Yes, Enphase's assessment of parasitic power is highly likely the correct explanation. Here's a breakdown of why, and what to consider

Why Parasitic Power is Likely:
Shared Conduit and Proximity

The key factor is the shared conduit containing 15 energized phase conductors, 5 neutrals, and a ground. These conductors are running in close proximity for a significant length.

Capacitive Coupling

The energized conductors create an electric field around them. Even with insulation, a small amount of capacitive coupling can occur between the energized conductors and the "dead" conductor connected to the microinverters with the breaker off. This capacitive coupling essentially forms a tiny capacitor between the conductors.

Inductive Coupling

Similar to capacitive coupling, inductive coupling (magnetic field interaction) can also induce a small current in the "dead" conductor. This is especially true with the number of conductors present and the length of the run.

Microinverter Sensitivity

Microinverters are designed to be very sensitive to even small amounts of current and voltage to efficiently harvest energy. Even a tiny induced current can be registered and amplified by the microinverter.

Load Dependency

The diminishing current as you turn off more breakers is consistent with parasitic coupling. As fewer conductors are energized, the overall electric and magnetic fields are reduced, leading to less induced current in the disconnected circuit.

Why You Might Not Have Seen It Before



Conduit Length

Shorter conduit runs will have less coupling.

Conductor Arrangement

How the conductors are physically arranged in the conduit can affect coupling. More tightly packed and parallel runs will have higher coupling.

Microinverter Model

Different microinverter models might have different sensitivities and impedance characteristics, influencing how they respond to parasitic currents.

Load Conditions

How heavily loaded the other strings are affects the electromagnetic fields and thus the induced current.

Measurement Precision

The sensitivity of the amp clamp or multimeter you're using matters. A less sensitive meter might not register the small parasitic current.

Grounding Practices

Solid and robust grounding is crucial, but even with good grounding, parasitic currents can still exist.

Is Something Weird Going On? (Probably Not)

While parasitic power is the most probable explanation, it's always wise to verify: Check Neutral-Ground Bond

Ensure the neutral-ground bond is only at the main service panel and not in the subpanel. A neutral-ground bond in the subpanel can create circulating currents.

Conduit Grounding

Verify the conduit is properly grounded at both ends and at any intermediate junction boxes. Poor grounding can exacerbate parasitic currents.

Insulation Resistance

While unlikely, you could perform an insulation resistance (megger) test on the conductors in the conduit to ensure there isn't a hidden insulation fault. This would involve disconnecting the conductors at both ends before testing. However, a megger test is more for finding true insulation breakdown, not the subtle effects of capacitive/inductive coupling.

Recommendations

1.

Accept Parasitic Power (Mostly)

Understand that this is likely unavoidable with the described configuration. A tiny current, as you're seeing, is generally not a safety concern.2.

Documentation

Document this phenomenon in the job records. This will help future troubleshooting.3.

Mitigation (If Necessary, Unlikely)



Dedicated Conduits

The best way to completely eliminate this is to use separate conduits for each circuit. This is often impractical and costly.

Metal Conduit

Using metal conduit helps contain electromagnetic fields and can reduce the amount of induced current. PVC is transparent to electromagnetic fields. However, you're likely already using metal conduit.*

Increased Spacing (Highly Impractical)

In theory, increasing the spacing between the conductors in the conduit would help, but this is usually not possible.4.

Monitor for Changes

Keep an eye on the current readings over time. If the current increases significantly, then further investigation might be needed.

In conclusion, Enphase's explanation of parasitic power is highly credible. The configuration (multiple conductors in a shared conduit) creates the ideal conditions for capacitive and inductive coupling. Unless the current is unusually high, or increases significantly, it's likely just a characteristic of the installation. Flag for review