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Energy Conservation in Low-Voltage Landscape Lighting Installation
Simple Ways to Reduce Energy Consumption in Landscaped Lighting Systems
Author: David Beausoleil and Steve Parrott
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The Example – Walking through the steps towards energy conservation In our review of other manufacturers' recommendations and from our discussions with installers, we find a number of practices that waste energy. To demonstrate the negative contributions of each of these practices we start with a sample installation that makes all the mistakes, then step-by-step we correct them until, at the end, we finish with the model energy-conserving system.
Step 1: Transformers A low voltage lighting transformer steps down a 120v current to 12v. This is accomplished in the transformer by winding the primary wire around a steel core in close proximity to a secondary winding leading to the low voltage taps. The primary winding creates a magnetic flux in the steel core that is converted to energy in the secondary wires. The size and shape of the core determines the efficiency of this transfer. In lighting transformers there are two such configurations – EI-type and Toroidal. The EI-type core is composed of many laminations (flat layers) of steel. The toroidal type has a ring-shaped core. Efficiency is determined by the sum of losses through wire resistance (copper loss) and losses through the core’s magnetic field (iron or core loss). EI-type transformers vary from 70% to 85% efficient in their conversion of 120v to 12v current, while efficiently designed toroidal types are about 95% efficient. (Note – not all toroidal transformers are well made, less expensive models may have have thinner core wires or be less robust in their assembly.) To demonstrate how this 10% difference in efficiency translates to energy cost, we compare the best EI-type with the CAST toroidal (95% efficient).
Step 2: Wire Sizing Another source of energy loss is wire. When AC current passes through wire, electrons collide randomly and transfer their energy (current) along the atoms in the wire. In addition to transferring current, these collisions encounter resistance from the copper atoms that result in the generation of heat. Less heat is generated (energy lost) in thicker wire because there is more space for the collisions. In thinner wire, resistance is greater and a greater proportion of energy is lost as heat. For this reason, heavier gauge wires conserve energy by allowing the electricity to flow with the least resistance. Ideally, lighting installers would use the heaviest gauge wires for all wire runs. Instead, many installers select smaller gauge wire out of habit, or to cut cost. To demonstrate the energy saving benefits of using heavier gauge wire, let’s simply look at replacing the five runs of #12/2 wire in our example with #10/2 wire:
As we see from this example, a responsible contractor would install 10/2 wire instead of 12/2 wire with the knowledge that the additional wire cost (that could be passed on to the homeowner) would translate into hundreds of dollars of energy cost savings over the life of the system. This savings is magnified in systems with longer wire runs. With this in mind, many contractors will only use 10/2 wire for every run for every project. However, if you want to continue using 12/2 for shorter runs and for runs with a low wattage load, you can use this simple wire sizing guide:
You will also want to pay attention to recommended and NEC guidelines that specify maximum load on each wire type:
Step 3: Wiring Configuration (Load per Wire Run) Voltage loss, hence energy loss, increases when a few wire runs carry many fixtures/run (higher loads), as opposed to when many wire runs carry few fixtures per run (lower loads). In this example, we reduce the loads on each of the five runs, adding two more (shorter) wire runs to the transformer. The actual numbers of fixtures on each run change from 5,5,5,5 and 5 to 4, 4, 4, 4, 3, 3 and 3.
Step 4: Lamp Wattage One of the biggest mistakes that lighting installers make is in using overly high wattage lamps. We should always remember that the ideal lighting design strategically distributes low levels of lighting throughout the property. When light levels are set too high, the eye adapts to those high levels and sees shadowed areas as black. Lighting designers should keep all illumination at the minimum brightness – just enough to engage the eyes, but not so much as to cause the pupils to contract losing sensitivity to unlit areas. 50w MR-16’s are rarely needed (usually only with very tall palm trees that have large canopies). Even with palm trees, 35w is usually sufficient. For smaller trees and bushes, 20w (or even 10w) may suffice. In the case of tall trees, narrowing the beam spread allows you to use a lower wattage. At 32 ft., a 35w narrow (12 degree) MR16 delivers 7.8 fc with a spread diameter of 8 ft. At the same distance, a 50w medium (36 degree) delivers only 4.3 fc. To demonstrate the significance in energy cost savings with reducing lamp wattage, we replace approximately half of the lamps in our example with 20w lamps:
Step 5: Operation Time The most influential factor in energy conservation is actual operating time. Every transformer installation should have both a time clock and photocell. The photocell turns the lights on at twilight, then the time clock turns off the lights at a time determined by the homeowner. Installers should make sure homeowners understand how to adjust the time clock and advise them to minimize operation time as much as possible. While the turn-off time is completely under the homeowner’s control, there are the following considerations for the contractor. In all lighting designs, there is lighting that serves primarily the goals of security and safety (e.g. path lighting on driveways and walkways and entrance lighting). The homeowner may want these lights on all night or well past bedtime. Other lights that primarily serve the goals of beauty (e.g. lighting on specimen trees, garden beds and architectural features) may not need to be on late into the night. With this in mind, the contractor can employ separate transformers for these two functions. Each one would be set to turn off at the desired time. The energy savings could be considerable.
Summary of Recommendations
Notes *Energy cost is calculated at $0.15 per kWh with fixtures powered 8 hours/day 365 days/yr. Energy calculations in these examples incorporate the transformer efficiencies; if you are checking these calculations with our online calculator, you will need to divide the final energy cost (from the calculator) by .95 for toroidal transformers and by .85 for EI-types for comparable results. - Note: the ideal voltage of 12v lamps is between 10.8v and 11.3v.
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