National Electrical Code (NEC) Section 230-2 states that a building or other structure served shall be supplied by only one service, with certain limited exceptions. This generally prohibits the installation of more than one set of service drop or lateral conductors to a building or structure, including multifamily and other multi-occupancy buildings.
The following definitions, based on NEC Article 100, are relevant to this requirement:
Adjoining dwelling units or other types of occupancies may be considered to be separate buildings or structures only where the separating walls between the occupancies meet the requirements for separation of adjoining structures, as set forth in the Uniform Building Code, which is adopted by reference in the State Building Code (SBC). The SBC sets fire resistance requirements for outer walls of structures which are dependent on the distance of the structure from the property line, including zero lot line conditions where structures adjoin the lot line. Where two structures with zero lot lines adjoin each other, the structures are required to be separated by two fire rated wall assemblies, one on each side of the property line. In the case of adjoining occupancies on the same property, a property line may be assumed to exist between each occupancy and those occupancies considered to be adjoining separate structures if the adjoining parts of the structures have the required fire resistance rating. A separate service drop or lateral is permitted to serve adjoining structures only where the fire-resistance rating requirements have been met.
The following types of separations may exist between adjoining occupancies:
Zero lot line single family attached Group R, Division 3 (R-3) occupancies or "townhouses", must have one-hour fire resistive walls on each side of the of the property line for each occupancy. Protection of openings is required. In this case each R-3 occupancy is a separate building. Example: A structure with four Group R-3 single family dwellings appears to be a single building but is actually four separate buildings on separate lots, which are attached at the foundation, exterior finish, and roof. Occupancies separated by exterior wall protection to property lines are separate buildings and are permitted to have separate service drops or laterals. Electrical wiring for or from one building is not permitted to pass through nor enter another building.
Two or more service drops or laterals are permitted to serve one building or structure only if the installation conforms to one or more of the Exceptions 1 through 7 to Section 230-2. Where an exception requires special permission, such permission may only be granted by written request to the Board of Electricity or other electrical inspection authority having jurisdiction, and will be granted only where there is a substantial reason to do so.
NEC Section 230-40 allows a service drop or lateral to supply only one set of service entrance conductors, with two exceptions. Exception No. 1 allows one set of service entrance conductors to be extended to each occupancy or several occupancies of a multiple occupancy building. Exception No. 2 allows one set of service entrance conductors to be extended to each of several service equipment enclosures that contain no more than a combined total of six service disconnects.
To summarize key points:
1. Only one service drop or lateral is allowed to serve a building, unless the installation falls under one or more of the Exceptions 1 through 7 of NEC Section 230-2.
2. Occupancies that are cut off from adjoining structures by fire walls as discussed above may have separate service drops or laterals extended to them.
3. Multiple occupancy buildings may have service entrance conductors extended from the service drop or lateral to one or several occupancies, but to only one location in each occupancy.
4. A service is permitted to have no more than six service disconnects grouped in one location.
5. The terms "condominium" and "townhome" used to describe properties are marketing terms and do not consistently indicate that separate adjoining structures are present.
6. Occupancies within structures meeting the SBC requirements for and designated as R-3 occupancies are one- or two-family dwellings. Each R-3 occupancy is a separate building and is required to have its own service, which must be a single service drop or lateral or not more than two sets of service entrance conductors.
7. Structures designated as R-1 are buildings comprised of three or more dwelling units. Each occupancy of an R-1 building is within the same building, and only one service drop or lateral is permitted to serve the entire building unless one or more of the exceptions to NEC Section 230-2 apply.
8. Unless a multi-occupancy building has management supervisory personnel continuously on-site, each tenant must have ready access to the service equipment serving their space. In all cases, each tenant must have ready access to the circuit breakers or fuses for wiring in their space. Circuit breakers or fuses for an occupant's space is not permitted to be located in another tenant's space.
These requirements apply to residential and non-residential properties, including strip malls, row housing, condominiums, townhomes, and multi-occupancy commercial/industrial office/warehouse buildings. Compliance problems may often be avoided by providing a common readily accessible service to serve the entire building.
Note that NEC Section 300-21 requires penetrations of fire-resistive assemblies to be firestopped using approved methods.