Ground Fault Calculator

A ground fault occurs when electrical current finds an unintended path to the ground — and knowing how much current flows determines whether your protective devices will trip fast enough to prevent fire or injury. Select your system voltage, transformer size, and %Z impedance, then enter your circuit length, conductor material, conductor size, EGC size, and connection resistance to calculate the Ground Fault Current. Secondary outputs include loop impedance (Zs), transformer impedance, conductor impedance, and EGC resistance.

%

Transformer percent impedance (typically 2-8% for distribution transformers)

ft
mΩ

Typical connection resistance (50-200 mΩ)

Results

Ground Fault Current

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Loop Impedance (Zs)

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Transformer Impedance

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Conductor Impedance

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EGC Resistance

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Frequently Asked Questions

How do I measure ground fault loop impedance?

Ground fault loop impedance is measured using a loop impedance tester that injects current between the line conductor and equipment grounding conductor. The measurement should be taken at the furthest point of the circuit to ensure adequate fault current for protective device operation.

What is a good loop impedance value?

A good loop impedance value depends on the protective device and system voltage. Generally, loop impedance should be low enough to ensure fault current exceeds the protective device's instantaneous trip setting, typically requiring fault currents of 25A or higher for standard breakers.

Why is my fault loop impedance too high?

High loop impedance can result from long circuit runs, undersized conductors, poor connections, or high transformer impedance. Consider using larger conductors, shorter runs, or improving connection quality to reduce impedance.

Does GFCI protection change the loop impedance requirements?

GFCI devices provide personnel protection at 4-6mA but don't eliminate the need for proper loop impedance. You still need adequate fault current to trip overcurrent devices for equipment protection and to clear high-current faults that exceed GFCI ratings.

What NEC requirements apply to ground fault calculations?

NEC 250.4 requires that the fault current path be capable of safely carrying fault current and have sufficiently low impedance to facilitate overcurrent device operation. NEC 250.122 specifies equipment grounding conductor sizing requirements.

How does conductor material affect loop impedance?

Aluminum conductors have higher resistance than copper (about 1.6 times), resulting in higher loop impedance. When using aluminum, consider upsizing conductors or using shorter runs to maintain adequate fault current levels.

What happens if fault current is too low?

If fault current is insufficient to trip the protective device, the fault may persist, creating fire and shock hazards. The circuit breaker may not trip instantaneously, or in worst cases, may not trip at all, leaving energized equipment cases.