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Understanding the Basics of Low-Voltage Lighting

Understanding the Basics of Low-Voltage Lighting

Tungsten-halogen and standard incandescent are the lamps of choice in the growing low-voltage lighting system world Each of the three basic types of light sources incandescent, fluorescent, and HID has its own particular applications and advantages. The proper selection of a lamp type will ultimately depend on the specific requirements of the space to be lighted, its function, and the economic factors

Tungsten-halogen and standard incandescent are the lamps of choice in the growing low-voltage lighting system world

Each of the three basic types of light sources — incandescent, fluorescent, and HID — has its own particular applications and advantages. The proper selection of a lamp type will ultimately depend on the specific requirements of the space to be lighted, its function, and the economic factors involved, such as the initial equipment cost, maintenance costs, and kWh consumption, so understanding how those different applications and advantages will affect your choice is an important factor as well.

Compared to other light sources, incandescent lamps are easier to install and less expensive in terms of initial cost. In addition, they've been the most versatile and widely used lamp type for years and have particular advantages in display lighting and in decorative mood-setting applications. The warm color appearance of the incandescent lamp is considered appealing since it's similar to candlelight or the illumination of a kerosene lamp.

Incandescent lamps are available in both line-voltage and low-voltage models. This article will concentrate on the low-voltage (LV) lamp, which is available as both a standard incandescent or tungsten-halogen lamp.

Light production.

All LV lamps produce less than 25 lumens/W, giving them relatively low efficacy. The transformers needed to reduce 120V or 277V power to a low-voltage level like 12V add a 1% to 5% loss to the input wattage.

However, because the filament in a LV lamp is smaller than standard-voltage R and PAR lamps, LV lamps offer the maximum optical control using only the lamp reflector. By focusing most of the lamp's lumens in the beam, less light spills out into the field or unfocused area of the beam (Fig. 1). This optical advantage is an energy saver, since light is concentrated where it's needed, allowing fewer fixtures to be used. Additionally, LV lamps are generally more resistant to shock and vibration, which translates into less lamp breakage.

Common applications include residential and commercial decorative/display lighting, task lighting, and retail product highlighting. Other special applications include swimming pool, fountain illumination, and landscape/garden lighting.

Development of a specialized incandescent lamp with a small quartz glass capsule that contains a halogen gas, usually iodine or bromine, serves to extend the lamp's life. A chemical regeneration process known as the “halogen cycle” greatly reduces filament evaporation within the capsule. The tungsten-halogen capsule is available in both line-voltage and LV lamp models.

Popular LV lamp types.

LV PAR 36 lamps that use the halogen capsule and are available in narrow spot, very narrow spot, and wide flood beam patterns, serve a variety of architectural illumination applications. All of these models have a 4,000-hour average life. The PAR 36 lamp is widely used on commercial and institutional projects since the lamp delivers a powerful oval beam over a long distance, making it especially useful in high ceilings. Additionally, a 120W LV PAR 64 lamp is available with a very narrow spot beam spread.

One of the most widely used tungsten-halogen LV lamps is the multi-faceted reflector (MR) model, which is the architectural lighting version of a slide-projector lamp. These lamps are available in 20W, 42W, 50W, 65W, and 75W ratings. Rim-mounted MR lamps consist of a small tungsten-halogen capsule set within a reflector of faceted glass. They're available with a variety of beam patterns in two styles: The 2-inch diameter reflector MR-16 lamp comes in 20W, 42W, 50W, and 70W ratings; and the 1.375-inch diameter reflector MR-11 lamp comes in 20W, 35W, and 50W ratings. The less popular MR-11 lamp offers similar performance as well as a choice of beam spreads.

Both MR lamp types, which use a BZ4 two-pin base, precisely place the tungsten capsule relative to the reflector, which is generally ellipsoidal in shape. Unlike MR-16 lamps, however, some manufacturers also make MR-11 lamps with a double-contact bayonet base. This type of base is sometimes recommended because the thin bi-pins of the standard MR-11 lamp can be easily damaged.

Most MR lamps have a dichroic reflector made out of glass coated with a special reflective/transmissive film coating. This dichroic coating reflects all visible light but allows the infrared and ultraviolet rays to go through the reflector. This means that more than 60% of these rays are radiated out through the back and sides of the lamp, rather than being directed forward with the light beam.

The dichroic reflector, combined with a borosilicate glass lens in front of the lamp, removes about 95% of both the ultraviolet and infrared wavelengths. (Note: all tungsten-halogen luminaires must have a cover glass protector over the lamp to receive UL Listing). This feature offers protection for light- or heat-sensitive objects, such as valuable paintings, drawings, or fabrics. For display work, color filters can be added to the clear lens of the MR lamp.

The following special MR-16 designs are also available:

  • A lamp with an aluminum reflector coating that eliminates the appearance of light coming through the rear of the lamp, or “back glow.” It also eliminates the infrared reduction feature.

  • A lamp with color dichroic coatings — red, yellow, green, and blue — can be used for special lighting effects.

  • A lamp with a more expensive dichroic coating provides constant color over lamp life, longer lamp life, and improved lumen maintenance.

