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UL 508 and NEC Code Updates for SCCR

Recent updates to the UL 508 and NEC codes have put a greater emphasis on making sure that electrical panels can maintain their integrity under major fault conditions. The safety of the system is determined by the short-circuit current rating (SCCR) of the panel, in conjunction with the available fault current where the panel is located. Local inspectors must now make sure the SCCR meets or exceeds the available fault current before they can approve a panel to be used. This makes it more important than ever to make sure your panel is designed properly, so that it meets the SCCR requirements, while not being over-engineered.

The short circuit current rating of a panel is the amount of energy, usually expressed as a value in kilo-Amperes (kA), that the panel can handle without causing fire, a shock hazard, or explosive danger. Misapplied panels can not only be a safety hazard in the facility, but could cause additional downtime as well.

The available fault current where the panel is located can be affected by many influences in the facility. Some influences include, the size of the upstream transformer, the size of the electrical conductors feeding the panel, and the length of the electrical conductors feeding the panel. Unless the end user has already completed an arc-flash analysis of their facility, it is unlikely that they will know the available fault current where the panel will be located. Most facilities have locations with available fault currents as high as 20 to 50 kA or higher, potentially causing an issue where a panel only meets default SCCR values of 5 or 10 kA. On the flip side, many panel builders will build every panel to meet 65 or 100 kA even though the available fault current where it is installed is much lower. In this case a competitive product, closely designed to the available fault current could come in at a lower price, potentially causing lost business, or lower margins

To give yourself the greatest competitive advantage, while maintaining your margins, it is important to have your customer provide the available fault current and design your panel accordingly.

The SCCR of a panel can be calculated in accordance to UL standard 508A, supplement SB. The National Electric Code also references UL 508A, supplement SB for SCCR calculation, so it is important even if the panel is not being designed to, or marked, with a UL label.

Determining the SCCR of a panel is accomplished in three steps:

  • First, establish the short circuit current ratings of individual power circuit components.
  • Second, modify (limit) the available short-circuit current within a portion of a circuit in the panel due to the presence of current limiting components.
  • Third, determine the overall panel short-circuit current rating.

Determining the SCCR of the individual power circuit components is accomplished in three ways. The first, and easiest, way is to evaluate the SCCR rating marked on each component. If an SCCR rating is not marked on the component, then the second way is to use a table provided by UL which provides default values for the components. The table ratings are generally low though, 10 kA or less. The third way is to use components that have been tested together to achieve higher SCCR ratings. Examples of the third way are two and three component starters which have had the components tested together, or power distribution blocks and terminal blocks tested with fusing or circuit breakers to achieve greater than 10 kA ratings. Rockwell Automation has made the process of finding their tested ratings very easy, just follow the weblink.

Though not part of the SCCR process, another aspect of component ratings to consider is the type of coordination. The different types refer to the amount of acceptable damage to the components that is allowed in the case of a short circuit event. Lower types can be less expensive upfront, but cause significant damage to the components resulting in increased downtime before the panel can be repaired and put back into service. Higher types, or “Self-protected” ratings, are often more expensive up front, but are generally damage free in the case of a short circuit, so that the panel can be put back into service as soon as the short is repaired. Understanding the level of risk your customer is willing to accept is another way to design a panel that specifically meets your customers’ demands while keeping a competitive advantage.

The second step of determining the SCCR rating of a panel is to use current-limiting devices, fuses or circuit breakers marked current-limiting, in the feeder circuit to reduce the available fault current non-interrupting devices in the branch circuit. The branch circuit is considered all devices from the load to the first protective device upstream from the load. Branch circuit protectors are not able to take advantage of the current limiting, so they must match the feeder protector SCCR to allow the branch circuit devices downstream to take advantage of the current limiting. In a current limiting panel, the branch components must only meet or exceed the peak let-through current of the feeder protection device.

Fused transformers are also a form of current limiting device. As an example, a 10 kVA transformer limits the available energy on the load side to less than 5 kA. Which is a value most devices can meet, even if using unmarked components and the UL table for the rating. This is especially important for control circuits, as most of the time it is those components that are limited to 5-10 kA.

After the individual component SCCR is determined, tested coordination ratings have been verified, and current limiting circuits have been evaluated, the complete panel can be assigned to the equivalent of the lowest rated device.

Building to a specific SCCR rating can be confusing at first, but Revere Electric has the resources to train and assist panel builders in ways to evaluate their panels properly and design to the most efficient rating for their customers.

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