Since the 2011 edition of the NEC^®, section 110.24(A) has required the maximum available fault current (AFC), and fault current calculation date, to be legibly field marked on service equipment in other than dwelling units.

The AFC marking is to ensure that overcurrent device AIC ratings and equipment short circuit current ratings (SCCR’s) are considered and equipment is properly selected based on the amount of AFC in order to comply with 110.9 and 110.10. These two code sections have been in the NEC^® for many years.

Section 110.24(B) requires the AFC to be recalculated and the marked value to be adjusted when a modification to the electrical installation changes the previous amount of AFC at the service equipment. This is to ensure that the service equipment ratings are still sufficient for the maximum available fault current at the line terminals of the equipment.

Some examples of modifications that can change the final amount of AFC at the service equipment can include:

• Changing the length of service entrance conductor by moving the service further from or closer to the utility transformer. A shorter service entrance conductor has less resistance which results in an increase of the AFC at the service.
• Increasing the size of the service and service entrance conductors to accommodate an increase from additional loads will increase the amount of AFC.
• If the utility company changes out their transformer to a more efficient unit with a lower impedance value, the amount of AFC at the service will increase (hopefully, they will be proactive and mention it to the local inspection authority so that the inspector can verify that the equipment ratings are still sufficient for any increase in the AFC).

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014-2017 Code Language:

110.24 Available Fault Current.

(B) Modifications. When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary 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) shall be adjusted to reflect the new level of maximum available fault current.