Cooper Bussmann Electronics Short-Circuit Current Rating What Is a Short-Circuit Current Rating

Are You Ready For The New SCCR Marking Requirements?

What Is A Short-Circuit Current Rating

(SCCR)?

SCCRs on components and equipment represent the maximum level of short-circuit current that the component or equipment can withstand and is used for determining compliance with NEC® 110.10. This rating can be marked on individual components or assemblies. The new SCCRs represent the maximum amount of fault current that the assembly can withstand under fault conditions. Assembly ratings take into account all components contained within the equipment rather than just the main overcurrent protective device.

CAUTION: Short-circuit current ratings (SCCRs) are different than interrupting ratings marked on overcurrent protective devices.

A common mistake is to assume that the interrupting rating of the overcurrent protective device protecting the circuit represents the SCCR for the entire circuit. Interrupting ratings, used for compliance with NEC® 110.9, apply solely to the overcurrent protective device. It is the characteristics of the overcurrent protective device (e.g. opening time, let-through energy) that need to be used in determining compliance with NEC® 110.10, not the interrupting rating.

Who Is Affected By The SCCR Markings

The 2005 NEC® has requirements for anyone building equipment listed to the 2005 NEC® or to UL508A and requires the following to be marked with an SCCR:

•Industrial Control Panels [409.110]

•Industrial Machinery Electrical Panels [670.3(A)]

•Certain HVAC Equipment [440.4(B)]

•Meter Disconnect Switches [230.82(3)]

•Certain Motor Controllers [430.8]

Effective April, 2006, all equipment listed to UL508A will be required to be marked with an assembly SCCR. Inspectors and installers need this information in order to ensure compliance with NEC® 110.10. Equipment installed where fault current levels exceed their short- circuit current limit can be hazardous to persons and property. SCCRs marked on components and equipment make it easier to verify proper protection for components and equipment for specific applications — whether it be the initial installation or relocation of equipment.

How Is SCCR Determined?

For meter disconnect switches and motor controllers, this withstand level, or SCCR, is often determined by product testing. For assemblies, the marking can be determined through product listing or by an approved method. With the release of the UL508A Industrial Control Panel standard, an industry method is now available. Any method used, whether UL508A or another approved method, should be based on the “weakest link” approach. In other words, the assembly should be limited to installation where fault levels do not exceed the withstand rating of devices with the lowest SCCR. The marking determined should represent the limits of the assembly for a safe installation. Current-limiting overcurrent protective devices can be used in the feeder or branch circuits to increase the assembly SCCR where lower rated components are used. When current-limiting devices (that limit fault current levels to within the lower rated components' ratings) are used, they provide protection for the assembly — provided the current-limiting Overcurrent Protective Device (OCPD) is used within its ratings.

Who Is Affected	How Are They Affected
Facility/Process Engineers/	Need to specify end use equipment with ratings adequate for available
Consulting Engineers	short-circuit current. Need adequate ratings so equipment has flexibility to be
	moved around.
Control Panel Builders/	Need to determine and mark the assembly short-circuit current rating on the
Machine Builders/	equipment they build. Need to market equipment with ratings as least as high
HVAC Manufacturers	as their competition.
Electrical Contractors	Need assurance that the equipment they install is adequate for the available
	short-circuit current at the point of installation, to avoid red tags, and lost time
	and labor.
Electrical Inspectors	Need to assure that the available short-circuit current where equipment is
	being installed does not exceed the rating marked on the equipment.