rooftop location, and determined the panels were only capable of a 5kA SCCR per UL 1995.
Because of the large panel manufacturer's unwillingness to take responsibility, they investigated modifying the panels already installed on the roof. The listing agency had a significant charge to make certain any field modifications would be performed correctly. The economics of this situation dictated that the panels be removed from their rooftop location. They were sent to a retrofit shop with the ability to modify the panels and provide the appropriate SCCR and labeling.
It was unclear if a single entity should be responsible for the extra labor and costs to fix this installation. The engineer required compliance to the Code in the bid documents. The original manufacturer built to UL 1995, and no SCCR values were in the bid documents. The contractor was just installing an engineered project.
The facility is also responsible for complying with OSHA regulations concerning safety for the life of the equipment. OSHA Code of Federal Regulations for General Industry Subpart S, 1910.303(b)(5) prohibits extensive damage caused by this improper overcurrent protection. If the facility had an injury resulting from inadequate SCCR, it would be an OSHA violation. Although the facility engineer approved the additional costs to move the project to completion, there could have been litigation and destruction of goodwill between parties.
How could this situation have been avoided? By specifying the required SCCR values before the project was bid. That makes the bidder clearly responsible for the correct SCCR values. In a new building, engineers or designers have to communicate the SCCR requirements, because only they would know the available fault currents. Without a consulting engineer involved, such as with a new control panel in an existing facility, the facility engineer would be responsible for knowing the available fault current.
If the contractor takes responsibility for control panel selection, he can specify the minimum SCCR to his supplier. If the contractor is assembling or modifying panels himself, then he has an obligation per the Code to accurately calculate the panel's SCCR and label it on its nameplate. The facility engineer has the final responsibility of ensuring safe electrical equipment. He can specify that all new panels have a sufficient SCCR on their nameplates. Control panel ratings of 35kA, 65kA, and 100kA are common in industrial facilities. If the facility engineer determines that a panel could be relocated later, he can specify a sufficient rating to avoid problems at any future location.
Walsh is a senior field engineer for Ferraz Shawmut, Newburyport, Mass. He is a member of IEEE, NFPA, and the International Association of Electrical Inspectors and can be reached at [email protected].