Selecting the Short Circuit Current Rating of Equipment

By: JADE Learning | Jul 15, 2014

Image 1: One-Line Diagram with AFC
Image 1: References on the one-line diagram that show each fault point and the AFC at each location

In the Summer Newsletter article, we discussed marking our service equipment with the maximum Available Fault Current (AFC) as well as the date the fault current calculation was performed to comply with NEC 110.24(A). We selected circuit breakers with an AIC rating equal to or greater than the AFC. So what’s next?

Is the service equipment the only item required to be rated for the maximum AFC? There must be a reason that these AFC values need to be marked on the service equipment. In some cases, fault current calculations must also be performed downstream of the service for feeders and branch circuits. How do we use these AFC values at the service to select proper ratings for equipment on the load side of the service?

NEC Section 110.10 requires that Overcurrent Protective Device (OCPD) ratings and equipment Short Circuit Current Ratings (SCCR) are properly selected so that during a fault, the OCPD can clear the fault without extensive damage to the equipment. The only way we can make sure that OCPD’s and equipment SCCR’s are chosen properly for items located downstream of the service is to first know what the AFC is at each sub panel or equipment location. If the job happens to be a new construction project and the plans are available, then hopefully the plans will have references on the one-line diagram that show each fault point and the AFC at each location (See Image 1).

If the job does not have engineered drawings, it may be up to the electrical contractor to determine what the AFC is at a given location so that the electrical equipment being installed can be selected properly with the correct SCCR or AIC rating. See Image 4 for the Typical Short-Circuit Current Ratings for Unmarked Components.

Image 2: Cooper Bussmann FC2 Calculator
Image 2: Cooper Bussmann FC2 Calculator

Performing short-circuit calculations is an engineering level task. However, there are online calculators that can be downloaded. The best one is called FC2 by Cooper Bussmann. It is also available for mobile devices in the Apple App Store and Android App Store.

Several pieces of information are necessary in order to perform a fault current calculation.
• AFC at the service, 110.24(A)
• Length of feeder or branch circuit conductors supplying equipment in question
• Type of conductors (Copper or Aluminum)
• Conductors in a raceway or in a cable
• If the raceway being used is magnetic or non-magnetic

Image 3: Time Clock with SCCR
Image 3: Time Clock with SCCR

 

The short-circuit current rating (SCCR) of the equipment must always be equal to or greater than the available fault current. If the results of the FC2 calculation shows that the AFC at the line terminals of a new subpanel was 9,000 amps, then circuit breakers with a 10,000 AIC rating could be installed in the panel and comply with both 110.9 and 110.10. If your results showed that there were 6,596 amps of available fault current at the line terminals of a time clock attached to the new subpanel after 3 ft. of No. 12 AWG cu. conductor, then in order to comply with NEC 110.10, any time clock installed at that point in the electrical system would need to have a SCCR of at least 6,596 amps (See Image 3). If the time clock you have chosen is not marked with a SCCR, then it will have a default value of 5,000 amps and would not comply with 110.10 for this installation. Other components such as meter socket enclosures or GFCI-type receptacles have a default SCCR of 10,000 amps if they are not marked otherwise (See Image 4).

Image 4: SCCR Ratings for Unmarked Components
Image 4: SCCR Ratings for Unmarked Components

2 thoughts on “Selecting the Short Circuit Current Rating of Equipment

  1. Just got my county plans rejected : reviewer wants to know SCCR of equipment to tap into conductors from service meter. we will be using ILSCO taps to tap into conductors, looked for SCCR ratings of ILSCO taps but found nothing… am I looking for the right info?

    1. It is unlikely that they are concerned with the actual splicing devices but more concerned with the equipment that will be supplied by a given length of wire. If there is 35,000 amps of available fault current (AFC) at a panelboard for example, and the plan is to nipple out of the panel into a contactor box or control panel, then in order to comply with NEC 110.10, the SCCR of the contactor or control panel needs to be evident so that the plans examiner knows if the SCCR is sufficient for the AFC.

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