Under NEC 110.24, the requirements for marking Available Fault Current have become a major focal point for inspectors ensuring equipment ratings are actually safe for the grid.
Here is the breakdown of what is required to keep your facility compliant and your gear protected.
1. Mandatory Field Marking
In all buildings other than dwelling units, service equipment must be legibly marked in the field with the available fault current. This isn't just about labels; it's about documenting the potential energy at the line terminals to ensure your equipment's interrupting rating can actually handle a fault without catastrophic failure.
2. The Two Required Data Points
A compliant fault current label is more than just a number. Per NEC 110.24(A), your marking must include:
- The Available Fault Current: The calculated value in Amperes.
- The Calculation Date: The specific date the fault current study was performed.
3. Modifications Trigger Recalculation
One of the most overlooked aspects of the code is NEC 110.24(B). If there are modifications to the electrical installation that affect the available fault current - such as a utility transformer upgrade - the current must be verified or recalculated. Once recalculated, the field markings must be updated to reflect the new levels.
4. Durability and Design Standards
Don't reach for a permanent marker. The code is very specific:
- Durability: Markings must be of sufficient durability to withstand the environment.
- No Handwriting: While variable data like the date can be handwritten if legible, the primary label must not be.
- Standardized Coloring: Following industry practice, these labels are typically specified as YELLOW background / BLACK text to denote a CAUTION-level rating.
Why Engraving is the Professional Standard
Using Durmark 2-ply acrylic ensures your labels meet the "permanently affixed" requirement of NEC 110.21(B) and stay legible. In industrial environments where only qualified persons service the equipment, having clear, permanent data is the difference between a safe maintenance window and a dangerous guessing game.
Is your service equipment up to date? Create this label now on our designer at panellabel.com/design-labels.