How to design your 12 volt outdoor lighting

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How to Design Your 12 Volt Outdoor Lighting System

• Vocabulary
• Fixture - a housing and the electrical components
  contained in that housing, a luminaire.Lamp - a
  light bulb.Transformer - an electrical power
  source this device is required to operate any
  low voltage fixture, in this case
  12-volts.Circuit - a closed, usually circular
  electrical line that leaves a transformer
  (positive) and returns to the transformer
  (common)Design Voltage - the amount of voltage a
  fixture is designed to operate on the actual
  voltage that you supply may be more or less
  depending on the wiring method.

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• Fixture and Lamp Selection
• The fixtures and lamps are selected after
  determining which features of your landscaping
  are to be used. Decorative fixtures will be
  placed in plain sight otherwise, conceal the
  light source when possible.
• Bullet/Directional Lights - These designs help
  focus and direct the light beams. Some also cut
  off glare and protect the lamp and socket from
  debris and moisture.

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• Area/Path/Bollard Lights - These low level units
  are designed to cast illumination in a broader
  pattern for flower beds, perimeter plantings,
  driveways, steps and paths.
• In-ground/Well Lights - Burying these fixtures
  flush with the ground conceals the light source.
  Use for up-lighting trees and shrubs, an grazing
  textured walls.
• Accent/Spot Lights - Versatile/adjustable
  fixtures used for up-lighting, cross lighting,
  accenting and grazing. When mounted high up
  provide focused down-lighting and moonlighting.

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• Deck/Step/Brick Lights - These wall mount
  fixtures light pathways, steps, garden walks,
  deck and pool areas.
• Incandescent lamps are the standard type commonly
  used around the home. They are inexpensive but
  yield less light than other bulbs. Halogen and
  Quartz lamps are a compact light source which
  provides a consistently bright light. These are
  available as a reflector (MR) and as a projector
  (PAR) type for controlling the direction of the
  light. These fit into smaller and less obtrusive
  fixtures.

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• Mounting Method
• The mounting method is determined by the location
  of the fixture. Metal canopies allow you to mount
  fixtures to a deck or soffit, while plastic
  stakes are for mounting into the ground. There is
  an extensive variety of selection in mounting
  hardware, so check to see which kind is offered
  with your light fixture.

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• Power Cable
• It is important to note that the cable referred
  to in this document is two-pronged i.e., there
  are two independent wires inside.
• The cable lengths in your design will determine
  many further aspects of your lighting system. You
  want a cable wire that will deliver the best
  voltage to each individual fixture, and the best
  method for selecting this cable begins with a
  layout of your project. Be sure to include all
  landscape features, buildings, individual light
  fixtures, and power supplies in your drawing.

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• Group your light fixtures into clusters of 2 to
  6 do not form a group whose total wattages
  exceed 240 Watts. One cluster at a time, select a
  fixture -or a point between two fixtures- that is
  the most centralized and draw a line from the
  power supply to this point (your center point).
  From the center point, draw a line to the nearest
  fixture on the right, and do the same for the
  nearest fixture to the left. Repeat this process
  from the two newly connected fixtures until the
  entire cluster is connected. Click here to view a
  sample layout.

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• To achieve this wiring method, you will need a
  common device known as a T-connector. The
  T-connector allows you to place your cable along
  the path of the lights, cut the cable at the last
  light, and then connect the middle of the
  laid-out cable to a new piece of cable that will
  run back to the power supply. Then you will use a
  similar item known as a Quick Connector to fasten
  the laid-out cable to each light fixture. These
  Quick Connectors are included with most
  low-voltage outdoor landscape lights. If quick
  connection devices are not available, you can use
  a common wire nut. Place a small bead of
  household silicone seal inside the wire nut to
  create a water-tight fit. Here is an example of a
  typical layout. The "T" stands for the
  Transformer, and the "35" represents a 35 Watt
  light fixture.

