15 Things You Need to Know About Low Voltage Lighting
1. 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 include recessed lights (cans), track lights, bendable rail lighting systems, landscape lighting, deck lighting, suspended cable lighting, pendant lights, and display lights.
A transformer is generally used with low voltage lighting systems to "transform" the higher standard line voltage to either 12 or 24 volts. Without a transformer the light bulbs in the circuit would be subjected to 120V or 277V and would burn out immediately. The transformer may be part of the fixture (integral to the fixture) or it may be located outside the fixture (an external transformer).
3. Type of Transformer
The transformer used in a low voltage lighting system may be either electronic or magnetic.
4. Maximum Transformer Wattage
The wattage rating of a 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 should never exceed the wattage rating of the transformer. If, however, a conventional EI magnetic transformer is used, then the maximum wattage of the lighting system should not exceed 80% of the transformer's wattage rating. This type of magnetic transformer gets its name from the stacks of E-shaped steel sheets capped with I-shaped pieces in its laminated core. 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%.
5. 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 at least 10 watts of lighting load. If only 5 watts of lighting load connected, the lighting system will not work.
6. Electronic vs. Magnetic Transformers
An electronic transformer is generally lighter in weight, smaller in size, cooler to operate, and quieter than a magnetic transformer, making it a good choice for indoor low voltage lighting systems. However, they can only provide up to 300 watts of power whereas some magnetic transformers can provide as much as 1200 watts.
7. 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 (the low voltage or secondary side). Instead, a "true RMS" voltmeter must be used. On the other hand, a standard voltmeter can be used to read the output voltage of a magnetic transformer.
8. AC Electronic Transformer Placement Recommendations
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 system will experience a lower voltage (known as voltage drop) and have dimmer lamps. Also, the longer the distance between the transformer and the lighting system, the greater the chance that it might create radio frequency interference (RFI) with other electronic components in the area.
9. Toroidal Magnetic Transformer
If a magnetic transformer is used to power a low voltage lighting system, a toroidal magnetic transformer should be considered. These are more efficient, lighter in weight, smaller in size, cooler to operate, and quieter than a conventional EI magnetic transformer.
10. Wiring the Transformer
A low voltage transformer usually has 4 connections or 4 wires attached to it. Occasionally, it 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. 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.
11.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 control devices like a dimmer (or an on/off switch or an occupancy sensor or a timer) it should always be installed so that it is controlling the line voltage side (primary or input side) of the transformer.
12. Selecting a Dimmer
Both electronic and magnetic transformers require dimmers that are specifically designed to control that type of transformer. Always make sure the dimmer you choose is compatible with its transformer type.
13. 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 loose or insecure, 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 light fixture, it (along with its splices) should be safely housed in a UL-approved junction box. Also, the light bulbs in a low voltage lighting kit should always be wired in parallel, not in series, so that each one is powered by 12 or 24 volts.
14. 24-Volt Lamps
When using a 24-volt transformer make certain that the lamps are also rated for 24 volts - not 12 volts. If 12-volt lamps are used with a 24V transformer they will burn out immediately and could even become a fire hazard.
15. Thicker Wires
The low voltage lighting circuits on the secondary (output) side of transformers require thicker wires (lower gauge; a lower AWG number) since they are conducting more electricity. 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 is carrying as much as 25 amps of current, then the wire should be rated for 30 amps. This way there is a built-in safety factor that avoids over-heating the wire.
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.