Okay, this is a challenging but interesting project! Let's break it down and analyze your sizing concerns. You're smart to be carefully considering voltage drop over that distance. Here's a breakdown of considerations and my thoughts

Understanding the Load and NEC Requirements:
Feeder to Subpanel

The primary feeder that runs out the pier to the gazebo/sub panel is your biggest concern from a voltage drop perspective.

Boat Lifts

Although not powered from the subpanel directly, you still need to account for the load of the boat lifts to determine the overall service size and potential future growth.

Shore Power (Future)

Keep in mind that if you eventually have shore power, you'll need to upgrade your feeder size to accommodate the additional load.

Marina Guard & GFPE

Using Bender's Marina Guard is an excellent choice for GFPE protection. Critical for safety in a marine environment.

Sizing Considerations and Calculations

Here's where the number crunching becomes important. Remember, this is for informational purposes, and a qualified electrical engineer should ultimately sign off on the final design.

Load Calculation



Gazebo Outlets/Lights

Estimate the connected load (in amps) of the outlets and lights at the gazebo. Even if they are rarely used simultaneously, you need a reasonable estimate. Let's assume 20 amps for now as a placeholder. This could be less with LED lighting and limited receptacle usage.

Boat Lifts

Each lift is 15A at 240V. Since you're not powering them directly from the subpanel on the pier, we don't include them in the calculation of the feeder to the subpanel. However, you'll need to account for them when sizing your new service.

Voltage Drop Calculation

Use voltage drop calculators to make sure you are within acceptable limits of 3%. Several online tools can help with this.

Feeder to Subpanel

This is the most important calculation for your question. You're already seeing a 7.2V drop with 250 kcmil. Let's assess the 3/0 THWN option.

3/0 Copper

Assuming a 100A load at 240V, a distance of 670', and using a voltage drop calculator, a 3/0 copper conductor shows to be around a 6% voltage drop! This is way over the 3% allowable limits.

300 KCMIL

This would put you at 3.3% voltage drop for the long feeder run.

Ampacity

Ensure the conductor ampacity is sufficient for the calculated load plus any future expansion plans.

Thoughts and Recommendations


Voltage Drop is King

In this situation, voltage drop is your driving factor. While you might be able to get away with smaller conductors initially based on the very limited gazebo load, it's not a good practice to design at the very edge of the code and you do not want to take away from available voltage to start.2.

Consider 300 KCMIL

The 300 kcmil copper would be recommended for future potential of shore power.3.

Aluminum Consideration

You may want to consider Aluminum for feeder conductors which can bring the cost down.4.

Transformer Option? A step-down transformer at the gazebo could be worth exploring. This would allow you to run higher voltage (and lower current) over the long distance, significantly reducing voltage drop. Then, transform back down to 120/240V at the gazebo. While it adds cost and complexity, it could be more economical in the long run than oversized conductors, especially if you plan for future heavy loads.5. Future Shore Power Planning

The future shore power will require its own feeder.

Other Important Considerations



Marine Environment

Use appropriately rated and listed marine-grade wiring, connectors, and enclosures. Corrosion resistance is paramount.

Grounding/Bonding

Follow NEC Article 555 and Article 250 meticulously for grounding and bonding in a marina environment. This is critical for safety. Pay special attention to equipotential bonding to minimize stray voltage issues.

Overcurrent Protection

Properly size and install overcurrent protection devices (circuit breakers) at both the service entrance and subpanel.

Load Management

Consider the possibility of load management devices if future shore power loads become a concern. This would allow for controlled sharing of power.

In summary, your initial concern about voltage drop is spot on. Oversizing the conductors to minimize voltage drop is crucial for this long run. A 300 KCMIL copper (or equivalent aluminum) is likely necessary. Consult with a qualified electrical engineer who can perform a detailed load calculation, voltage drop analysis, and ensure compliance with all applicable codes and standards. Flag for review