Interrupting Ratings and Short Circuit Current Ratings[1]

The 2005 National Electrical Code (NEC) [Article 409.110]addressed the need to accurately identify and label an industrial control panel’s Short Circuit Current Rating (SCCR). This requirement is necessary to ensure the integrity of the panel and the safety of personnel during a short circuit situation.

The NEC also requires an SCCR for:

• Industrial machinery electrical panels [Article 670]
• Multiple motor HVAC equipment [Article 440]
• Meter disconnect switches [Article 230]
• Multiple motor controllers [Article 430]

Article 409 was added due to the dangerous and common misconception by equipment manufacturers that the circuit breaker’s interrupting capacity or the fuse’s interrupting rating in the panel’s feeder circuit is also the panel’s SCCR. In reality, the panel’s SCCR is directly related to the current-limiting capabilities (opening time) of the protective device in the feeder circuit and the individual panel components’ SCCR. In most cases, a panel’s SCCR is less than the interrupting capacity or interrupting rating of the over-current device.

Installation Considerations

Also, keep in mind that an existing panel being moved to anew location within a facility still requires verification that its SCCR is greater than the new available fault current. As the panel is moved closer to the electrical service, the available fault current can increase, and the panel needs to withstand this new level. Even if the panel isn’t moved and the available fault current increases from the utility company, this verification needs to be done to ensure a safe environment.

How is a panel’s SCCR determined?

NEC Article 409.110(3) states that the SCCR shall be based on a listed (i.e. UL tested) assembly or an approved method of calculation.

Since this NEC rule in 2005, Cerus has been working with UL to verify all of our drives, starters and control panels carry a UL approved SCCR rating. For example, Cerus Drives are rated for up to 100,000 ampere interrupting capacity when installed with a UL approved breaker. A Cerus BAS combination starter exclusively uses Cerus UL tested and approved components that provide a UL rated SCCR of up to 100,000 amperes.

For more information, or concerns about interrupting ratings for your particular installation, please contact our knowledgeable Cerus sales representative who will be happy to answer your questions.