LOW VOLTAGE DC LED LIGHTING SYSTEM

SUMMARY

In this white paper we will review the code requirements for installation of a Class 2 Low Voltage Direct Current (DC) LED lighting system and address common questions.

INTRODUCTION

What is low voltage lighting? Low voltage lighting, as defined by the National Electrical Code or the NEC, is comprised of Class 2 circuits. Class 2 circuits are power limited to 100 Watts and can be installed using more relaxed Class 2 wiring methods. This is because they present less of a shock, arc, and fire hazard than traditional higher voltage circuits. Why 24VDC? Power over 60 Volts DC is not Class 2 and would be subject to the same requirements as standard 120VAC circuits. So, you do not receive the benefit of the more relaxed requirements inherent to Class 2 circuits. Systems with voltages lower than 24VDC (i.e. 12 Volt DC systems) generally have greater power losses in the wiring. Utilization of 24VDC thus provides the benefit of Class 2 standards without having to worry about significant losses during transmission. Additionally, there are already many digital devices used within commercial interiors today that utilize 24VDC power – such as occupancy and daylight sensors. Are AC circuits more efficient? Alternating Current (AC) – is the power that comes from standard wall outlets and can be distributed over long distances very efficiently (i.e., from a power station to a home). A low voltage DC circuit that would traverse long distances would be much less efficient. However, the low voltage lighting applications discussed here are not designed or implemented that way. In system configurations at 24VDC, with power distribution in NEC Class 2 limited circuits, DC wiring should be as efficient as high voltage AC at the shorter distances that are found inside a typical home.

Is a class 2 lighting system as reliable as traditional LED lighting?

In 2019 the US Department of Energy (DOE) released its third report on LED driver reliability. The DOE identified that the first stage of the drivers, where the AC to DC conversion takes place, is where most failures occur. By utilizing a Class 2 distributed DC system, the opportunity for such failures is greatly reduced and increase the probability that the LED light fixture would fulfill its long life potential.

Figure 1: Traditional 120VAC System vs Low Voltage 24VDC System

As illustrated above in Figure 1, the typical LED installation has four potential failure points, while the Class 2 Distributed DC System has only one. The driver used for the Distributed DC System is constructed more robustly and is usually comprised of much higher quality components vs a traditional LED commodity lighting fixtures.

DO LOW VOLTAGE SPLICES NEED TO BE MADE INSIDE AN ELECTRICAL ENCLOSURE?

Class 2 Low voltage wiring does not require use of an electrical enclosure, commonly known as a junction box. However, for convenience and for future access it is recommended that they be used. Carlon makes low voltage, semi closed junction boxes that provides a means of attaching a cover, either blank or containing the necessary hardware to connect to the cabling. This also allows for generous bend radii that some structured wiring requires.

Figure 2: Carlon Structured Wiring Electrical Boxes

Since a box is not required, splices can be made in the open. This is not recommended for several reasons: 1) Splices are the weak link in the system so having them protected is advantageous; 2) Having splices in junction boxes will facilitate trouble shooting a system; 3) Facilitates re-routing/adding to/or removing part of the circuit.

In order to differentiate standard AC high voltage systems from Class 2 Low Voltage systems, it is recommended to use different color junction boxes.

The four main types of boxes usually needed for new construction utilizing Class 2 DC Distributive Lighting Systems are:

1. Singe Gang Box – 22 cubic inches

2. Two Gang Box – 32 cubic inches

3. Four-inch Round Ceiling Box

4. 20 cubic inch round ceiling box with bracket

Figure 3: Recommended Box Types (Available from CLEANLIFE)

Can you wire cables and conductors from a 120VAC light in the same enclosure as the Class 2 distributed DC system?

The short answer is NO but there are some exceptions. This is covered in Article 725.136 of the National Electric Code (NEC 70 2020). The most common instance is using a barrier or divider to separate the circuits. As noted in Article 725.136(B): Class 2 and Class 3 circuits shall be permitted to be installed, together with the conductors of electric light, power, Class 1, non-power-limited fire alarm and medium power network-powered broadband communications circuits where they are separated by a barrier, the electrical box must be rated appropriately.

An example of this is shown in Figure 4.

Figure 4: Carlon 2 Gang Box wigh Low Volagbe Divider Plate

Are inspections required?

The National Electric Code (NEC) requires that low voltage power supplies and electrical wiring be Class 2 compliant. The code was created to prevent electrical fires and shock. For these installations inspectors will specifically check to see if the wiring is installed via industry standard practices (per NEC). Basically, the rough-in installation will be reviewed to verify the gauge and rating of the wire are correct for the given application and that it meets all applicable standards.