2011 National Electrical Code Changes

1. 110.24 Available Fault Current

A new section requires some equipment to be marked with the available fault current and requires updating of that marking if modifications of the electrical system occur.

110.24 Available Fault Current.

(A) Field Marking. 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. (click here to see Fig. 1)

(B) Modifications. 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.

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

Analysis: All equipment must have an interrupting rating or short circuit current rating that’s equal to or greater than the available fault current [110.9 and 110.10]. As premises wiring systems age, utilities may change transformers in an effort to become more efficient or to increase capacity. When this occurs, the available fault current increases, many times resulting in noncompliant (and dangerous) wiring systems. This NEC change is intended to alert Code users to the fact that when utilities change transformers — or when emergency or standby systems are installed — the ratings of equipment must be re-evaluated.

Opponents of this NEC change argue that oftentimes the ratings of equipment are based on a “worst-case” scenario. While this is suitable for designing a system, it isn’t suitable for performing the calculations required to establish the proper personal protective equipment (PPE) necessary to work on the equipment. When artificially high values of fault current are used for equipment ratings, a lower PPE rating is often the result of the calculations.