Short circuit current calculations and labeling

For the person whose tasks is the specification, coordination or maintenance of an electrical system, the available short circuit fault current is a major consideration in the selection of circuit protective devices. Knowing the available fault current and applying available fault current labels on new equipment is also now mandated by National Electrical Code (NEC). The NEC 110.24(A) states:

" Field Marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved."

Any time there is a change in equipment the fault current calculation must be redone. This is specified in NEC 110.24(B):

" When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) shall be adjusted to reflect the new level of maximum available fault current."

110.16 has been evolving and the latest iteration has very specific requirements for not only the available short circuit current but also the clearing time of the upstream device based on the short circuit current.

" 110.16 (B) Service Equipment. In other than dwelling units, in addition to the requirements in (A), a permanent label shall be field or factory applied to service equipment rated 1200 amps or more. The label shall meet the requirements of 110.21(B) and contain the following information:
(1) Nominal system voltage
(2) Available fault current at the service overcurrent protective devices
(3) The clearing time of service overcurrent protective devices based on the available fault current at the service equipment
(4) The date the label was applied "

Also, NEC's 430.99 Available Fault Current reads:

" The available short circuit current at the motor control center and the date the short circuit current calculation was performed shall be documented and made available to those authorized to inspect the installation."

A study must be done to identify the maximum amount of the available short circuit fault current. The Short-Circuit-Analytic (SCA) software performs available fault currents calculations in three-phase electric power systems you are working with. The program takes into consideration electrical parameters of the power supply as well as the power distribution system including utility, cables, bus ducts, transformers, generators, motors etc.

SCA V1.0 Dashboard

Once the fault current has been calculated, labels made giving the available short circuit fault current, should be applied to the equipment. That information, and the date it was determined, must be put on a label that is field applied to the equipment. The purpose of this requirement is to allow the short circuit current rating (SCCR) of the equipment to be easily compared with the maximum available fault current.

Available Short Circuit Fault Current Label

We also believe it is critical to include system X/R rating at point of fault along with the available short circuit fault current data. The belief is based on the fact that all low-voltage protective devices are tested at predetermined X/R ratios. If the calculated X/R ratio at any given point in the electrical distribution system exceeds the tested X/R ratio of the overcurrent protective device, then you must de-rate the effective rating of the gear. This can be a very critical issue if your fault current calculation doesn't include the X/R ratio. A fault condition with a high reactive component can potentially require a de-rating of the gear to below the symmetric fault current value calculated at the gear. The SCA short circuit calculations software factors in both resistance and reactance components for every part composing the power system and provides an extremely accurate analysis of the maximum available short circuit fault current and X/R rating any part can be subject to.

SCA V1.0 Calculation Results Screen

If you lack the time or expensive equipment to print the required labels internally, let us do it for you. Our vinyl labels are 100% waterproof and are suitable for use on equipment that will be subjected to the heavy wear and tear ordinarily inflicted by corrosive gases, liquids, dust, sunlight or other environmental conditions. Check Arc Flash Label Printing Services for more information. ARCAD's software resources and label printing services make NEC, NFPA 70, OSHA and CSA Z462 compliance simpler and very affordable.