An increasing number of occupancy sensor switches, involving infrared sensing, are being installed in commercial, residential and industrial installations to cut power usage and comply with energy conservation codes. In the 2005, 2008 and 2011 National Electrical Code (NEC) cycles, Panel 9 has struggled with text dealing with electronic lighting control (sensor) and other devices that require standby current to maintain ready-state function as well as the motion or sensing function of the device. Standby current may be more accurately called operational or functional current, since this current is necessary for the normal operation of the switch or control device.
Can an equipment-grounding conductor run with or enclosing the circuit conductors, such as a metal conduit, tubing or cable assembly, act as the current path for the standby or operational current for the switch, or must an insulated-grounded or neutral conductor be installed at the switch location? Should a grounded or neutral conductor be installed at all switch locations, even where a sensor switch is not initially installed, so a sensor or other switch using functional or standby current could be installed in the future? These questions and many others concerning sensor-switch installations may be answered by a change suggested for the 2011 NEC.
Occupancy sensor switches often use a passive infrared motion and heat detection system (passive means the infrared device does not emit an infrared beam but passively accepts incoming infrared radiation). Many infrared switches also provide passive dual technology (PDT) where the passive infrared device first sees motion and then uses a microphonic sensing device with a filter to reject background noise to detect only human sounds, indicating there is still an occupant within the room. For example, the infrared motion sensor detects the body heat of a person entering the room. After the person exits the room, the detector turns the lights off within a certain time (usually between one and 10 minutes) Many of these sensors have a built-in time delay of 10 to 50 seconds after turning off the lights; this delay permits an occupant who may still be in the room but shielded from the infrared sensor, to make a noise and reactivate the sensor, thus turning the lights back on.
In many residential, commercial and industrial electrical installations, a grounded or neutral conductor is not provided within the switch box, since only the hot and the switched leg is provided. This lack of a grounded or neutral conductor within the switch box requires the sensor-switch installer to provide a new conductor, unless the installer uses the equipment-grounding conductor to carry the standby current, which the NEC does not permit. The standby or functional current is frequently a very low value (typically no more than 0.5 milliamps) but the amount of standby current for each device is additive, based on the number of devices in the circuit. Therefore, the cumulative amount of current on a circuit with 10 devices may be 5 milliamps or more, depending on the function of the devices at a point in time. Permitting 5 milliamps or more on the equipment-grounding system could be hazardous, so Panel 9 has proposed new text and an exception in 404.2(C) for the 2011 NEC to address this problem.
The new text, if passed through the full NFPA process, including any challenges during the NFPA Annual Meeting, will read as follows:
“404.2(C) Switches Controlling Lighting Loads. Where switches control lighting loads supplied by a grounded general-purpose branch circuit, the grounded circuit conductor for the controlled lighting circuit shall be provided at the switch location.
“Exception: The grounded circuit conductor shall be permitted to be omitted from the switch enclosure where either of the conditions in (1) or (2) apply:
“(1) Conductors for switches controlling lighting loads enter the box through a raceway. The raceway shall have sufficient cross-sectional area to accommodate the extension of the grounded circuit conductor of the lighting circuit to the switch location whether or not the conductors in the raceway are required to be increased in size to comply with 310.15(B)(2)(a).
“(2) Cable assemblies for switches controlling lighting loads enter the box through a framing cavity that is open at the top or bottom on the same floor level, or through a wall, floor, or ceiling that is unfinished on one side.
“Informational note: The provision for a (future) grounded conductor is to complete a circuit path for electronic lighting control devices.”
The intent of this change is to provide a grounded-circuit conductor at each switch location within dwelling, commercial or industrial buildings, so occupancy sensors and similar switches that require functional-neutral current for operation can be installed and connected to a grounded conductor and not rely on a connection to a grounding conductor.
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and email@example.com.