Okay, I understand. You're referencing Section 220.82 of the 2008 National Electrical Code (NEC) which deals with an optional calculation method for determining the service or feeder load of a dwelling unit. You specifically are asking about the condition requiring a single 120/208 or 120/240V, 3-wire service or feeder with a minimum ampacity of 100 amps.To clarify, you're likely asking a question about eligibility for using the optional calculation method. Here's a breakdown of the key points related to this requirement and why it's important: Eligibility Requirement

Section 220.82(A) of the 2008 NEC, and similar sections in subsequent editions, requires the entire connected load of the dwelling unit to be served by a single 120/240V or 120/208V, 3-wire service or feeder. It also stipulates a minimum ampacity rating of 100 amps for that service or feeder.

Why this requirement exists

The optional calculation method is a simplified approach. It relies on certain assumptions about the diversity of loads within a typical dwelling. To ensure these assumptions remain valid and a reasonably sized service or feeder is provided, the NEC limits its application to relatively "normal" single-family homes, as determined by voltage and minimum service size. If you have a very small dwelling, or a very large one that may require a service larger than the calculation will provide, this standard may not be appropriate.

Understanding the 3-Wire Service

The 3-wire service (or feeder) is crucial. It consists of two ungrounded (hot) conductors and one neutral conductor, allowing for both 120V and 240V (or 208V) circuits.

"Entire Connected Load"

This means everything connected to the electrical system within the dwelling unit must be supplied by this single service. This includes: Lighting Receptacles Appliances (ranges, ovens, water heaters, dryers, air conditioners, etc.) Electric space heating

Why 100 Amps Minimum? The 100-amp requirement represents a reasonable minimum capacity for a dwelling unit capable of supporting a range of typical household appliances and loads.To help me understand your question more specifically, please tell me: What are you trying to determine? Are you trying to figure out if a specific dwelling unit qualifies for the optional calculation? What specific load is causing you concern? Is it a large appliance, electric heat, etc.? What is the existing (or proposed) service size and voltage? Is it actually 100 amps (or more)? Is it 120/240V or 120/208V?The more details you provide, the more helpful I can be in addressing your specific situation. Flag for review