Safety & Protection calculator
Ground Fault Loop Calculator
Enter system voltage, transformer kVA, conductor size, circuit length, equipment grounding conductor size, and connection resistance before using this calculator to screen ground-fault loop impedance (Zs) and prospective fault current. The result should stay tied to the selected conductor and equipment-grounding path, then be reviewed against the protective device curve, equipment documentation, the adopted NEC edition, and AHJ requirements.
Updated August 4, 2026
Example Calculations
How to Use
Use the calculator first
Zs = Z_transformer + Z_line + Z_EGC + Z_connections
Ground fault loop impedance determines how much fault current can flow during a phase-to-ground fault. Higher impedance usually means less fault current and slower protective-device operation, but the acceptable value depends on the actual voltage, source, conductor path, device curve, and installation requirements.
- Enter the system voltage and transformer kVA or source basis.
- Select the line conductor and equipment grounding conductor sizes.
- Enter the one-way circuit length and any known connection resistance.
- Calculate Zs and prospective fault current, then compare the result with breaker trip data and equipment ratings.
NEC Requirements
NEC 250.4 - Effective Ground Fault Current Path
The ground fault path must have sufficiently low impedance to:
- Facilitate operation of overcurrent protective device
- Limit voltage to ground during fault condition
- Clear the fault in minimum time
Fault Current Rule of Thumb
Some preliminary checks compare calculated fault current with a multiple of the circuit breaker rating, but the final judgment should use the actual protective-device curve or manufacturer trip data for the installed device.
Ground-Fault Loop Review Notes
| Review Item | What to Check |
|---|---|
| Loop impedance | Line conductor, EGC, transformer impedance, and connection resistance |
| Available fault current | Compare the calculated current with breaker and equipment ratings |
| Protective device | Review the time-current curve or manufacturer trip data |
For EGC sizing context, see the Wire Size Calculator with NEC 250.122 references. For available fault current beyond the branch-circuit loop, use the Short Circuit Calculator.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
How do I measure ground fault loop impedance?
Use a loop impedance tester that injects a test current and measures voltage drop. The instrument calculates Zs automatically; follow the tester manual, site safety procedures, and qualified-person requirements.
What is a good loop impedance value?
Lower is generally better, but the usable value depends on the voltage, conductor path, source impedance, equipment grounding conductor, and protective-device curve. Use the calculator result as the project-specific basis, then confirm trip behavior against the actual device data.
Why is my fault loop impedance too high?
Common causes include: 1) Undersized EGC for circuit length, 2) High resistance connections (corrosion, loose terminals), 3) Long circuit runs, 4) Aluminum conductors (higher resistance than copper), 5) Small transformer serving large loads.
Does GFCI protection change the requirements?
GFCI devices trip at 4-6mA ground fault current, far below the amount needed to trip a breaker. They provide electrocution protection but do not protect against line-to-line faults. A proper low-impedance fault loop is still needed for overcurrent protection during phase-to-ground faults. GFCI adds a layer of safety but does not replace proper grounding.
How often should ground fault loop impedance be tested?
Ground fault loop testing is commonly reviewed at commissioning, after grounding-system or service-equipment changes, after a ground fault event, and whenever a site safety concern is reported. Set the recurring interval through the facility maintenance program, applicable standards, and AHJ requirements.
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