Q. When is equipment required to be "field" marked with available fault current?

A. Service equipment in other than dwelling units must be legibly field marked with the maximum available fault current, including the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved [110.24(A)].

When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated 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) must be adjusted to reflect the new level of maximum available fault current [110.24(B)].

Exception: Field markings aren't required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.