Four-wire, 0–10V dimming is emerging as the industry standard for control of electronic lighting. The addition of low voltage wires to the line voltage conductors powering the fixtures requires considerations that were not necessary with traditional two-wire dimming. Per the NEC, there are different classifications and installation requirements for wiring of different systems. The purple and gray conductors of a 0–10V dimming system are typically considered low voltage and can be installed like other low voltage wiring, without the need to meet the requirements for line voltage wiring. However, when low voltage wiring is installed in conduit or enclosures with line voltage wiring, all conductors must meet the installation requirements of line voltage wiring for the entire length of the circuit.
This is particularly relevant with wall-box 0–10V dimming devices. These are devices that are installed in place of a standard wall switch that have both line voltage and 0–10V connections in the same junction box. Since the connections are made in the same box, all wiring, including the 0–10V conductors, must be treated as line voltage conductors for the entire circuit run from the dimmer to the light fixture. Due to this installation requirement, UL testing does not preclude AC voltage from being present on the purple and gray dimming conductors. Many manufacturers have devices with non-isolated power supplies which have the result of AC voltage on what are typically considered low voltage wires. This voltage is at a very low current, but will still provide a shock if contact is made with the conductors.
The National Electrical Code (NEC) Article 725 deals with classifications for different wiring systems. In general, there are two classifications that are relevant to 0–10V dimming installations. At a high level, Class 1 is the classification for line voltage conductors carrying current with a voltage up to 600V. Class 2 is for low voltage conductors with low current. Basically, Class 2 circuits do not have high enough voltage or current to present a hazard to personnel and have less stringent installation requirements in regards to protection of the wiring. Class 1 installations require wire with insulation that is rated for the voltage carried (most wiring has 600V rated insulation) and must be installed in conduit or a protective cable assembly. Class 2 wiring may be installed free within the wall or ceiling without line voltage rated insulation or protection of conduit or a cable assembly.
Based on the characteristics of the maximum of 10V DC on the 0–10V dimming lines, they are considered Class 2 which allows free cable installation as mentioned above. There are specific requirements governing separation distances, barriers, and other methods of installation for Class 2 conductors when installed with Class 1 conductors. If Class 2 conductors are to be installed together with Class 1 conductors in a conduit, junction box, or other enclosure, the NEC allows the Class 2 wiring to be reclassified as Class 1. This reclassification involves removing any labels on the conductors indicating they are Class 2, as well as following the installation requirements for Class 1 circuits.
It is important to note that these requirements are for the entire circuit. For instance, it is not allowable to install the 0–10V wiring from the wall junction box to above the ceiling and then install the 0–10V wiring across the ceiling as Class 2 after it has been separated from the line voltage wiring. The rationale is that whenever the conductors occupy the same conduit or enclosure, there is the chance of short which would energize the entire circuit at the higher voltage. Class 2 wiring methods are not safe for conductors that have any chance of carrying line voltage.
Wall-Box 0–10V Dimming Devices
Dimming devices that are installed in the wall are inside a single junction box where the terminations for the line voltage and 0–10V dimming conductors are made. For this application, the 0–10V wires must then be considered Class 1, meeting the same installation requirements as the line voltage conductors. Additionally, most manufacturers note on the device documentation that the 0–10V conductors must be treated as Class 1 and be terminated or capped off prior to energizing the device.
Due to the requirement to treat the dimming conductors as Class 1, it is acceptable, per the NEC and UL, to have AC voltage on these conductors. In testing devices from multiple manufacturers, AC voltage was measured on the 0–10V conductors for most manufacturers. This voltage can be measured between either the purple or gray wire and ground. The 0–10V DC voltage can be considered to float on top of the AC voltage carried on the wire. There is not an AC voltage between the purple and gray wire — they both are at the same voltage when referenced to ground. Since they are at the same voltage, the wires still operate the fixture as intended and do not damage drivers or ballasts, with the exception of drivers that also have a non-isolated power supply. These drivers also will create an AC voltage on the 0–10V conductors. Since the voltage from the wall-box device and the driver is not the same, damage will occur to one or both devices.
Most people would not expect the AC voltage on the purple and gray wires since it cannot be present for Class 2 installations. However, if contact is made with the conductor in this case, a shock will occur. There is no current behind the voltage in this case, so the shock will not be enough to cause any significant physical harm from the electricity. However, if you are on a ladder working in the ceiling, the unexpected shock could be enough to knock a person off the ladder and cause injury. Treating the dimming conductors as Class 1 and following all the same safety precautions as for line voltage conductors will ensure a safe installation.
As the use of low voltage dimming evolves, manufacturers are now working to change their wall-box sensors to have isolated power supplies. This ensures that the sensor will be compatible with all driver types (both isolated and non-isolated). The manufacturers that MH represents are releasing version 2 sensors that no longer have an AC voltage on the 0-10V conductors.
All Class 1 conductors need to be treated as current carrying conductors through all installation practices. This includes not energizing the circuit until all conductors are terminated and not working on the circuit without the power turned off. Many wall-box sensors on the market have an AC voltage on the 0-10V conductors, but whether or not this is the case, low-voltage conductors installed with line voltage conductors should always be considered current carrying to ensure a safe workplace.