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.

Ground Fault Loop Impedance Calculator

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.

How it works in an example

A 20 A breaker feeds a 100 ft run of 12 AWG copper with a 12 AWG ground at 120 V. Each conductor adds about 1.98 Ω/1000 ft × 100 / 1000 = 0.198 Ω, so the loop is roughly 0.198 + 0.198 = 0.396 Ω plus source impedance. The fault current is about 120 / 0.45 ≈ 267 A, well above the 10 × 20 = 200 A instantaneous threshold, so it passes.

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. To fix a failing result, increase the equipment grounding conductor size, shorten the circuit, or add ground-fault protection. 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.