The use of low voltage lighting systems has become increasingly popular over the past several years. However, maybe you are asking yourself "What is low voltage lighting?" or "How to wire low voltage lighting?" We answer these two questions below and tell you 15 things that you should know about low voltage light fixtures.
Low Voltage A low voltage lighting system usually operates on 12 or 24 volts whereas a line voltage system usually operates on 120 or 277 volts. Examples of low voltage lighting are recessed lights (cans), track lights, bendable rail lighting systems, landscape lighting, suspended cable lighting, pendant lights, and low voltage display lights to name several.
Transformer A transformer is generally used with low voltage lighting systems to "transform" the standard line voltage that is normally available (usually 120 volts or 277 volts) to either 12 volts or 24 volts. Without a transformer the low voltage light bulbs in the lighting circuit would be subjected to a voltage of 120 or 277 volts and would, therefore, burn out immediately. Furthermore, the low voltage transformer may be part of the fixture (integral to the fixture) or it may be located outside the fixture (an external transformer).
Type of Transformer The transformer used in a low voltage lighting system may be either electronic or magnetic.
Maximum Transformer Wattage The wattage rating of the 12v transformer should always be equal to or greater than the total wattage of the lighting system. If an electronic transformer is used, then the maximum wattage of the lighting system may be equal to but not greater than the wattage rating of the electronic transformer. If, however, a conventional EI magnetic transformer is used, then the maximum wattage of the lighting system should be equal to but not greater than 80% of the wattage rating of the EI magnetic transformer. This type of magnetic transformer gets its name from the stacks of E-shaped steel sheets capped with I-shaped pieces in the transformer's laminated core. With this design, an EI magnetic transformer is somewhat less expensive to produce and creates additional loss which is why the maximum wattage should not be greater than 80%.
Minimum Transformer Wattage Transformers usually have a minimum wattage that they must power before they work. For example, it is not uncommon for a 60-watt electronic transformer to require that there be at least 10 watts of lighting load and if there are only 5 watts of lighting load connected to this 60W electronic transformer, the lighting system will not work.
Electronic Transformers An electronic transformer is generally lighter in weight, smaller in size, cooler to operate, and quieter than a magnetic transformer. However, electronic transformers cannot provide more than 300 watts of power whereas some magnetic transformers can provide as much as 1200 watts of power.
Voltmeter Readings Because an electronic transformer provides its power at very high frequencies (usually greater than 20,000 Hertz) a standard voltmeter cannot be used to accurately measure the output voltage. Instead, a "true RMS" voltmeter must be used to measure the secondary voltage of an electronic transformer. On the other hand, a standard voltmeter can be used to read the voltage on the secondary side (low voltage side or output side) of a magnetic transformer.
AC Electronic Transformer An AC (alternating current) electronic transformer should not be placed any farther than 10 feet from the low voltage lighting system. If it should be placed farther away than 10 feet, part or all of the low voltage system will experience a lower voltage (known as voltage drop) and have dimmer lamps. Also, the longer the distance from the AC electronic transformer and the lighting system, the greater the chance that it might create radio frequency interference (RFI) with other electronic components in the area.
Toroidal Magnetic Transformer If a magnetic transformer is used to power a low voltage lighting system, a toroidal magnetic transformer should be considered. This type of magnetic transformer is more efficient, lighter in weight, smaller in size, cooler to operate, and quieter than a conventional EI magnetic transformer.
Wiring the Transformer A low voltage transformer usually has 4 connections or 4 wires attached to it. Occasionally, a transformer may also have a green wire that should be attached to "ground". Remember that the power should always be turned off before making any electrical connections to the transformer. The primary side of a transformer is often labeled "Input" and is made up of one black wire and one white wire for the incoming line voltage. Always connect the white wire of the transformer to the white wire (neutral wire) of the incoming line voltage and the black wire of the transformer to the black wire (hot wire) of the incoming line voltage. The secondary side of a transformer is often labeled "Output" and is made up of two similarly colored wires (often two red or two blue wires) and they will supply power to the low voltage circuit. The polarity of these wires is not important; that is, either wire of the secondary side of the transformer may be connected to either wire of the low voltage circuit.
Wiring a Dimmer A low voltage light fixture can be dimmed or controlled whether it is powered by an electronic or a magnetic transformer. When using a dimmer (or an on/off switch or an occupancy sensor or a timer) with low voltage lighting the control device should always be installed so that it is controlling the line voltage side of the transformer, which is also called the primary side (input side) of the transformer.
Type of Dimmer The dimmer selected to control an electronic transformer should be specifically designed to control that type of transformer whereas a dimmer selected to control a magnetic transformer should be specifically designed to control that type of transformer.
Electrical Connections All of the electrical connections in the secondary circuit of a low voltage lighting system must be very tight and secure. If an electrical splice is not very tight and secure, the wires may arc, create a great deal of heat, cause the entire lighting system to fail, and possibly become a fire hazard. If the transformer is an external transformer and not integral within the low voltage light fixture, it along with its splices should be safely housed in a UL-approved junction box. Also, the light bulbs in the low voltage circuit should always be wired in parallel, not in series, so that each light bulb is powered by 12 or 24 volts.
24-Volt Lamps When using a 24-volt transformer make certain that the lamps are rated for 24 volts - not 12 volts. If 12-volt lamps are used with a 24V transformer the lamps will burn out immediately and possibly become a fire hazard.
Thicker Wires The low voltage lighting circuits on the secondary side of transformers require thicker wires (lower gauge; a lower AWG number) since more electricity is being conducted by those wires. For example, a 300-watt lighting system operating at 12 volts uses 25 amps of electricity on the low voltage side of the transformer (300 watts/12 volts = 25 amps) whereas this same transformer may be powered by 120 volts and have only 2.5 amps of electricity on the line voltage side of the transformer (300 watts/120 volts = 2.5 amps). It is generally recommended that if the low voltage circuit of a transformer is carrying as much as 25 amps of current, then that low voltage wire should be rated for 30 amps so that there is a built-in safety factor that avoids over-heating the wire in the low voltage circuit.
An abbreviated version of this article appears in the print and electronic version of the June 2008 issue of EC&M magazine. The EC&M (Electrical Construction & Maintenance) magazine is the technical authority for over 140,000 electrical professionals involved in large electrical contracting companies, industrial plants with 100+ employees, and large engineering firms. The National Electrical Code, power quality, motors and controllers, lighting, installation methods, and more are covered with in-depth articles and easy-to-understand instructions in the EC&M magazine.