Source input assistant
Available fault current, with the source attached.
Available (bolted) fault current at the panel from transformer nameplate data — free, no account. Then send the value straight into an arc-flash calculation.
Common questions
How do you calculate available fault current?
From the transformer nameplate: full-load current is kVA × 1000 ÷ (√3 × secondary volts); the infinite-bus fault current is that divided by %Z ÷ 100. Conductor runs reduce it (point-to-point per NEC Chapter 9), and motor loads add contribution for the first cycles.
What is the formula for available fault current at a transformer secondary?
Isc = (kVA × 1000) ÷ (√3 × V × %Z ÷ 100). A 1000 kVA, 5.75 %Z, 480 V transformer gives about 20.9 kA before conductor attenuation and motor contribution.
What is the difference between fault current and short-circuit current?
They are the same quantity: the current that would flow into a bolted three-phase fault at that point. "Available" stresses that it is the maximum the source can deliver there.
What is NEC 110.24 available fault current labeling?
NEC 110.24(B) requires service equipment to be field-marked with the maximum available fault current and the date the calculation was performed, in a durable way that withstands the environment.