You're right, this is a common and tricky situation. Here's a breakdown of techniques to approach this, combining code compliance, practical considerations, and risk assessment, addressing your concerns

1. Conservative Load Calculations & Code Minimums (NEC 220):
NEC 220.11 (General Lighting Loads)

Start with the code minimums for general lighting and receptacle outlets. This is your absolute floor. Even the most energy-efficient apartments will have some basic lighting load. Apply demand factors as per NEC 220.42.

NEC 220.12 (Small Appliance & Laundry Circuits)

NEC requires at least two 20A small appliance circuits and one 20A laundry circuit per dwelling unit. These are required. Calculate the demand factor per NEC 220.42

NEC 220.14 (Fixed Appliance Loads)

Estimate the average size and power of common fixed appliances most likely to be installed. This includes things like water heaters, dishwashers, garbage disposals, and built-in microwaves. Remember:

Include HVAC Loads

This is critical. Consider window A/C units, portable heaters, or, even better, prepare for split systems.

"Worst Case Scenario" Appliance

Instead of aiming for the average, think about what could be installed. Are electric ranges or dryers likely? Factor in a significant portion of apartments having these.

NEC 220.84 (Optional Method - Dwelling Unit)

You're already using this, but it's crucial to apply it correctly. This method helps account for the diversity of loads in multiple dwelling units.

2. Gather Historical Data (If Possible)



Talk to the Utility Company

See if they have any data on peak demand for similar apartment buildings in the area, especially those with recent renovations. They might be able to provide general averages or ranges.

Review Existing Meter Data

If you can get access to the consumption data from the old meter banks (even if it's only for a short period), that will be helpful. You can't rely on it directly for future loads, but it gives you a baseline.

Surveys

If it's feasible, survey the current residents about their appliances and energy use. This won't predict the future, but it gives you a sense of current usage patterns.

3. "Tiered" Approach to Capacity



Base Load (Floor)

This is calculated from the absolute minimum NEC requirements. This is the load you cannot go below.

Likely Renovation Scenario

Assume a reasonable level of renovation. For example: 50% of apartments get upgraded kitchens with new appliances (dishwasher, microwave, upgraded refrigerators). 25% of apartments get full HVAC (split systems or high-capacity window units). 10% of apartments get electric ranges or dryers.

High-End Renovation Scenario

This is your "worst-case" estimate. Assume a significant number of apartments will go all-electric, with electric ranges, dryers, water heaters, and high-end HVAC.

4. Capacity Buffer & Future-Proofing



Add a Safety Factor

After calculating your load for each scenario (base, likely, high-end), add a buffer. 15-25% is common. This accounts for unknown variables and future technological advancements (e.g., EV charging stations).

Consider Spare Capacity

If possible, include additional spare capacity for future expansion or unexpected needs. This can be in the form of empty conduit and space for additional breakers in the meter bank.

5. Communication and Planning



Landlord/Owner Communication

Explain the different scenarios and the rationale behind your chosen capacity. Make sure they understand the potential consequences of undersizing (overloading, nuisance tripping, limitations on renovations).

Consider Staged Upgrades

If the initial budget is tight, explore the possibility of planning for future upgrades. For example, installing larger conduit now, even if you don't pull all the conductors, can make a future expansion much easier and cheaper.

6. Document Assumptions & Justifications



Detailed Load Calculations

Keep detailed records of all your load calculations, including all assumptions, demand factors, and sources of information.

Document Rationale

Explain why you chose a particular safety factor or renovation scenario.

This protects you if issues arise later.Addressing Your Specific Concerns



"Each apartment renovation is different"

You can't predict every renovation, but you can model common scenarios and their associated loads. Focus on the likely range of renovations and ensure the meter bank can handle the upper end of that range with a safety buffer.

Space Constraints

If space is limited, prioritize the most critical factors: Adequate amperage for likely renovations + safety factor. Sufficient spare breaker space for future loads. Conduit sized for future expansion.

Rule of Thumb

Your "rule of thumb" of 15-25 apartments per 600A meter bank is a starting point, but it should be validated with load calculations and historical data (if available). Don't blindly rely on it without considering the potential for increased loads due to renovations.

Future EV Charging

It's becoming increasingly common to plan for EV charging capabilities. Consider adding additional service capacity to accommodate EV charging stations in the future.

Example Scenario

Let's say you're installing a meter bank for 20 apartments.1.

NEC Minimums

You calculate the minimum lighting, receptacle, and small appliance loads per NEC.2.

Likely Renovation

You assume 50% of apartments will get new appliances, 25% will get HVAC, and 10% will get electric ranges. You estimate the load increase for each of these renovations.3.

High-End Renovation

You assume a smaller percentage (e.g., 10%) could go "all electric."4.

Total Load Calculation

You calculate the total load for each scenario (minimum, likely, high-end).5.

Safety Factor

You add a 20% safety factor to the "likely" scenario.6.

Meter Bank Sizing

You select a meter bank with the capacity to handle the "likely" scenario + safety factor, and consider if there is extra breaker space.

Key Takeaways



Err on the Side of Caution

It's always better to oversize the meter bank slightly than to undersize it. The cost of upgrading a meter bank later is significantly higher than the initial cost of a larger unit.

Document Everything

Keep detailed records of your calculations and assumptions.

Communicate with Stakeholders: Make sure the building owner understands the risks and benefits of different capacity options.By combining conservative load calculations, historical data, a tiered approach to capacity, and clear communication, you can make informed decisions about meter bank sizing that minimize risk and maximize future flexibility. Remember to consult with a qualified electrical engineer if you have any doubts or concerns. They can provide expert advice and ensure your design meets all applicable codes and standards. Flag for review