Available Fault Current Calculator

Available fault current is the prospective short-circuit current at a defined point in an electrical system. The worksheet produces an Adjusted available fault current from entered transformer source values, then compares that result against an entered equipment rating.

The result is used during protection and equipment-duty review. A breaker’s interrupting rating, a fuse’s interrupting capability, and the short-circuit rating of applicable distribution equipment must be evaluated against the fault current available at the equipment line terminals. Available fault current is not a load-current value, conductor ampacity value, voltage-drop result, or raceway-fill result. It is a fault-duty value that can be many times normal transformer full-load current. Equipment interrupting rating must be at least equal to the available fault current at its line terminals.

The worksheet is intended for a source-impedance screen when the transformer kVA, secondary voltage, impedance, phase, and source multiplier are already known.

Entered Source Values

The calculation uses these interface fields:

Field	Electrical use in the screen
Transformer size (kVA)	Transformer rated apparent power used to establish secondary full-load current
Secondary voltage (V)	Transformer secondary voltage used in the current calculation
Phase count (phase)	Establishes the phase basis for the source-current screen
Transformer impedance (%)	Nameplate impedance used to limit transformer secondary fault current
Entered source multiplier (%)	Multiplier applied to transformer-limited fault current
Source multiplier basis	Documentation field recording the basis supplied for the multiplier; it does not select a study method
Entered equipment rating (A)	Equipment rating used to calculate the remaining rating margin

For a three-phase secondary, the voltage entered as Secondary voltage (V) is the line-to-line voltage. Transformer impedance is entered as a percentage, such as 5.75 rather than 0.0575.

A lower transformer impedance produces a higher fault-current result. A larger transformer kVA at the same secondary voltage and impedance also produces a higher result. Transformer secondary fault current is commonly calculated by determining full-load current and dividing it by impedance expressed in per unit.

Transformer Fault-Current Formula

For a three-phase transformer:

\(\displaystyle I_\text{FL} = \frac{\text{Transformer size (kVA)} \times 1000} {\sqrt{3} \times \text{Secondary voltage (V)}}\)

For a single-phase transformer:

\(\displaystyle I_\text{FL} = \frac{\text{Transformer size (kVA)} \times 1000} {\text{Secondary voltage (V)}}\)

Where I_text{FL} is Full-load current.

The transformer-limited fault current is then:

\(\displaystyle I_\text{TF} = \frac{I_\text{FL}} {\text{Transformer impedance (\%)} / 100}\)

The screen applies the entered source adjustment as:

\(\displaystyle I_\text{AFC} = I_\text{TF} \times \frac{\text{Entered source multiplier (\%)}}{100}\)

Where I_text{AFC} is Adjusted available fault current.

Finally, the equipment comparison is:

\(\displaystyle \text{Equipment rating margin} = \text{Entered equipment rating (A)} - \text{Adjusted available fault current}\)

A positive margin means the entered rating exceeds the screen’s calculated fault current. A negative margin means the entered rating is below the calculated value and requires engineering or design review.

Calculation Example

Using the entered values:

• Transformer size (kVA): 500
• Secondary voltage (V): 480
• Phase count (phase): Three phase
• Transformer impedance (%): 5.75
• Entered source multiplier (%): 100
• Source multiplier basis: Entered screen
• Entered equipment rating (A): 22,000

Three-phase full-load current:

\(\displaystyle I_\text{FL} = \frac{500 \times 1000} {\sqrt{3} \times 480} = 601.4065\text{ A}\)

Transformer-limited fault current:

\(\displaystyle I_\text{TF} = \frac{601.4065}{0.0575} = 10{,}459.244\text{ A}\)

Because Entered source multiplier (%) is 100:

\(\displaystyle I_\text{AFC} = 10{,}459.244 \times 1.00 = 10{,}459.244\text{ A}\)

Equipment rating margin:

\(\displaystyle 22{,}000 - 10{,}459.244 = 11{,}540.756\text{ A}\)

Result	Calculated value
Full-load current	601.4065 A
Transformer-limited fault current	10459.244 A
Adjusted available fault current	10459.244 A
Equipment rating margin	11540.756 A

The calculated adjusted value is approximately 10.46 kA. Against an entered 22,000 A equipment rating, the screen shows 11.54 kA of positive rating margin.

Equipment-Duty Review

The calculated fault-current value supports an initial review of equipment exposed to a short circuit at the evaluated source point, including service equipment, switchboards, switchgear, panelboards, breakers, fusible disconnects, industrial control panels, and feeder protection.

The entered equipment rating must correspond to the actual rating being checked. Depending on the equipment and application, that may involve an overcurrent protective device interrupting rating, an assembly short-circuit current rating, or another manufacturer-marked short-circuit rating. Do not substitute normal ampere rating, feeder ampacity, conductor AWG or kcmil size, terminal rating, insulation temperature rating, or branch-circuit load for a short-circuit rating.

Available-fault-current information may also feed a larger engineering workflow:

• Selecting protective devices with suitable interrupting capability
• Reviewing panelboard, switchgear, or control-panel short-circuit ratings
• Supporting short-circuit coordination and protective-device studies
• Identifying when available fault current could change after transformer, utility, generator, or service modifications
• Providing an input for a separate arc-flash analysis

For applicable non-dwelling service equipment, available fault current field marking includes the maximum available fault current and the date of the calculation; modifications affecting the fault-current level require recalculation and label updates.[fluke]

Calculation Boundary

This is an impedance screen only. It uses entered transformer source values and the Entered source multiplier (%) to calculate the displayed results.

It does not model utility-source impedance, primary conductors, secondary feeder impedance, parallel paths, generators, motor contribution, X/R ratio, asymmetrical duty, fault type, protective-device clearing time, arc-flash incident energy, equipment withstand ratings, or code approval. It also does not determine conductor ampacity, adjustment factor, correction factor, current-carrying conductor count, terminal temperature limitation, voltage drop, raceway fill, bending-space compliance, branch-circuit sizing, or feeder sizing.

For downstream equipment, the actual available fault current may differ from the transformer-terminal result because feeder conductor impedance, conductor length, conductor material, conductor size, raceway characteristics, motors, generators, and other available sources affect the fault-current path. Final equipment selection and installation acceptance require the applicable equipment markings, manufacturer data, project fault-current study where needed, and AHJ requirements.

For adjacent protection checks, compare the result with the Short Circuit Current Calculator and the Breaker Interrupting Rating Calculator.