Continuing fault current calculations to X3 is sometimes necessary to determine the available fault current parameter value for a 480-volt panelboard.

• A. It is always necessary
• B. It is not necessary
• C. Sometimes necessary
• D. Only for DC systems

Answer: (B)

The main idea is that available fault current at a panel is determined by the impedance from the source up to that panel. For a 480-volt panelboard, you only need the upstream source impedance and the feeders up to the panel to find how much current could flow if a fault occurred at the panel’s terminals. Extending the calculation beyond the panel to X3 would bring in impedances that lie downstream of the fault location, which do not affect the fault current entering the panel. Including those downstream elements would incorrectly alter the calculated AFC at the panel.

You would, of course, extend calculations to downstream points if you needed the fault current at those downstream locations or were performing a full-system short-circuit study across multiple points. But for determining the panel’s available fault current, stopping at the panel is appropriate.