What is ground-fault loop impedance?

It is the total impedance of the path a line-to-ground fault current follows: out along the phase conductor and back through the equipment grounding conductor and the supply. A low loop impedance lets a large fault current flow, which is what trips the breaker quickly and safely.

Why must the fault current be large enough?

NEC 250.4(A)(5) requires an effective ground-fault current path that allows the overcurrent device to operate. If loop impedance is too high, the fault current may fall below the breaker's instantaneous trip and the fault could persist, leaving exposed metal energized.

What instantaneous trip multiple should I use?

A typical thermal-magnetic breaker has an instantaneous magnetic pickup around 10 times its rating, though it can range from about 5 to 13 times depending on the type. Use the value from the breaker's trip curve when known; the default of 10 is a common general estimate.

Does this account for conductor reactance?

It uses the DC resistance of the phase and grounding conductors from NEC Chapter 9 Table 8, which is accurate for small conductors and short runs. For long runs or large feeders, reactance becomes significant and you should use the full alternating-current impedance method.

What does a fail result mean?

A fail means the prospective fault current is below the breaker's instantaneous trip threshold, so the ground-fault path is not effective enough. Fixes include increasing the equipment grounding conductor size, shortening the run, or adding ground-fault protection to clear faults at lower current.

What is the Ground Fault Loop Impedance Calculator?

Calculates ground-fault loop impedance from phase and equipment-grounding-conductor lengths and resistances, derives the prospective fault current, and checks whether it exceeds the breaker's instantaneous trip threshold per NEC 250.4(A)(5). It runs free in your browser on Gera Tools, with nothing uploaded.

Ground Fault Loop Impedance Calculator

Name: Ground Fault Loop Impedance Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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For a metal enclosure to stay safe during a fault, enough current must flow back through the equipment grounding conductor to trip the breaker instantly. This calculator builds the ground-fault loop impedance from the phase and grounding conductors, derives the prospective fault current, and checks it against the breaker’s instantaneous trip threshold per NEC 250.4(A)(5).

How it works

The fault current is limited by the round-trip loop impedance:

Z_loop  = R_phase + R_egc + Z_source
I_fault = V_line-to-ground / Z_loop
trip threshold = instantaneous multiple × OCPD rating
pass if I_fault ≥ trip threshold

Conductor resistances come from the copper DC values in NEC Chapter 9 Table 8, scaled by the one-way run length. The instantaneous multiple models the breaker’s magnetic trip, where a standard thermal-magnetic breaker picks up around 10 times its rating.

Worked example

A 20 A breaker feeds a 100 ft run of 12 AWG copper with a 12 AWG equipment grounding conductor at 120 V line-to-ground.

From NEC Chapter 9 Table 8, 12 AWG copper has a DC resistance of approximately 1.98 Ω per 1000 ft:

R_phase = 1.98 × (100 / 1000) = 0.198 Ω
R_egc   = 1.98 × (100 / 1000) = 0.198 Ω
Z_source = 0.05 Ω (typical transformer secondary impedance for a small panel)
Z_loop  = 0.198 + 0.198 + 0.05 = 0.446 Ω
I_fault = 120 / 0.446 ≈ 269 A
trip threshold = 10 × 20 = 200 A
Result: PASS — 269 A exceeds 200 A

Now extend the run to 400 ft with the same conductors:

R_phase = 1.98 × (400 / 1000) = 0.792 Ω
R_egc   = 1.98 × (400 / 1000) = 0.792 Ω
Z_loop  = 0.792 + 0.792 + 0.05 = 1.634 Ω
I_fault = 120 / 1.634 ≈ 73 A
trip threshold = 200 A
Result: FAIL — 73 A does not reach the 200 A instantaneous threshold

This is why long branch circuits need their equipment grounding conductors upsized or require GFCI/AFCI protection to clear faults at lower currents.

Common fixes for failing circuits

Increase the EGC size. A larger equipment grounding conductor has lower resistance. The NEC allows an EGC upsized beyond Table 250.122 minimum when the circuit is long.
Add GFCI protection. GFCI devices trip at as little as 4 to 6 mA, regardless of impedance, so a high-impedance path cannot defeat them the way it can defeat a thermal-magnetic breaker’s instantaneous element.
Reduce the run length. Adding a sub-panel closer to the load cuts both conductor lengths and the loop impedance proportionally.

Tips and notes

Lengthen the run or shrink the grounding conductor and the loop impedance climbs until the fault current can no longer reach the trip threshold. Remember this uses DC resistance, which suits small conductors and short branch circuits; for long feeders or large conductors, switch to the full alternating-current impedance method that includes reactance.