Main Rule:
A building or structure shall be supplied by only one service unless permitted under specific exceptions in 230.2(A) through (D).
Why This Matters:
The default rule is meant to:
Simplify electrical service design
Improve safety by consolidating disconnects
Prevent multiple points of utility feed that may confuse first responders or maintenance crews
However, NEC 230.2 recognizes that some buildings or circumstances require more than one service, and it provides four main exceptions:
| Exception | Description | Typical Use Case |
|---|---|---|
| (A) Special Conditions | Firewalls, different voltages, emergency systems | Hospitals, large commercial campuses |
| (B) Special Occupancies | Multiple occupancies like malls, office buildings | Multi-tenant buildings with no shared service area |
| (C) Capacity Requirements | A single service is insufficient for the building’s load | Industrial plants, high-rise buildings |
| (D) Different Characteristics | Services with different voltages, frequencies, or phases | Data centers, mixed-use electrical needs |
Two-Family Dwellings and 230.2, and 100(Fire wall creates separate building)
Per Definition of “Building” (Article 100) A building is defined as: A structure that stands alone or is separated from adjoining structures by fire walls.
This means that if one physical structure is divided by code-compliant fire walls, each section is considered a separate building under NEC.
Exhibit 230.6 illustrates this clearly:
The building is physically continuous, but internally separated by fire walls.
These fire walls legally create four separate buildings.
Therefore, per NEC 230.2, four separate services (with separate service laterals and equipment) are allowed.
Each of these “separate buildings” is permitted its own set of service-entrance conductors per NEC 230.40.
A two-family dwelling (e.g., a duplex) may fall under definition of building if internally separate by firewall 230.2(B) – Special Occupancies, if each unit is considered a separate occupancy and there’s no common accessible area for shared service equipment.
Then, per 100 (Fire wall creates separate building) or 230.40 Exception No. 1, we are allowed a separate set of service-entrance conductors for each occupancy, if compliant with 230.2.
| Code Section | Requirement | Explanation |
|---|---|---|
| Article 100 | Fire wall creates separate building | A properly rated fire wall creates a new “building” per NEC |
| 230.2(A)(1) | Separate services permitted for separate buildings | As each fire-wall-divided section is a “building,” each can have its own service |
| 230.40 Exception No. 1 | One set of service-entrance conductors per occupancy/building | Allows multiple sets if each occupancy qualifies per 230.2 |
| Section | Main Rule | Exceptions Allow |
|---|---|---|
| 230.2 | One service per building | Four types of exceptions (A–D) |
| 230.40 | One set of service-entrance conductors | But allows more for multiple occupancies |
| Application to Two-Family Dwelling | One service typically | Two services allowed under 230.2(B) and 230.40 Exception 1 |