  • A square-shaped MR lamp offers features for several applications.

LV lamp fixtures.

Because of their small size, MR lamps can be used in compact fixtures and recessed fixtures with small diameter apertures. A typical full-featured, recessed, adjustable accent light for an MR-16 lamp has a 5.5-inch tall housing and a 2.875-inch wide aperture. A similar fixture for a PAR 36 accent light is 10.125-inches tall with a 5.6875-inch wide aperture. And a similar fixture for a line voltage PAR 38 lamp is 13.375-inches tall with a 5.6875-inch wide aperture.

Generally, two types of recessed fixture constructions are used with MR-16 lamps. Where the space above the ceiling is accessible, which is typically the case during new construction, the transformer is mounted outside the lamp enclosure. If it's not possible to access the area above the ceiling, which is typically the case during a retrofit project, the transformer is mounted on the lamp housing and is accessed from below. Spot and flood MR-16 fixtures are well suited for highlighting decorative wall elements, furniture, and tables.

A family of 12V rated aluminum reflector (AR) lamps is used for specific architectural lighting effects. These include: AR48 (1.9-inch diameter) at 20W; AR70 (2.75-inch diameter) at 20W, 50W, and 75W; and AT-111 (4.4-inch diameter) available in 35W, 50W, 75W, and 100W ratings. They're available in narrow spot, spot, flood, and wide flood beam shapes.

Bi-pin halogen lamps with a T3 or T4 glass envelope are used for display applications. Available wattages range from 5W to 100W.

Dimming concerns.

LV lighting systems (and individual fixtures) use a transformer to step the 120VAC line voltage down to 12VAC. A magnetic transformer reduces the operating voltage using copper wire wound around a steel core, which is inductive by nature. This type of transformer is fairly large and heavy. An electronic transformer reduces the operating voltage with electronic circuitry, which is capacitive by nature. This type of transformer is compact and fairly lightweight. Before selecting a dimming control, determine what type of transformer is used in the lighting fixture you're working with. The different characteristics of the two transformer types require special dimming considerations.

As is the case with line voltage incandescent lamps, dimming increases the operating life of LV lamps. However, the MR-16 and other halogen capsule lamps should be operated at full voltage periodically to activate the halogen cleaning cycle and thus get full-rated lamp life.

LV lighting and the Code.

Lighting systems that operate below 30V are subject to the provisions of NEC Art. 411 and 725. UL Standard 2108 also covers these types of systems.

Special requirements cover swimming pool lighting and landscape lighting. A particular issue with these systems is the location of the power supply. An installer should be aware that the various markings on these power supplies impose specific restrictions. For example, the “Outdoor Use Only” power unit can't be used indoors. In this case, only a power unit marked “Indoor Use Only” or “Indoor/Outdoor Use” can be used indoors.

A requirement in 411.4(2) defers to Art. 680 on the use of lighting within 10 feet of a pool, spa, or fountain. This type of lighting operates on Class 2 power-limited circuits evaluated for wet-contact applications.

The step-down transformer, with either a 120V or 277V supply, is usually located within the fixture. However, it may also be remotely mounted (Sidebar on page C18). In all cases, the length of the secondary conductors should be limited to minimize voltage drop.

Most of the time the voltage drop is insignificant, but where multiple fixtures are connected to a single transformer, or where the remote transformer is some distance away from the fixture — say 30 feet or 40 feet — voltage drop could noticeably lower the light output level. A voltage drop chart is usually included on the installation instruction sheet, allowing the selection of the proper wire size during installation.

Incandescent lamps — particularly LV — are often a wise choice for economical lighting installations. Their versatility makes them a viable option for a variety of applications, but adherence to proper voltage step-down installation practices and NEC requirements are important to the complete package.

Sidebar: Low-Voltage Transformers: Integral or Remote?

Transformers for low-voltage lighting systems can be of the integral type, which is part of the fixture, or remote type, which is located in a service area. Some remote transformers can be recessed in wall cavities or direct-buried. If you choose to use a remote transformer with primary and secondary boost taps, you can manually adjust the voltage at the time of installation to ensure adequate voltage is supplied to each lamp — even if the lamps are installed at varying lengths from the transformer.

However, you must realize that even though a single boost on the primary side compensates for voltage drop caused by low line voltage, this adjustment isn't enough to compensate for every variable that might arise on-site. Other variables include different lengths of wire run to lamps or dimmers, and the use of a choke to reduce dimming-generated hum. These other variables must be resolved through the installation of boost taps on the secondary side of the remote transformer.

Some remote transformers offer as many as five separate voltages on the secondary, which can be tapped simultaneously. These taps simplify installation and ensure that the voltage to each lamp falls between 11.5V and 12V for a 12V system, and between 23V and 24V for a 24V system.

Interestingly, there is no cost-per-fixture difference between the uses of either type of transformer because low-voltage lighting fixtures without integral transformers are far less expensive than those with them. This reduction in fixture costs easily offsets the cost of a remote transformer.
David Pitts, president, Semper Fi Power Supply

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