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• Now you must determine the amount of voltage
  being delivered to each fixture of each cluster.
  Though you may start with 12 Volts, there is a
  phenomenon called voltage drop that will reduce
  the voltage being delivered to each fixture.
  Voltage drop on your system will affect lamp life
  and the amount of light the lamps will deliver.
  Too much voltage drop lowers the light output and
  changes the color of the light. Too little
  voltage drop burns the lamps too hot and shortens
  the lamp life. A good rule of thumb to prevent
  voltage drop is no more than 100 Watts on 100
  feet of 12/2 cable. Here is a handy tool for
  calculating the correct wire size for lighting
  design.

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• Another very effective way to eliminate voltage
  drop is to use continuous loop wiring. This
  entails the same preparation as the T-connector
  method, except you lay the cable from the
  transformer, along side each light fixture (one
  cluster per cable), and back to the transformer.
  To achieve continuous loop, simply connect both
  ends of one wire of the cable to the Positive
  lead and both ends of the other wire of the cable
  to the Common lead. Be very careful to not mix
  these connections! The two internal wires can be
  distinguished by their markings. One will have a
  solid color and the other will have writing, a
  stripe, or a different solid color.

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• Sometimes, a lighting designer or a home owner
  will want to take advantage of voltage drop. Here
  is a table showing the effects voltage drop has
  on standard non-halogen lamps. With a halogen
  lamp, a voltage feed of less than 10.8 Volts may
  have a negative effect.

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• As you can see, a lamp with a design voltage of
  12 Volts will deliver 80 of its light output and
  its life will double by reducing the voltage to
  11.5 Volts. The light loss is hardly noticeable
  to the naked eye while lamp life is generously
  extended. Therefore, you want to experience a
  slight voltage drop to all lamps in your low
  voltage lighting system. The recommended feed to
  each lamp is between 10.8 Volts and 11.5
  Volts.Voltage drop is determined by multiplying
  the total watts on the cable by the total length
  of cable and dividing by the cable constant
  listed below. A smaller number gauge equals a
  larger sized wire.

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• Cable length - length of cable used from the
  transformer to the fixture on which you are
  measuring the voltage drop (in feet).Total watts
  - sum the wattages of every lamp along the cable
  length.Cable constant - indicates the thickness
  of copper wire a thicker wire results in less
  voltage drop.

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• Cable length X Total watts  Voltage DropCable
  constant IMPORTANT! When most of your lights are
  at the far end of the run, multiply your answer
  by 1.5 (i.e., multiply the voltage drop by 150).
  Click here to view some example calculations.

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• Always start your calculations with 12-gauge
  wire. The voltage at a fixture is the actual
  voltage supplied (12 Volts) minus the voltage
  drop. The voltage supplied may be increased by
  use of a multi-tap transformer. These multi-taps
  have alternate voltage sources to counteract a
  long cable run with too high of a voltage drop.
  Another way to counteract a high voltage drop is
  change to a thicker wire, such as 10-guage. If
  you need more voltage drop than you are getting,
  you may change to a thinner wire, such as
  14-gauge, but a thinner wire supports less
  wattage. Here is a chart to determine the maximum
  wattage allowed for each buried wire.

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• Transformer Guidelines
• A low-voltage transformer is an electrical device
  that changes voltage by inversely adjusting the
  current. A transformer contains a positive
  terminal and a common terminal. Your supply cable
  is connected to one of each terminal thus making
  a circuit as soon as the first quick connector is
  attached. The transformer is critical to your
  low-voltage landscape lighting.

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• The low voltage transformer is selected by first
  determining the total wattage being used in your
  plan. Select a transformer that has a higher
  wattage capacity than the total wattage being
  used this will give you potential to expand or
  alter your design. You may need multiple
  transformers to achieve your landscape look. No
  single circuit may have more than 300 Watts of
  power, but one transformer can handle multiple
  runs of cable. The transformer will be placed
  close to the power source, and some are even
  rated for indoor installation. Your transformer
  must be at least one foot off the ground to avoid
  potentially rising water. All transformers must
  dissipate heat to cool you should leave a 3-inch
  clearance around the transformer housing for
  indoor transformers.