What Is Available Fault Current? (And How to Find Yours)

Available fault current (AFC) is the maximum current the electrical system can deliver into a bolted — zero-impedance — fault at a given point. It is set by three things: the utility source behind your service, the transformer (its kVA rating and percent impedance), and every conductor between the source and the point you are measuring.

You need this number for two separate reasons: the code requires it on your service equipment, and every arc-flash calculation depends on it.

Last reviewed: July 18, 2026.

Why NEC requires it on the equipment

NEC 110.24 requires service equipment in other-than-dwelling units to be field-marked with the maximum available fault current and the date the calculation was performed — and the marking must be updated if the installation changes. The reason is practical: the inspector and the next electrician both need to know whether the equipment's ratings hold.

That check is a comparison against a second number:

• Available fault current (AFC) — what the system can deliver into a fault.
• AIC (ampere interrupting capacity) — what a breaker or fuse can safely interrupt. NEC 110.9 requires equipment intended to interrupt fault current to have an interrupting rating sufficient for the current available at its line terminals.

In one line: AIC must meet or exceed AFC. A 10 kAIC breaker on a service that can deliver 18 kA is a code violation and a genuine hazard — the device can fail to clear the fault.

Why it drives the arc-flash math

Incident energy is not set by fault current alone. IEEE 1584-2018 works from the arcing current — a fraction of the bolted fault current — together with the protective device's clearing time at that current. That pairing creates the trap that catches people:

A higher assumed fault current can produce a lower calculated hazard. If you assume more current than the system can really deliver, the protective device appears to trip faster, the assumed arc duration shrinks, and the calculated incident energy drops. A NEMA technical paper on utility fault-current parameters states it directly: calculations based on conservatively high available fault current values can underestimate the actual flash hazard.

This is why the utility's first answer is dangerous. Utilities commonly quote the infinite-bus value — the transformer's nameplate maximum, ignoring primary system impedance. In a Mike Holt's Forum thread on study workflow, engineers report that the infinite-bus number is usually the first thing the utility sends, that meaningful fault-current data for services at 480 V and below arrives less than 10% of the time, and that waits of months are normal. One engineer described filing a state utility commission complaint to force the data out.

Three ways to find your number

In order of accuracy:

1. Ask the utility (best)

Request the data in writing, and ask for the right things: available fault current and X/R ratio at the primary protective device, the primary fuse type, riser cable data, and the padmount transformer data. If the answer comes back as only the transformer nameplate maximum, it is the infinite-bus value — treat it as an upper bound, not the answer.

2. Estimate from the transformer nameplate (fast, conservative)

With only the nameplate you get the infinite-bus secondary current:

I = (kVA × 1000) ÷ (√3 × V × (%Z ÷ 100))

Worked example — a 500 kVA transformer, 480 V secondary, 5.75% impedance:

I = (500 × 1000) ÷ (1.732 × 480 × 0.0575)
  = 500,000 ÷ 47.8
  ≈ 10,460 A  (about 10.5 kA)

For single-phase, drop the √3 term. Treat the result as "no more than this" — the real value is lower once primary and conductor impedance are counted. Running an arc-flash calculation on an upper-bound value is not automatically conservative, per the NEMA warning above, so check the result at reduced currents too.

3. Calculate point to point (for downstream panels)

Between the transformer and a downstream panel, the conductors add impedance and knock the value down. A point-to-point calculation applies conductor impedance step by step from the source to your panel. Eaton's free FC2 calculator does this for single- and three-phase systems and generates the NEC 110.24 service label. One more adjustment: rotating motors feed current back into a fault for the first few cycles, so a complete number adds motor contribution on top of the utility-and-transformer figure.

What to do with the number

• Mark it on the service equipment with the calculation date (NEC 110.24), and update the marking when the system changes.
• Feed it into the arc-flash calculation. Our free arc flash calculator asks for available fault current as a core input — the wizard walks you through where the number comes from — and reports the incident energy at your entered value so you can see how sensitive your panel is to it.
• Put it on the arc-flash label with the date obtained. It is standard practice on spec'd jobs, and it documents the input the rest of the label depends on. The full field list is on the arc flash label requirements page.

If the whole exercise starts to look like a facility-wide job, the arc flash study cost page breaks down what the engineering firms charge and why.