Free tool
Available fault current calculator
The point to point method, the same one used on plan review sheets. Enter the
transformer and the run of conductor, and see the available fault current at the panel, plus the
interrupting rating your equipment needs.
The transformer
Assumes an infinite primary bus, which is the conservative case.
The run to the fault point
From the transformer secondary to the panel you are rating.
Measure the actual conductor length, not the
straight line distance.
available fault current at the panel
rate for 10 kA
The method. This is the point to point method published by Cooper Bussmann, now Eaton,
and it is what most plan reviewers expect to see.
C is the conductor constant from the Bussmann tables at 600 V, and it changes with the
material and with whether the raceway is magnetic. n is the number of conductors per phase.
Why the tolerance box matters. Transformers of 25 kVA and larger listed to UL 1561
carry an impedance tolerance of plus or minus 10 percent. A transformer marked 2 percent may
really be 1.8 percent, which raises the available fault current by about 11 percent. Leaving
the box ticked gives you the worst case, which is what you want when you are choosing an
interrupting rating.
What this does not include. Motor contribution, which can add roughly four times the
running current of the connected motors for the first cycles, and any impedance on the primary
side. The infinite bus assumption makes the result conservative, so the real number is usually
lower, but if the utility gives you a figure for the primary, use that instead.
Two code points worth knowing. NEC 110.9 requires equipment to have an interrupting
rating at least equal to the available fault current, and NEC 110.24 requires service equipment
in other than dwelling units to be field marked with the available fault current and the date
of the calculation.