Okay, this is a great question and gets to the heart of applying demand factors correctly. You're right to be thinking critically about how these factors are used.

The Correct Approach: Apply the Demand Factor to the Total Connected Load

Your intuition is correct. You need to:1. Calculate the Total Connected Load

156 kVA/building 5 buildings = 780 kVA2.

Apply the Demand Factor from Table 220.84 for 20 dwelling units

780 kVA 0.38 = 296.4 kVA

Explanation and Why Your First Option is Correct



Demand Factors Account for Diversity

Demand factors are designed to account for the fact that not all appliances and loads in all dwelling units will be operating at their maximum rating simultaneously. The more units you have, the higher the statistical likelihood that usage will be staggered, and the lower the overall peak demand relative to the total connected load.

Transformer Perspective

The transformer "sees" the aggregate load of all five buildings. It doesn't know which building is drawing power, only the total current it's supplying. Therefore, you need to consider the total connected load before applying the demand factor.

Table 220.84 is Aggregated Data

The demand factors in Table 220.84 (and similar tables) are based on real-world measurements of multiple dwellings. They represent the statistical reduction in load observed across all the units, not a reduction within a single unit that is then multiplied out.

Why Multiplying the Demand Load for a Single Building is Incorrect



Double-Dipping

Multiplying the already demand-factored load of one building by the number of buildings would be like applying the demand factor multiple times. You've already accounted for some diversity within the single-building calculation using 220.84. Applying it again, by simply multiplying the result, wouldn't be statistically sound.

Underestimation

It could lead to significantly underestimating the peak demand on the transformer.

In summary

You're right. Calculate the total connected load across all buildings, then apply the demand factor from Table 220.84 that corresponds to the total number of dwelling units (20 in your case).

Additional Considerations



Feeder Calculations

Remember that once you determine the transformer demand, you'll still need to perform separate feeder calculations for the service conductors to each individual building. While the transformer is sized based on the diversity of all 20 units, the feeders to each building need to handle the potential load of that specific building (using its individual demand calculation, 70.2 kVA in your example, but you still need to check the results of your calculations).

Future Expansion

Consider any potential future expansion or increased load in the dwelling units. It's prudent to add a safety margin to your transformer size.

Motor Loads

Be mindful of any significant motor loads (e.g., HVAC systems) in the dwelling units. Motor starting currents can briefly create higher demand than the calculated demand.*

Local AHJ: It's always a good practice to discuss your load calculations with the local Authority Having Jurisdiction (AHJ) to ensure they are comfortable with your approach. They may have specific interpretations or requirements.I hope this clarifies things! Good luck with your project. Flag for review