The panelboard, with an unknown X3 available fault current at its line terminals, is being investigated to determine if the arc flash PPE category method can be used. A fault current calculation determines the available fault current at the service transformer secondary X1 to be as shown. Is it necessary to continue doing the fault current calculations to X3 in order to determine whether the available fault current parameter value in Table 130.7(C)(15)(a) for a 480-volt panelboard is satisfied?

The panelboard, with an unknown X3 available fault current at its line terminals, is being investigated to determine if the arc flash PPE category method can be used. A fault current calculation determines the available fault current at the service transformer secondary X1 to be as shown. Is it necessary to continue doing the fault current calculations to X3 in order to determine whether the available fault current parameter value in Table 130.7(C)(15)(a) for a 480-volt panelboard is satisfied?

• A. Yes
• B. No
• C. Only if X3 is unknown
• D. Cannot be determined

Answer: (B)

When applying the arc flash PPE category method, you compare the available fault current at the equipment location to the values in the table. The panelboard will see fault current that is at most the fault current available at the source (the service-transformer secondary) and will typically be lower because of impedance in the feeders. Therefore, if the fault current at the service transformer secondary is already known and used to assess the panelboard against the table’s thresholds, continuing the calculation all the way to the panel’s line terminals would not raise the fault current; it can only stay the same or decrease. Since the table’s parameter value for a 480-volt panelboard is determined by that available fault current at the point of interest, the known X1 value is sufficient to decide whether the PPE category can be used. Calculating down to X3 isn’t required unless X1 is unknown or insufficient to determine compliance.

When applying the arc flash PPE category method, you compare the available fault current at the equipment location to the values in the table. The panelboard will see fault current that is at most the fault current available at the source (the service-transformer secondary) and will typically be lower because of impedance in the feeders. Therefore, if the fault current at the service transformer secondary is already known and used to assess the panelboard against the table’s thresholds, continuing the calculation all the way to the panel’s line terminals would not raise the fault current; it can only stay the same or decrease. Since the table’s parameter value for a 480-volt panelboard is determined by that available fault current at the point of interest, the known X1 value is sufficient to decide whether the PPE category can be used. Calculating down to X3 isn’t required unless X1 is unknown or insufficient to determine compliance.