You've identified a serious safety concern and your reasoning is sound. Stray voltage on docks is a well-documented problem, and your solution using an isolation transformer is a recognized method for addressing it. Let's break down the situation and discuss your approach

Understanding the Problem: Stray Voltage and Galvanic Corrosion
What is Stray Voltage? Stray voltage, also known as neutral-to-earth voltage, occurs when there is a potential difference between the neutral and ground conductors. This potential difference can be caused by: Improper Wiring

Faulty connections, loose neutrals, or unbalanced loads in the electrical system.

Shared Neutral Conductors

Multiple circuits sharing a neutral conductor can lead to neutral current flowing through the ground path.

Ground Faults

Current leaking to ground due to insulation breakdown or other faults.

Grid-Induced Current

In your case, the voltage increases with the load on the power grid, often correlating with hotter weather. This strongly suggests the stray voltage is being induced into your grounding system due to voltage gradients in the earth itself, caused by high ground currents in the power grid.

Galvanic Corrosion

While not the immediate safety concern you're addressing, stray voltage also contributes to galvanic corrosion of underwater metal components. DC current (even very small amounts) flowing through the water between dissimilar metals causes one metal to corrode more rapidly.

Why the Dock Grounding System Doesn't Solve the Problem

You're doing everything right according to the NEC in terms of bonding and grounding. However, the purpose of the NEC required ground is to provide a low impedance path for fault current, enabling breakers to trip and prevent shock hazards during fault conditions. It doesn't eliminate the stray voltage caused by neutral-to-earth potential differences. In fact, by grounding the dock, you're potentially introducing the stray voltage to the dock structure, making it accessible to swimmers. The ground rod on shore, while code-compliant, may be contributing to the problem by providing a path for earth currents to flow.

Your Isolation Transformer Solution: The Right Approach

Using an isolation transformer is an excellent strategy. Here's why: Electrical Isolation

The primary and secondary windings of an isolation transformer are electrically isolated from each other. This breaks the direct connection between the utility ground and the dock's electrical system, preventing stray voltage from migrating onto the dock.

Creating a Clean Ground

By creating a new, isolated ground reference on the secondary side of the transformer, you establish a "local" ground for the dock's electrical system.

Floating vs. Bonded Configuration: The Right Choice

You are choosing the correct configuration by bonding the neutral and ground on the secondary side of the transformer at the shore panel. Why Bond? This configuration essentially creates a separately derived system, mimicking the way power is supplied to a boat floating in the water. It ensures that a ground fault on the dock will trip a breaker, providing the necessary overcurrent protection. You are providing the required low-impedance path for fault current back to the source (the transformer's secondary winding), ensuring the breaker trips quickly. Why Not "Floating"? Leaving the neutral unbonded to ground on the secondary side of the transformer would create a truly "floating" system. While this might further reduce stray voltage, it eliminates the ability of breakers to trip during a ground fault. This is extremely dangerous because a single ground fault could energize the entire dock structure without tripping a breaker. The next person touching the energized structure and any other ground (like the water!) would become the path to ground, creating a potentially lethal shock.

Things to Consider and Questions to Answer1. Transformer Sizing

Ensure the transformer is adequately sized for the total load on the dock. Overloading the transformer will reduce its effectiveness and can lead to overheating. Calculate the maximum possible load, and then add a safety margin (e.g., 20%) to determine the required transformer VA (volt-ampere) rating. You may want to consider future expansions as well.2.

Transformer Location and Enclosure

The transformer needs to be installed in a location protected from the elements. Use a weatherproof enclosure rated for outdoor use. Consider placement for ease of access for maintenance.3.

Proper Wiring and Connections

Use properly sized conductors and make sure all connections are tight and corrosion-resistant. Pay close attention to grounding connections, as they are crucial for safety.4.

Ground Fault Circuit Interrupters (GFCIs)

All circuits on the dock should be protected by GFCIs. GFCIs are designed to detect even small amounts of current leakage to ground and will trip, providing an additional layer of protection. This is an absolute must.5.

Professional Inspection



Crucially, have your installation inspected and approved by a qualified and licensed electrician or electrical inspector. While you have a good understanding of the problem and solution, a professional can verify that the installation meets all applicable codes and safety standards.6. Documentation

Keep thorough records of the installation, including transformer specifications, wiring diagrams, and inspection reports. This documentation will be helpful for future maintenance and troubleshooting.7.

Regular Maintenance

Periodically inspect the transformer, wiring, and grounding connections for any signs of damage or corrosion. Test GFCIs monthly to ensure they are functioning properly.8.

Voltage Monitoring

Consider installing a permanent voltmeter to monitor the voltage between the dock structure and ground. This will allow you to quickly identify any changes in stray voltage levels.9.

Alternative Solutions (Explore, but secondary to your isolation transformer)



Equipotential Plane

While expensive and difficult to implement on existing docks, an equipotential plane is a large, buried grid of conductive material that provides a very low-resistance path to ground. This can help minimize voltage differences in the area.

Improve Utility Grounding (Less likely to be successful)

While less likely to be successful, you could contact your utility provider and ask them to inspect their grounding system in your area. Poor grounding on their side could be contributing to the problem.

Recommended Isolation Transformers

Several reputable manufacturers offer isolation transformers suitable for marine and dock applications. Look for transformers that are specifically designed for outdoor use, marine-grade, and meet relevant safety standards (e.g., UL Listed). Examples (but do your own research): Victron Energy

You're already using Victron, and their isolation transformers are a good choice.

Charles Industries

Charles Industries offers a range of marine-grade isolation transformers.

Xantrex

Xantrex makes inverters and transformers suitable for off-grid and marine applications.

Critical Safety Recommendations (Beyond the Isolation Transformer) Signage

Post clear and conspicuous warning signs on the dock indicating that the electrical system is grounded and that swimmers should avoid touching metal components while in the water.*

Education

Educate dock users about the potential hazards of stray voltage and the importance of following safety precautions.

In Summary

Your approach using an isolation transformer, wired with the secondary neutral bonded to ground at the shore panel, is a sound and recognized method for mitigating stray voltage on docks. Ensure proper sizing, installation, and grounding, use GFCIs on all circuits, and most importantly, have your installation inspected and approved by a qualified electrician. Regular maintenance and voltage monitoring will help ensure long-term safety. With these precautions, you can significantly reduce the risk of electrical shock hazards and galvanic corrosion on your docks. Flag for review