You're right to be curious about the demand factors in Table 220.84(B). They seem counterintuitive at first glance, but they're based on years of statistical data and real-world observation of how household appliances are actually used. Here's a breakdown of the thought process behind those demand factors and why they decrease as the number of units increases

The Underlying Principle: Diversity and the Law of Large Numbers

The core concept at play is diversity of use. Think about it: One Unit

If you only have one electric range, oven, or cooking appliance, you have to assume it's going to be used frequently and potentially at its maximum capacity. There's no way to spread the load out. You must plan for its full load demand.

Multiple Units

However, when you have multiple units within a dwelling (or multifamily complex), the probability that all of them will be running at their maximum load simultaneously decreases significantly. People cook at different times, use different appliances, and for different durations.This principle is related to the

Law of Large Numbers. As you add more units, the average load becomes more predictable. The extremes (all appliances running at max) are less likely to dominate the overall demand.

Factors Driving the Decreasing Demand Factors1. Coincidence Factor

This is the heart of the matter. It's the statistical probability that multiple units will be operating at peak load at the exact same time. With one or two units, the coincidence factor is relatively high because it's not uncommon for both to be used at or near their full capacity simultaneously (even if only for short periods like preheating an oven). As the number of units increases, the coincidence factor drops. It becomes statistically less likely that all of them will be drawing their maximum power concurrently. People have different cooking schedules, different cooking styles, and will use different appliances.2.

Usage Patterns

Studies have shown that the overall usage of cooking appliances (including ranges, ovens, cooktops, etc.) in multi-family dwellings is more diverse than in single-family homes. This translates to less peak demand per unit.3.

Diversity of Appliances

Multifamily dwellings are more likely to contain a wider variety of cooking appliances (microwave ovens, slow cookers, air fryers) in addition to the standard range. These appliances often have lower power demands compared to a full-size range.4.

Statistical Modeling

The demand factors in Table 220.84(B) aren't just pulled out of thin air. They're derived from statistical analysis of actual electrical usage data collected from numerous multifamily buildings over extended periods. Engineers and statisticians analyze the data to determine the maximum demand likely to be experienced for different numbers of units.

Why Not Just Use Individual Appliance Ratings?

Ignoring the diversity of use and simply summing the individual ratings of each appliance in each dwelling would lead to significant overestimation of the actual load. This would result in: Oversized and Over-Expensive Electrical Service

Larger transformers, heavier wiring, and higher capacity distribution equipment would be needlessly installed.

Increased Cost for Consumers

Apartment complexes would have higher operating costs due to the larger electrical infrastructure, which would likely be passed on to tenants.

In summary: The decreasing demand factors in Table 220.84(B) are a practical application of statistical probability and observed usage patterns. They acknowledge that the likelihood of all cooking appliances in a multifamily dwelling being used at their maximum capacity simultaneously decreases as the number of units increases. This allows for a more accurate and cost-effective estimation of the electrical demand, without compromising safety. Flag for review