You're right to question this! It's a source of confusion for many. Here's a breakdown of why the NEC handles multifamily dwellings differently under the Optional Calculation Method (220.84) compared to the Standard Calculation Method

Key Differences and Reasoning
Optional Calculation (220.84)

Specifically designed for

multifamily dwellings (apartment buildings, condos, etc.). It allows for a more lenient calculation based on actual observed load profiles of similar buildings. The rationale is that not every unit uses all its appliances at the same time, leading to diversity. This often results in a smaller service size. Standard Calculation Method (Article 220, various parts)

A more conservative approach. It calculates the load based on maximum potential usage, assuming simultaneous operation of many appliances. It applies to

all types of occupancies, including single-family homes, commercial buildings, and, yes, could be used for multifamily dwellings.

Why the Explicit Section Under 220.84? Specificity and Simplicity

The NEC provides a dedicated section for multifamily dwellings under the Optional Method because it provides a streamlined and simpler method to calculate the service load. Without it, the standard calculation can lead to oversizing.

Recognizing Diversity

The Optional Method recognizes that in multifamily dwellings, there is a degree of load diversity (not all units are consuming peak power at the same time). The Standard Method doesn't inherently account for this.

Real-World Data

The Optional Method is based on empirical data from studies on actual load profiles of similar buildings. This allows for a more realistic (and often smaller) service size.

Code History

The optional calculation method was introduced to address the need for a more accurate load calculation for multifamily dwellings, which were often oversized using the standard method.

Why No Similar Explicit Section Under the Standard Calculation for Multifamily? Universality of the Standard Method

The Standard Calculation is designed to be a universally applicable method. It works for all types of buildings. There is no need for a separate section specifically for multifamily dwellings under the standard method because it is already designed to handle them.

Application of General Rules

When using the Standard Calculation for a multifamily dwelling, you apply the same rules and tables as you would for any other occupancy (e.g., applying the demand factors in Table 220.42 for ranges and ovens).

Important Considerations Permission

Using the Optional Method usually requires permission from the local Authority Having Jurisdiction (AHJ). They may want to see load studies or other data to justify its use.

Compliance

Regardless of the method used, the calculated service size must be sufficient to handle the actual connected load.

Diversity

The Optional Method relies heavily on diversity. If the building's occupancy or load profile deviates significantly from the assumptions used in the Optional Method, it may not be appropriate.

Feeder and Service Calculations

It's crucial to understand whether you're calculating the load for an individual feeder serving a single unit or the entire service for the whole building. The rules and demand factors are different.

In Summary

You're not missing anything! The NEC has a specific section (220.84) for calculating the service load of multifamily dwellings using an Optional Method because it's a recognized and accepted way to account for load diversity and often results in a more appropriate (and smaller) service size. The Standard Calculation Method can be used, but it will usually result in a larger service size because it's more conservative. There isn't a specific section under the standard calculation dedicated to multifamily dwellings because the standard calculation itself is universally applicable to any type of occupancy.Think of it this way: Standard Method

The "one-size-fits-all" approach.

Optional Method: The "tailored" approach for multifamily dwellings, based on observed load characteristics. Flag for review