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Ground Fault Loop Calculator

Calculate ground fault loop impedance (Zs) and prospective fault current. For the 120 V example with a 50 kVA transformer, 100 ft of #12 copper line conductor and #12 EGC, plus 50 mΩ connection resistance, Zs is 0.453 Ω and prospective fault current is 265 A. Use the result for protective-device and equipment-grounding review against the adopted NEC edition, equipment documentation, and AHJ requirements.

Updated June 21, 2026

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Example Calculations

120V Residential, 100 ft

Typical home branch circuit.

Inputs
  • System Voltage: 120 V
  • Transformer Rating: 50
  • Conductor Size: 12
  • Circuit Length: 100
  • Equipment Grounding Conductor Size: 12

How to Use

Quick Answer

What is ground fault loop impedance?

Zs = Z_transformer + Z_line + Z_EGC + Z_connections

Ground fault loop impedance determines how much fault current will flow during a ground fault. Higher impedance = less fault current = slower or failed protective device operation.

Zs (Ω) Fault Current @ 120V Status
0.1 1,200A Excellent
0.5 240A Good
1.0 120A Check CB rating
2.0 60A May not trip instantaneously

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

Fault current should be ≥ 5× the circuit breaker rating for instantaneous trip.

Example: 20A CB needs at least 100A fault current for fast trip.

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, see Wire Size Calculator with NEC 250.122 references.

Common Applications

Circuit Verification - Confirm protective devices will operate on fault

Long Run Analysis - Verify impedance on extended circuits

Safety Review - Review NEC 250.4, project criteria, and device documentation

Troubleshooting - Diagnose nuisance tripping or non-tripping

Design Validation - Pre-construction fault analysis

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 better. For 120V circuits with 15-20A breakers, Zs under about 0.5Ω usually provides much higher fault current than the breaker rating; always confirm trip behavior against the actual device curve.
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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