Providing mechanical protection for branch circuits supplying critical loads in a hospital is extremely important. Any potential damage to these circuits could cost lives. Patients rely on life support equipment and other electro-medical devices as part of their treatment, and any interruption of power could have dire results.
Section 517.30(C)(3) in the 2002 National Electrical Code (NEC) requires emergency branch circuits to be mechanically protected by installation in a nonflexible metal raceway or cable with minor exceptions. While this seems to be a fairly simple statement, it is not an easy task to execute. When reviewing the types of wiring methods to provide mechanical protection, the issue becomes difficult.
Can flexible metal conduit or armored cable provide sufficient protection for emergency circuits or would installing the circuit in electrical metallic tubing or rigid metal conduit provide better protection? Is it even possible to use nonflexible metal raceways in all installations or is some flexibility necessary in some situations? The electrical and the medical community must consider these and other questions before relaxing any requirements that might already have been in place in the NEC.
These questions were raised by various proposals and comments for the 2005 NEC process that proposed relaxing the requirements for only nonflexible metal raceways or mineral insulated (MI) cable. Panel 15 has jurisdiction over the requirements in Article 517 covering electrical installations in healthcare occupancies but must rely heavily on the requirements in NFPA 99, the Standard for Health Care Facilities for guidance pertaining to healthcare applications and NFPA 110, the Standard for Emergency and Standby Power Systems.
While the NEC handles the electrical installation requirements for healthcare occupancies and emergency power used within these facilities, the other two NFPA committees provide the performance and maintenance requirements. Before a change can occur in Article 517, a change may be required in NFPA 99 or NFPA 110.
Upon examining the requirements in 517.30(C)(3) in the 2002 NEC, it is clear that providing nonflexible metal raceways or MI cable is the preferred method of protection. There are five exceptions to this rule where certain installations and applications might permit an alternate wiring method from the general rule. In the 2005 NEC, these five exceptions were converted into positive text, and some additional wiring methods were added. Let’s cover each exception in the 2002 NEC and any changes in the wiring methods permitted by the 2005 NEC.
The first exception permits flexible power cords for appliances or other electrical equipment to not be enclosed in a raceway. Requiring a raceway to enclose the power cords of an appliance would defeat the portability of a cord-and-plug connection.
The second exception permits communications or signaling circuits powered by the secondary side of a transformer to not be enclosed in a raceway, although there may be requirements to provide additional protection for these circuits in Chapters 7 or 8. An example of this extra protection requirement is found in Section 725.11, covering physical protection of remote-control circuits for safety-control equipment where failure of the equipment to operate introduces a direct fire or life hazard.
The third exception permits Schedule 80 rigid nonmetallic conduit where it is not prohibited elsewhere in the Code. Remember, nonmetallic raceways cannot be used for branch circuits that supply patient care areas since Section 517.13 requires a metal raceway and a separate insulated equipment-grounding conductor as a redundant path for grounding.
The fourth exception permits Schedule 40 rigid nonmetallic conduit or electrical nonmetallic tubing where encased in not less than two inches of concrete but with the same requirement of not supplying a patient care area. In the 2005 NEC, flexible nonmetallic, jacketed-metallic raceways or jacketed-metallic cable assemblies that are listed for installation in concrete have been added to the acceptable wiring methods with the same redundant path restriction.
Finally, the fifth exception permits flexible metal raceways and cable assemblies to supply emergency circuits in listed prefabricated medical headwalls, in listed office furnishings or where necessary for flexible connection to equipment.
In the 2005 NEC, flexible metal raceways and listed metal-sheathed cable assemblies enclosing emergency circuits can now be fished into existing walls or ceilings where not subject to physical damage and, after installation, these wiring methods are not otherwise accessible. Since flexible metal conduit is not an acceptable grounding path in lengths longer than 6 feet, it would not meet the requirements in 517.13 for redundancy and would be limited to 6 feet or shorter with an additional insulated equipment-grounding conductor. EC
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 at email@example.com.