The five exceptions to providing nonflexible metal raceways
Providing alternate power for essential electrical circuits in a hospital is extremely important during loss of normal power. Providing protection from damage will provide additional assurance of the continuity of power to circuits that are vital to the protection of life and safety in a hospital.
An essential electrical system is defined in the National Electrical Code as a system of alternate sources of power and all connected distribution systems and ancillary equipment, designed to ensure continuity of power to designated areas and functions of a healthcare facility during disruption of normal power sources, and designed to minimize disruption within the internal wiring system.
The essential electrical system for a hospital is made up of two separate systems capable of supplying a limited amount of lighting and power for the operation of equipment necessary for the life safety of patients and for the effective operation of the hospital. The two separate essential systems are the equipment system and the emergency system.
The equipment system supplies major electrical equipment for basic hospital operation and power for electrical equipment for patient care. Some examples of equipment connected to the essential equipment system are central suction or compressor air systems serving medical and surgical functions, smoke and stair-pressurization systems, and heating or cooling equipment for general or critical care patient areas.
The emergency system has been divided into the life safety branch and the critical branch. The life-safety branch supplies power to electrical circuits that are necessary for illumination for corridors, passageways, stairways and landings at exits from the building. Fire alarm systems and medical gas alarms are connected to the life-safety branch, as well as hospital communications systems used for instructions during emergency conditions. Cab lighting and the control, communications and signaling systems for elevators can be connected to the life-safety branch since one or more elevators may be used for patient evacuation during a fire or other catastrophe. Automatic electric doors have been added to this list of life-safety-branch equipment, since having power to the automatic doors will help in evacuation.
The critical branch supplies power for task illumination, selected receptacles, fixed electric equipment and special power circuits serving patient care areas, as well as special environments and functions related to patient care in the hospital. Operating rooms, postoperative rooms, medication preparation areas, pharmacy dispensing areas and general and critical care patient bed locations are just a few locations within a hospital with emergency circuits supplying receptacles and task lighting.
Section 517.30(C)(3) requires the wiring of both the life-safety and the critical branch of the emergency system to be mechanically protected from potential damage by installing the emergency feeder and branch circuits with mineral-insulated, metal-sheathed cable (Type MI cable) or in nonflexible metal raceways. Electrical metallic tubing (EMT), rigid metal conduit (RMC) and intermediate metal conduit (IMC) are a few examples of the more common metal raceways used for protection of these most important power systems.
There are five exceptions to this general rule of providing nonflexible metal raceways for physical protection of these circuits. Where the cords are supplying emergency power to appliances or other electrical utilization equipment, flexible cords are not required to be installed in raceways. Any signaling systems or communications systems powered by the emergency system but supplied by the secondary of a transformer does not require a raceway, unless required by Chapters 7 or 8 of the NEC. Flexible metal raceways and flexible cable assemblies can also be used in listed prefabricated medical headwalls at patient beds, in listed office furnishings or where necessary for flexible connection to equipment. Always ask the inspector for special permission in writing to use a flexible metal raceway or cable assembly before connecting to electrical equipment since a judgment call will be necessary for a final determination of flexibility.
The last two exceptions, involving the use of rigid nonmetallic conduit (RNC), need clarification. The text almost seems to imply that neither the feeder conductors nor the branch circuit conductors can be installed in nonmetallic conduit where the branch circuits supply power to fixed electrical equipment or receptacles in a patient care area. Branch circuits supplying fixed electrical equipment or receptacles in a patient care area must have redundant grounding, which cannot be accomplished using nonmetallic conduit. This restrictive text only applies to the branch circuit for a patient care area in accordance with Section 517.13 and not to the feeder.
A change in the 2005 NEC will clarify that only the branch circuit supplying power to fixed electrical equipment and receptacles must comply with the grounding redundancy in 517.13. Feeders can be installed in nonmetallic conduit if the restrictions in the two exceptions are followed. 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.