What is the point-to-point method?

It is the standard Bussmann/Cooper procedure for hand-calculating available fault current. It starts with the fault current at the transformer secondary from the transformer impedance, then applies a multiplier that reduces the current as conductor impedance is added between the source and the point of interest.

Why assume an infinite primary bus?

Assuming the utility primary can deliver unlimited current gives the highest possible secondary fault current, which is the conservative worst case. Real available fault current is slightly lower once the finite primary contribution is included, so designing to the infinite-bus figure is safe.

What is the difference between SCCR and AIC?

AIC (ampere interrupting capacity) is the maximum fault current a single overcurrent device can safely interrupt. SCCR (short-circuit current rating) is the rating of an assembly such as a panelboard or industrial control panel. Both must equal or exceed the available fault current at their location per NEC 110.10 and 110.24.

How does conductor length reduce fault current?

Conductor impedance adds series resistance and reactance, dropping voltage under fault and limiting current. Longer runs and smaller conductors increase impedance, so the multiplier M shrinks and the available fault current at the downstream point is lower than at the source.

What is the C value in the formula?

C is a conductor constant from the Bussmann point-to-point tables that bundles the conductor's resistance and reactance for a given size, material, and conduit type. Larger conductors and copper have higher C values, meaning lower impedance and less fault-current reduction over the same length.

Short-Circuit Current Rating (SCCR) Calculator

Every piece of electrical equipment must be rated to withstand the fault current available where it is installed, per NEC 110.10. This calculator uses the industry-standard Bussmann point-to-point method to find that available short-circuit current at any node — a switchboard, panelboard, or motor control center — so you can compare it against equipment SCCR and breaker AIC ratings.

How it works

First, the fault current at the transformer secondary, assuming an infinite primary bus:

I_fla = kVA × 1000 / (V × √3)        (three-phase)
I_sca = I_fla / (%Z / 100)

Then a conductor run reduces the current. The point-to-point factor f and multiplier M propagate the fault through the conductor:

f = (√3 × L × I_sca) / (C × n × V)
M = 1 / (1 + f)
I_at_point = I_sca × M

Here L is the one-way length in feet, C is the Bussmann conductor constant for the size and material, n is the number of parallel conductors per phase, and V is the line-to-line voltage.

Reading the result

The tool reports the transformer full-load current, the secondary fault current, the conductor multiplier, and the final available fault current at the point. Your equipment SCCR and each breaker’s AIC must equal or exceed that final figure.

Example and notes

A 500 kVA, 5.75%Z, 480 V three-phase transformer delivers about 10,460 A full-load and roughly 18,200 A of fault current at its secondary terminals. Running 100 ft of 4/0 copper to a downstream panel drops the available fault current meaningfully through the multiplier. The infinite-bus assumption is deliberately conservative; supplying a known primary fault current would lower the result, so the value shown is the safe worst case for selecting rated equipment.