What Does Available Fault Current Mean?

What does the term Available Fault Current mean? It’s not a term we hear terribly frequently in the industry unless you are having to mark the panel you are installing, or you hear about it while studying for an exam. It is not as baffling or confusing as it may sound! In this episode of Electrician U, Dustin talks about what the term stands for and how it is used.

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To describe the term Fault, we would be talking about an accidental occurrence of something. Something that isn’t meant to happen but did. A ground fault for example, is the accidental joining of an ungrounded conductor and a grounding conductor. The NEC defines Fault Current as “The current delivered at a point on the system during a short-circuit condition”. Furthermore, the NEC defines Available Fault Current as “The largest amount of current Capable of being delivered at a point on the system during a short-circuit condition”. A short circuit is when the current in a circuit attempts to take another path back to its source other than the path intended. This can be either via another hot (ungrounded) conductor or a neutral (grounded conductor), but not a ground conductor (this would be a ground fault).
So, Available Fault Current is the largest among of Current that is CAPABLE of flowing during a Short-Circuit. The biggest reason that we label the service with the Available Fault Current is due to the size of the wire. For example, a branch circuit within a building will definitely have much smaller size wire than the incoming service. This smaller wire will be able to contain MUCH less current than the service entrance conductors from utility. So, the hazard downstream in the potential of current is less than at the service where the potential for current is much greater. If you had several parallel runs of 500KCMIL conductors for each phase at the service, the amount of potential current being able to flow during a short circuit are massive compared to the amount that could flow on a set of #12 conductors. Length is something that also comes into play when factoring the amount of available fault current. The longer the length of the cable is, the more opposition to current flow (or impedance) there will be.
NEC article 110.24(A) states that service equipment (other than dwelling units) shall be marked in the field with the available fault current. It also states that it shall be durably marked (so the weather or environment doesn’t make it fade or wash off) and be documented/available so to anyone authorized to design, install, inspect, maintain, or operate the system. 110.24(B) goes on to say that if you were to modify the original system, you must update the available fault current to be accurate with the new work.
We hope this clears up the term Available Fault Current! Continue to follow Electrician U and Dustin’s travels as we are constantly adding new and updated information for our viewers!

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