Fault Loop Impedance Calculator

A fault loop impedance calculator converts a test-voltage and fault-current reading into a measured loop impedance, then subtracts the entered Lead impedance to display a corrected worksheet value.

The resulting impedance is useful when documenting test-lead correction during electrical troubleshooting, commissioning records, or comparative circuit testing. It can help organize measured data for a branch circuit, feeder, raceway installation, or equipment connection where the continuity of the fault-current return path is under review.

The calculator produces two impedance values:

• Measured loop impedance — the direct ratio of Test voltage to Measured fault current
• Corrected loop impedance — measured loop impedance less the entered Lead impedance

It does not determine whether an overcurrent protective device will clear a fault within a required time, whether a grounding and bonding arrangement is compliant, or whether an installation is acceptable to the AHJ.

Test Voltage and Measured Fault Current

The worksheet uses three inputs:

Field	Electrical meaning	Unit
Test voltage	Voltage applied or used as the basis for the measurement	V
Measured fault current	Current recorded during the test	A
Lead impedance	Impedance attributed to the test leads and entered as a correction	ohm

Test voltage and Measured fault current establish the direct impedance reading through Ohm’s law. If measured current decreases while test voltage remains unchanged, calculated loop impedance rises. Conversely, higher measured current at the same voltage produces a lower impedance value.

Lead impedance is not measured or derived by this worksheet. It is an entered correction value. The calculator subtracts it from the measured loop impedance so the record can show both the original measured value and the adjustment applied.

Use consistent test conditions and record the actual test setup separately. Changes in lead arrangement, connection pressure, oxidation, conductor temperature, parallel paths, energized equipment, or instrument test method can affect field readings independently of this worksheet arithmetic.

Loop Impedance Calculation

The calculator applies the following formulas:

\(\displaystyle \text{measured impedance} = \frac{\text{Test voltage}}{\text{Measured fault current}}\)

\(\displaystyle \text{corrected impedance} = \text{Measured loop impedance} - \text{Lead impedance}\)

Impedance is displayed in ohms. The calculation treats the entered values as a simple voltage-to-current ratio and a direct lead-impedance subtraction.

A corrected result may be useful for comparing readings taken with the same leads across multiple circuits. It does not identify which portion of impedance comes from phase conductors, equipment grounding conductors, bonding jumpers, terminations, raceways, transformer windings, source impedance, or test leads.

Calculation Example

Using the worksheet values:

Input	Value
Test voltage	24 V
Measured fault current	10 A
Lead impedance	0.1 ohm

First, calculate the Measured loop impedance:

\(\displaystyle \frac{24\text{ V}}{10\text{ A}} = 2.4\text{ ohm}\)

Then apply the entered Lead impedance:

\(\displaystyle 2.4\text{ ohm} - 0.1\text{ ohm} = 2.3\text{ ohm}\)

Result	Value
Measured loop impedance	2.4 ohm
Lead impedance used	0.1 ohm
Corrected loop impedance	2.3 ohm
Measured current used	10 A

The corrected loop impedance for this worksheet is 2.3 ohm.

Field Verification Limits

Fault-loop impedance affects the available fault-current path, but this worksheet does not perform fault-current, protective-device, or grounding-system analysis. It makes no determination regarding:

• Available fault current at the service, feeder, branch circuit, or equipment terminals
• Breaker or fuse interrupting rating
• Circuit-breaker instantaneous operation, fuse clearing behavior, or trip time
• Equipment grounding conductor sizing, bonding continuity, or grounding electrode performance
• Touch voltage, step voltage, or personnel-protection conditions
• Voltage-drop compliance or design suitability
• Conductor ampacity, AWG or kcmil selection, insulation temperature rating, terminal rating, adjustment factor, or correction factor
• Raceway fill, conductor bending space, conductor routing, or installation workmanship
• NEC compliance, manufacturer requirements, project specifications, or AHJ acceptance

For a fault-current or protective-device decision, verify the actual source characteristics, conductor sizes and lengths, conductor material, terminations, equipment grounding and bonding path, overcurrent protective device data, and applicable code and project requirements separately.