In the 1996 National Electrical Code (NEC), Section 225-8, the requirements for installing a separate disconnecting means for a branch circuit or feeder to a separate building or structure on the same property were fairly simple.
If a separate building required electrical power, a disconnecting means was required at the separate building or structure to disconnect the ungrounded (hot) conductors feeding the building. The disconnecting means could be located either outside or inside the building at a readily accessible point nearest the point of entrance of the conductors into the building. The number of branch circuits or feeders to the separate building or structure was not limited, but the number of disconnects at each location could not exceed a maximum of six.
There were a number of exceptions to these general rules. For example, large-capacity multi-building industrial installations under single management could have the disconnecting means for the separate building(s) located at some remote location, rather than at the building(s). These industrial facilities needed a safe disconnect procedure to disconnect power to the building(s).
Another exception permitted lighting poles or standards to have the disconnecting means located remotely from the poles, even though many considered the poles to be separate structures. This allowed the disconnecting means to be located at a safe and secure location, rather than one subject to vandalism or tampering. If the building was supplied by more than one feeder or branch circuit, or any combination of branch circuits, feeders, and services, a permanent plaque was required to be installed at each feeder and branch circuit disconnect location denoting all other feeders, branch circuits, or services supplying the building.
There were two exceptions where a plaque was not required:
- for large capacity multi-building installations under single management where qualified plant personnel accomplished a safe switching procedure, and
- for branch circuits installed from a dwelling unit to a second building or structure. The owner would be familiar with the location of disconnects for all buildings on the property.
In 1999, Article 225 of the NEC was significantly revised to treat feeders and branch circuits to separate buildings or structures more like electrical services, as covered by Article 230. These new requirements are located in a new Part B in Article 225.
The general rule of permitting any number of branch circuits or feeders to a building or structure has been changed to permit only one branch circuit or feeder to a building, unless there are special conditions, special occupancies, or other special considerations as follows:
* For fire pumps, emergency circuits, standby power systems, or for parallel power production.
* By special permission, to supply multiple occupancy buildings where space is not available to locate the disconnecting means at a readily accessible location for all occupants.
* Where the building or structure is sufficiently large to make more than one power supply necessary or where the amount of current required is in excess of 2000 amperes. The size and capacity specifications in these requirements will likely limit the use of this exception to extremely large facilities.
* For additional buildings or structures that require different voltages, frequencies, or phases or have different uses for the power, multiple branch circuits or feeders. For example, a feeder supplying 277/480 Volt, three-phase power and a feeder supplying 120/208 Volt, three-phase power could be installed to additional buildings.
* For multiple buildings that are under single management, if a procedure is developed and maintained that allows the buildings to be safely shut down in an emergency situation.
The disconnecting means required for each branch circuit or feeder must consist of no more than six switches or circuit breakers mounted in a single enclosure, in a group of separate enclosures or in or on a switchboard. Each switch or circuit breaker must be marked to identify its load.
If a circuit passes directly through a building without supplying loads within that building, a disconnect must be provided for the feed-through circuit at the second building, plus an additional disconnect for the circuit as it enters the third building. Again, a plaque or directory must be provided so that power shutdown can occur.
Certain disconnects, such as those for fire pumps, emergency, legally required standby, and optional standby systems, are required to be remotely located from those for normal power circuits to ensure that these critical systems are not inadvertently shut down with the normal power disconnects. The locations of these “emergency” disconnects must be indicated on the plaques or directories provided to ensure that power can be turned off for the entire building when necessary.
Many other changes in Part B of Article 225 are also very important. Be sure to review all of Part B before designing or installing power circuits for a multiple building installation.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at email@example.com.