Wiring and Protection
Fault Loop Impedance Calculator
Fault Loop Impedance helps you find maximum loop impedance from nominal voltage and required trip current. The page keeps the arithmetic visible so you can check the result against the original measurements.
Run the Fault Loop Impedance scenario
Enter ratings or measurements that describe one scenario.
Maximum loop impedance: calculation method
For this worksheet, the governing relationship is Zloop = V ÷ Itrip. The formula draws on Nominal voltage and Required trip current.
With the current entries, the primary answer is 1.44 Ω. Preserve the raw inputs when replacing this example.
Calculate maximum loop impedance from nominal voltage and required trip current. A separate required impedance result is available from Current-Limiting Impedance Calculator.
Reading Maximum loop impedance
Read Maximum loop impedance as the outcome of this equation, not as automatic equipment approval. Compare it with the selected conductor and protective-device ratings.
Label alternate cases rather than overwriting the baseline.
Before entering values
Record ambient temperature, bundling, insulation class, and termination conditions. Nameplate and meter values are not interchangeable unless they describe the same condition.
A denominator entry cannot be zero. Use the displayed unit for every field and document conversions separately.
- Nominal voltage
- Default example: 230 V. Enter nominal voltage in V.
- Required trip current
- Default example: 160 A. Enter required trip current in A.
Checks before using the answer
Verify disconnection requirements with the applicable protection standard.
The surrounding system may introduce fault duration, protective-device curves, and conductor thermal limits. Resolve material omissions before selecting a standard size or rating.
Record ambient temperature, bundling, insulation class, and termination conditions.
Keep a reproducible record
Save nominal voltage and required trip current with maximum loop impedance. Add the date and any unit conversion.
Pass maximum loop impedance unchanged when passing it to the next worksheet. Do not round between dependent calculations.
Common questions before using the result
Should I use measured or nameplate values?
Measured and rated values can represent different states. Keep one-way and round-trip lengths clearly identified.
What is not captured by this equation?
Verify disconnection requirements with the applicable protection standard. A broader review should include code rules, manufacturer instructions, and field installation details.
What explains a gap between calculated and observed maximum loop impedance?
First confirm that the measurements match the equation. Then review fault duration, protective-device curves, and conductor thermal limits.
How do I run a useful sensitivity check?
Change one uncertain entry at a time and preserve the baseline. Keep one-way and round-trip lengths clearly identified.
What else is required before final selection?
Use the number as one check, then review ampacity, voltage-drop, fault-current, and protection limits.
Should intermediate values be rounded?
Keep full precision while transferring maximum loop impedance; round only at the final reporting or selection step.