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.

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.

What is the Short-Circuit Current Rating (SCCR) Calculator?

Use the Bussmann point-to-point method — transformer impedance plus conductor impedance — to compute available short-circuit current at switchboards, panelboards, and MCCs for SCCR and AIC compliance per NEC 110.10. For electrical engineers and inspectors. It runs free in your browser on Gera Tools, with nothing uploaded.

Short-Circuit Current Rating (SCCR) Calculator

Name: Short-Circuit Current Rating (SCCR) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Worked example

A 500 kVA, 5.75%Z, 480 V three-phase transformer:

Full-load current: 500,000 / (480 × √3) ≈ 601 A
Secondary fault current: 601 / 0.0575 ≈ 10,452 A at the secondary terminals

Now run 75 ft of 3/0 AWG copper (C ≈ 14,900 from Bussmann tables, single conductor per phase):

f = (1.732 × 75 × 10,452) / (14,900 × 1 × 480) ≈ 0.190
M = 1 / (1 + 0.190) = 0.840
Available fault current at panel = 10,452 × 0.840 ≈ 8,780 A

The panel’s SCCR and every breaker’s AIC rating must be at or above 8,780 A. If the panel is rated 10,000 A and breakers are 10 kAIC, they are adequate. If a breaker is rated 5 kAIC, it is under-rated for this location.

SCCR vs AIC — why the distinction matters

These two ratings are often confused but address different levels of the system:

AIC (Ampere Interrupting Capacity): The maximum fault current a single overcurrent protective device (circuit breaker or fuse) can safely interrupt without damage, arcing, or failure. A breaker that interrupts a fault beyond its AIC rating may fail violently.
SCCR (Short-Circuit Current Rating): The rating of an electrical assembly — panelboard, industrial control panel (ICP), switchboard — for the maximum fault current it can sustain without component failure. Determined by the weakest component in the assembly. Required on industrial control panels by UL 508A and NEC 409.

Both must be verified at every point in the distribution system. A high-SCCR panelboard that feeds low-AIC branch breakers creates a hidden hazard.

NEC compliance notes

NEC 110.10 requires that overcurrent protective devices be selected considering the available fault current at their point of application.
NEC 110.24 requires the available fault current to be marked on service equipment in commercial and industrial installations.
NEC 409.22 requires SCCRs to be marked on industrial control panels and verified against the supply.

This calculator uses the infinite-bus assumption, which is the conservative worst case: real available fault current is slightly lower once the finite primary impedance is included. For safety-critical design, always use the conservative figure unless the utility has provided a documented maximum available fault current for the service point.