How to Calculate the Short Circuit Current Rating of a Control Panel
Failing to calculate the short circuit current rating (SCCR) of a control panel could lead to catastrophe involving damage and injury if the control panel is connected to an electrical system with an available fault current higher than the SCCR of the panel. For that reason, SCCR marking on the nameplate of a control panel is required by the National Electrical Code 409.110.
There are two ways to find the SCCR of a control panel. The first method is to test the entire panel by building multiple panels and testing them by repeatedly exposing them to higher fault currents until you get a failure. This process is very costly and time consuming. Therefore, these tests are usually only done on individual components and common combinations of components rather than complete panels.
The second method is by calculation. NEC 409.110 allows for calculation of SCCR using an approved method. The most common method in the United State is UL 508A supplement SB.
UL 508A Calculation Method
Step 1: Determine the short circuit current rating all individual power circuit components by using one of the following methods:
- Use the short circuit current rating marked on the component or on the specifications provided with the component.
- For unmarked components, use the assumed short circuit current rating from Table SB4.1 of Supplement SB.
- Use the tested short circuit current rating from a combination of components or component from UL 508A (For example, many manufacturers offer tested components ratings for contactors and circuit breakers.)
Step 2: Calculate the short circuit current rating of each branch circuit in the panel. The SCCR of a branch circuit is equal to the smallest SCCR of its individual components.
Using current limiting components (fuses, circuit breakers, transformers) in the feeder circuit can increase the SCCR of the branch circuit. These components must be specified and tested as current limiting by the manufacturer. All current limiting devices have a let-through current rating. This is the amount of current that devices downstream of the current limiting device will be exposed to in the case of a short circuit. If all the components on the load side of the current limiting device have higher SCCR ratings than the let-through current of the current-limiting device, the SCCR of the current limiting device can be used as the SCCR of the branch circuit.
Step 3: Determine the overall panel short circuit current rating according to SB4.4. The panel SCCR is the smallest branch circuit SCCR.
By calculating the short circuit current rating of a control panel, operators can avert potential disaster and comply with regulations. Matrix Technologies can assist you in ensuring control panels meet the standards.
Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our multidiscipline engineering capabilities and manufacturing process control solutions, contact Brian Haury, PE, Discipline Manager.
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