SCCR code change Webcast: Your questions answered

Thomas Domitrovich and Derek Strittmatter from Eaton answer unanswered questions from the Sept. 21, 2016, webcast on how SCCR code changes will impact procurement, installation, and safety.
By Thomas Domitrovich, Derek Strittmatter October 4, 2016

Thomas Domitrovich (left), P.E., Director of Technical Sales, Eaton; Derek Strittmatter, Product Manager, IEC Control Components, Eaton. Courtesy: EatonThomas Domitrovich and Derek Strittmatter from Eaton answer unanswered questions from the Sept. 21, 2016, webcast on how SCCR code changes will impact procurement, installation, and safety.

Question: Who is the governing body/enforcement agency for verifying compliance? 

Answer: The Authority Having Jurisdiction (AHJ) is responsible to verify correct application and compliance with the NEC. According to NEC article 100 definition for AHJ, in the absence of an inspector, the AHJ responsibility belongs to the property owner or their designated agent (likely the purchaser of the equipment). It is critical that the available short-circuit current be determined and communicated prior to equipment design/installation to avoid improper equipment application and potential safety hazards.  

Q: What can be done, in lieu of equipment replacement, to increase SCCR of an industrial control panel that has an SCCR less than the calculated fault current?

A: When equipment is found to have insufficient SCCR, either the available short-circuit current must be lowered and/or the equipment SCCR must be raised. In some cases an isolation transformer or additional conductor length may lower the available short-circuit current to a level sufficient for the existing equipment SCCR. Raising the SCCR of existing equipment may be accomplished, but a qualified resource will need to evaluate the panel and determine how the panel or circuit can be modified to raise the SCCR. Any modifications may also require recertification of the panel.

Q: Are programmable logic controller (PLC) panels, operating at 120 Vac or less, consisting of PLC hardware, I/O modules, 120 Vac to 24 Vdc power supplies, Ethernet switches, etc considered "industrial control panels?"

A: This meets the definition of an industrial control panel, but an exception exsists for the SCCR marking if the panel contains only control items. See NEC 409.110 (4) Exception.

Q: Will the end-user be the responsible party with respect to providing the local AHJ with the available short circuit calculation/documentation? Will a PE be required to sign off, similar to series ratings?

A: The party responsible to provide the available short-circuit current documentation/marking is not restricted or defined to a particular part in the 2017 NEC, only that the information must be provided. The party that provides this may differ depending on the type of equipment or those involved.

Q: Does NEC say if the calculation has to use available fault current from the utility or can a worst-case calculation using infinite bus be used?

A: If unavailable, an infinite supply can be used to determine the available short-circuit current on the secondary. The actual available short-circuit current provided from the utility may not be made available, and is subject to change.

Q: Does we need to provide the AFC data to vendor during the time of ordering the equipment?

A: Correct. In order for the equipment/machinery to have sufficient SCCR at the time of design/manufacturing, it is necessary to understand and communicate the available short-circuit current to the manufacturer prior. It can be very challenging to address inadequate SCCR once the equipment is on the job site.

Q: Is there a difference between the phrases "available fault current’ and "available short circuit current?"

A: The 2017 NEC replaced "available fault current" with "available short-circuit current" in several locations. There was nothing written to indicate a change in the meaning/intention/application of this term as a reason for the respective code changes.

Q: How are existing panels handled under the revised code in cases where equipment is relocated from one location to another?

A: Equipment that is relocated to another location should be installed in compliance with current governing codes and standards.

Q: How do we find the available SCC?

A: There are several methods to attain the available short-circuit current:

  • Review existing facility electrical schematics to determine if fault current was calculate and documented. Verify that no changes have occurred since the calculation that may impact the calculated value.
  • Use a fault current calculator app.
  • Consult a qualified resource capable of calculating the available short-circuit current.

Q: What do we do if the equipment does not state the available SCC?

A: Given the hazards associated with inadequate SCCR, it is important that the equipment SCCR be sufficient. By NEC code, the available short-circuit current must be known, as it must be marked/documented. However, if this information is not available or attainable at the time of equipment design/manufacturing, consider a higher SCCR design sufficient for a worst-case condition. It is much easier and cost effective to plan ahead than to attempt to address inadequate SCCR on the job site.

Q: What if we build our own cabinets?

A: As per 409.110 (4), use an approved method for determining the SCCR value for control panels. UL 508A is listed as an example of an approved method. Also consider leveraging SCCR tools when selecting and applying components.

Q: Must the control panel be label with the SCCR of the lowest rated device in the panel? For example, a relay in the panel is rated at 10K, but the available fault current is 22K.

A: If using UL 508A to determine the panel SCCR, then the overall panel SCCR is determined based on the weakest SCCR or interrupting rating. However, UL 508A has guidelines for raising the SCCR under certain conditions. Be sure to following applicable standards when evaluating SCCR.

Q: What safety factor is typically being applied to SCCR ratings?

A: There is not a recognized or established safety factor for SCCR. However, it is a good practice to choose an SCCR level that will allow for future changes such as equipment relocation or changes to the electrical distribution system that may impact the available short-circuit current. Some equipment manufacturers standardize on common component SCCR values, such as 35, 65, or 100kA.

Q: Can you clarify AIC and SCCR?

A: Amps interrupting capacity (AIC) is the amount of short-circuit current an overcurrent protective device is rated to interrupt in the event of a short-circuit. This rating applies to breakers and fuses.

Assembly SCCR indicates the amount of short-circuit current an assembly can safely handle during a short-circuit event.

Component SCCR indicates the amount of short-circuit current a component can safely handle when paired with the proper overcurrent protective device.

This rating applies to components that are not designed to interrupt short-circuit current, such as motor starters, distributions blocks, switches, and others.

The key observation is that both overcurrent protective devices and other devices not designed to interrupt a short-circuit must be properly applied according to their ratings, and must be sufficient for the available short-circuit current at the installation point.