Okay, let's break down this fairly common and often misunderstood solar service upgrade scenario. You've described a situation that requires careful analysis to ensure NEC compliance and safety.

Understanding the Setup 200A Service

You have a standard 200A service panel with a main breaker. Neutral and ground are bonded within this panel (the "main bonding jumper"). This is typical for a service entrance.

Solar Tap

The solar system is connected using a 100A service disconnect that taps into the line side of the 200A main panel. This is a common method when the solar system requires a higher amperage OCPD then a backfed breaker in the 200 amp panel can provide.

Grounding Electrode System (GES)

Both the 200A main panel and the 100A solar disconnect share the same grounding electrode system.

Shared Conduit

The conduit carrying conductors from the 200A main and the 100A solar disconnect is metallic and grounded.

Analysis of the Issues1. Neutral-Ground Bonding



Incorrect Bonding (Generally)

You are correct that generally, a service disconnect needs a neutral-ground bond. However, in this specific scenario, it's usually

NOT required (and often prohibited) in the 100A solar disconnect. Here's why: Non-Separately Derived System (SDS)

The solar inverter in most residential installations is not a separately derived system. This is the key! An SDS has its own independent neutral. In grid-tied solar, the solar inverter synchronizes with the utility grid and uses the utility's neutral. Because the solar inverter is not an SDS it should not have it's own grounded electrode. In this case the grounded electrode is only connected to one of the services in a multi service setup. This is acceptable by the code and prevents a parallel path on the shared ground.

Line-Side Tap

The 100A solar disconnect taps the line side of the 200A main. This means that the neutral from the utility already passes through the 200A main, where it's bonded to ground. Adding another neutral-ground bond at the 100A disconnect would create an intentional parallel neutral path, which is a major safety hazard (see #2 below). This means the neutral wire is terminated in the 200 amp panel and the neutral and ground are only bonded at the 200 amp panel.

NEC Requirements

NEC 690.41 specifically addresses this for PV systems. It generally prohibits bonding the neutral to ground at the PV disconnecting means when the PV system is interconnected on the line side of the service disconnecting means.

When a Second Bond is Required

If, and only if, the solar inverter is a separately derived system (e.g., in some off-grid or backup power systems) and the code requires it, then you would bond the neutral and ground at the 100A solar disconnect and isolate the neutrals from the 200 amp service. This scenario is much less common in grid-tied residential systems.2.

Parallel Neutral Path through Conduit



The Danger

A parallel neutral path is a serious concern. It can cause:

Overheating and Fire

Neutral current can flow through the metallic conduit, causing it to overheat due to the resistance of the metal.

Nuisance Tripping

GFCI breakers can trip because they sense the current imbalance created by current flowing on the conduit.

Electric Shock

The conduit can become energized if a significant amount of current is flowing through it.

Why It Might Not Be Happening (But Still Needs Checking)

Because the solar system is not supposed to have its own neutral-ground bond in this line-side tap configuration, the shared conduit should not be creating a parallel neutral path if the 200 amp and 100 amp services have shared the same point of neutral bonding. However, any unintentional neutral-to-ground fault anywhere on the system could create a parallel path through the conduit.

Your Concern about Downstream Bonds

You are correct that if there were an accidental or incorrect neutral-to-ground bond after the 200A main panel (e.g., in a subpanel), that would create a parallel path back to the 200A panel through the conduit. This is why subpanels must never have a neutral-ground bond; the bond is only at the service entrance. This would not be a problem in the 100 amp disconnect because the ground is not supposed to be bonded to the neutral. The ground is ran separate to the 200 amp panel.3.

Code Compliance and Safety Checklist



Verify Inverter Type

Absolutely confirm whether the solar inverter is a separately derived system. The entire wiring configuration depends on this. You may also consider the voltage and amperage coming from the inverter if the system is not a typical setup.

Bonding Jumper

Make sure the main bonding jumper is properly sized and installed in the 200A panel.

Equipment Grounding Conductor (EGC)

Ensure a properly sized EGC runs from the 100A solar disconnect back to the 200A main panel. This is essential for fault current protection.

Conduit as EGC

If the metallic conduit is used as the EGC, it must be properly bonded at each end (e.g., with grounding bushings and bonding jumpers) to ensure a low-impedance path for fault current. Consider pulling an additional EGC wire if you have concerns about the conduit's integrity as a grounding path.

NEC 690

Carefully review Article 690 of the NEC for solar-specific requirements.

Inspection

Most importantly, this type of installation must be inspected by a qualified electrical inspector.

In summary:

What you're seeing is likely the correct configuration for a common grid-tied solar system with a line-side tap if the solar inverter is not* a separately derived system. However, it's absolutely critical to verify that the inverter is not an SDS, that the grounding and bonding are done correctly, and that there are no unintended neutral-to-ground connections downstream of the service entrance. The shared conduit presents a potential hazard if anything is wired incorrectly.I strongly recommend consulting with a licensed electrical engineer or a qualified solar installer with expertise in NEC compliance to review the installation and ensure it is safe and code-compliant. The information provided here is for general informational purposes only and should not be considered a substitute for professional advice. Flag for review