The NEC’s short-circuit current rating changes

With changes to the National Electrical Code (NEC) coming in 2017, equipment manufacturers and system owners will be required to provide additional short-circuit current rating (SCCR) labeling on new and existing control panels. Variable frequency drives (VFDs) and other pump/ motor control components will need to be reviewed to determine if the panels are in conformance. This article explores some of the nuances of the changes and how to be ready.

First defined by the NEC in 2008, SCCR is the maximum short-circuit current a component or assembly can safely withstand when protected by overcurrent protective devices. Also known as "withstand ratings," they define the current levels at which the system can mitigate fire and shock hazards from the short-circuit event.

SCCR is not the same as "interrupting rating" and the two must not be confused. For example, a circuit breaker might have an interrupting rating of 100 amperes (A) and a short-circuit rating of 65,000 A. The interrupting rating is the load current at which a device trips or opens. The short-circuit rating is for a more severe failure during which the available energy from the feeding transformer surges through a load because of sustained short in the circuit. These component SCCRs are established and certified through actual short-circuit testing to validate the SCCR and the conditions by which the rating is achieved.

The ability of these devices to handle short-circuit current levels depends greatly on the overcurrent protective device attributes and sometimes external factors (wire size, length, enclosure volume, etc.). Requirements are specified in the component SCCR and must be applied properly to obtain the proper level as part of SCCR protection.

When analyzing assemblies for SCCR, the interrupting rating of overcurrent protective devices and the SCCR of all other components affect the overall equipment SCCR. In some cases, the use of flexible circuit protection options can be helpful in achieving higher ratings or balancing SCCR in drives and motor control applications.

SCCR — Assessing risk & assuring safety

In essence, SCCR is still all about protection. Without an adequate assembly SCCR, it is likely that the devices inside the panel will sustain, and even create, damage within the panel. Damage may extend outside the control panel. Additionally, panels with an insufficient assembly SCCR may result in commissioning delays or shut down until the nonconformance is resolved.

An insufficient assembly SCCR poses the following hazards: Electric shock and burns; burns related to arc flash and contact with heated surfaces; injury associated with flying debris; damage to equipment or the facility; arc blast (shock waves, shrapnel, etc.); and vaporized metal.

In the worst-cast scenario, fault currents resulting from inadequate SCCRs can cause dangerous arc flashes and fires that put personnel at significant risk of injury or even death.

SCCR rating as governed by UL testing

SCCRs apply to almost all industrial control panels used for operating machinery and equipment. But while protection from fault current events is often properly specified and applied in electrical switchgear and distribution equipment (such as panel boards and switchboards), it is often misunderstood or misapplied when it comes to machinery and industrial control panels.

Recent survey data indicates that it is not uncommon for equipment to be installed with insufficient assembly SCCR. This may be because of several factors:

Unknown available fault current at each circuit location where equipment is located
Lack of assembly SCCR requirements on new equipment specifications
Difficulty locating SCCRs of components used in industrial control panels
Misinterpretation/misapplication of the Underwriters Laboratories (UL) 508A standard for control panels
Changes in the electrical system that can raise available fault current, thus potentially affecting the adequacy of the existing equipment’s assembly SCCR

Without the available fault current and SCCR, it is impossible to determine if components or equipment can be safely installed.

The Occupational Safety and Health Administration and NEC require sufficient short-circuit current protection of industrial control panels to protect equipment and personnel from certain risks in the event of a short-circuit (fault) event. In some cases, SCCR may also facilitate lower insurance rates through compliance with codes such as the National Fire Protection Association’s NFPA 70E code and the UL 508A Supplement SB.

The 2017 NEC SCCR code changes

The 2017 NEC includes several sections with new short-circuit protection requirements. The most significant changes require that the available short-circuit current at the location where the equipment is installed is marked/documented and dated. These changes allow installers, inspectors and approvers to verify that the installed equipment SCCR is equal to or greater than the available short-circuit current, and that it is compliant with the other code sections dealing with equipment installations.

Generally the 2017 NEC will make it easier for system owners, maintenance teams and inspectors to verify proper equipment SCCR protection. Of special note are two revised sections of the code pertinent to the marking of SCCR. A brief summary of these critical changes includes:

Labeling requirements for panels

The NEC requires certain components and equipment (including motor control panels) to be labeled with the available fault current and the panel SCCR.

430.130(A) — Circuits Containing Power Conversion Equipment and VFDs

These standards pertain to open or enclosed power electronic conversion equipment that supply power to control a motor(s), operating at a frequency or voltage different than that of the input supply.

Most adjustable speed drives are listed with specific overcurrent protective devices (OCPD) types. For safety and compliance to 110.3(B), the adjustable speed drive manufacturer’s markings and instructions concerning the specific OCPDs and corresponding SCCRs must be followed.
Where the manufacturer’s instructions for their listed power electronic conversion equipment permits semiconductor fuses (high-speed fuses) or instantaneous trip circuit breakers (motor circuit protectors or MCPs) for use as the branch circuit shortcircuit and ground fault protective device, they must be integrated in the same assembly.
In addition, when semiconductor fuses or MCPs are used, UL 508C and UL 61800-5-1 require the drives to be marked with the specific fuse manufacturer and fuse model number or MCP manufacturer and MCP model number. The marking must state these protective devices should be integrated in the same assembly with the adjustable speed drive controller.
These precautions are necessary since MCPs and most semiconductor fuses are not branch circuit overcurrent protective devices as defined in Article 100, but application-limited devices.

Revisions to 440.4(B) — Multimotor and Combination-Load Equipment

This code change concerns multimotor and combination-load hermetic refrigeration equipment rated 60 A or less.

Previous NEC editions exempted this equipment’s nameplates from being marked with their SCCRs. This exception has been removed. Now this type of equipment must have its short-circuit current rating marked on its nameplate, the same as all other heating, ventilation and air conditioning equipment in 440.4(B).

A specific VFD model number may have multiple SCCRs conditional on the OCPD type and corresponding SCCR level achieved in the product standard testing evaluation, e.g., a drive may have two SCCRs: 5 kiloAmperes (kA) with MCPs, and 100 kA when protected by a Class J or T fuse. This information will be marked on the drive and/or noted in the installation instructions.

To easily update systems and have the maximum flexibility, components with the highest SCCR ratings and most options are best. Look for manufacturers that provide VFDs with SCCRs for fuses, molded case circuit breakers, miniature circuit breakers, MCPs and/or manual motor protectors to have the most flexibility.

Determining the SCCR of the circuit

While the new code requirements will make it easier for system owners, maintenance teams and inspectors to verify proper equipment SCCR protection, the chance increases that many equipment installations will not be code-compliant or approved.

Because of the complexity involved in achieving higher SCCR levels for control panels, manufacturers often provide standard designs with the minimum default rating of 5 kA. Research indicates more than half of original equipment manufacturers design to this standard. But 5 kA SCCR is not always sufficient. The available short-circuit current must be determined prior to installation, and the equipment SCCR must not be less than the available short-circuit current. Without clear guidance, equipment SCCR may be misapplied during design and installation. Once installed, it is often difficult and costly to raise the equipment’s SCCR.

Conclusion

Developing an equipment SCCR plan or implementing an SCCR solution can be easier with fault current calculation tools and support of a qualified third party. Consult qualified resources and adhere to prevailing agency, code and regulatory requirements when evaluating and executing an SCCR plan.

Charles Tahara is the global product manager for industrial drives at Eaton. For more information and resources on SCCR, including Eaton’s free Fault Current Calculator (FC2), visit eaton.com/sccr.