Okay, let's break down this boat lift load calculation challenge and see if we can arrive at a defensible solution. You're right; simply treating the motor as an outlet and applying 555.6 doesn't feel right either. 555.6 is specifically for shore power receptacles.

Understanding the Problem Existing System

You're being asked to provide a load calculation for an existing system, even though you're only changing sub-panels. This is often a trigger to ensure the existing feeder is adequate.

Boat Lift Motors

The core issue is how to determine a realistic demand factor for multiple boat lift motors, which likely won't all be operating simultaneously.

Cost Impact

A feeder upgrade is prohibitively expensive, making a reasonable load calculation crucial.

Here's a strategy to approach the load calculation, combining Code with practical considerations



1. Code Foundation (NEC Article 430)



430.24 (Conductor Ampacity)

As you've noted, this is the starting point. You've calculated it correctly. 8.625A + (25 6.9A) = 181.125A. This is the maximum theoretical load if all motors were running at their full-load current simultaneously. This is highly unlikely.

430.26 (Feeder Demand Factor)

This is the key to getting a more realistic number. It states the AHJ (Authority Having Jurisdiction - the permitting department) can grant permission for a reduced feeder ampacity if it's unlikely all motors will operate at once. This is where the argument for a demand factor comes in.

2. Building a Case for a Demand Factor (This is where you need to convince the AHJ)



Operational Characteristics

This is where you collect data and make your argument. Consider these factors:

Usage Patterns

How often are the boat lifts actually used? Talk to the marina/dock owner. Are most used only on weekends? Are usage times staggered?

Lift Time

How long does it typically take to raise or lower a boat? You mentioned 3 minutes, which is a good starting point, but confirm this.

Diversity

Are there peak times? For example, are more boats lifted on Friday evenings, and then lowered on Sunday evenings?

Interlocking/Control Systems (if any)

Is there any system in place to prevent multiple lifts from operating simultaneously (unlikely, but worth checking)?

Realistic Scenario

Based on the above, develop a scenario of the maximum probable simultaneous operation. For example: "Based on observation and discussions with the dock owner, the maximum realistic scenario is that 3 boat lifts might be operated simultaneously for a maximum of 3 minutes, with the rest idle."

3. Load Calculation with Demand Factor


Identify the largest motor

In your case, it's still a 3/4 HP motor. FLA = 6.9A. 125% FLA = 8.625A.
2.

Determine the number of simultaneously operating motors

Let's use the example of 3 simultaneous lifts. That's 6 motors.3.

Calculate the load for the simultaneous motors

8.625A (largest) + (5 motors 6.9A) = 43.125A4.

Calculate the load for the remaining motors (not operating)

You can include a small percentage (e.g., 10-20%) of the remaining motors to account for occasional, short-duration use. Let's say you include 10%: (20 motors 10% 6.9A) = 13.8A5.

Total Calculated Load

43.125A + 13.8A = 56.925A

In this example, the calculated load is 56.925A, which is well within the 100A feeder capacity.4. Documentation and Presentation to the AHJ



Formal Letter

Prepare a formal letter to the AHJ outlining your load calculation.

Justification

Clearly explain the operational characteristics and your rationale for the demand factor. Provide any data you've gathered (e.g., statements from the dock owner).

Code References

Cite NEC 430.26 to support your request.

Safety

Emphasize that you are proposing a safe and reliable installation based on a realistic assessment of the actual load.

Alternative

if they still require a feeder upgrade, have a plan b with the customer on what the cheapest option would be, such as only replacing a portion of the feeder

Important Considerations



AHJ Discretion

Ultimately, the AHJ has the final say. Be prepared to negotiate and compromise.

Overload Protection

Ensure that the overcurrent protection for the feeder is sized appropriately based on your calculated load.

Future Expansion

Consider whether there are any plans for future expansion or increased usage. If so, you may need to be more conservative in your demand factor.

Voltage Drop

Always check voltage drop calculations to ensure adequate voltage at the furthest boat lift motor.

Why the AHJ is Asking



Safety

The AHJ is primarily concerned with safety. They want to ensure that the feeder is not overloaded and that the electrical system is adequate for the intended use.

Liability

The AHJ also wants to avoid liability. If a fire or other electrical problem occurs, they want to be able to demonstrate that they followed the Code and took reasonable precautions.

In Summary:

You need to build a strong, data-driven case to justify a demand factor to the AHJ. Focus on the actual* operational characteristics of the boat lifts and present a realistic scenario of simultaneous usage. This is the only way to avoid an expensive and potentially unnecessary feeder upgrade. It might be good to schedule a meeting with the inspector to talk this out in person. Flag for review