Energy Conservation in Low-Voltage Landscape Lighting Installation

 Energy Conservation in Low-Voltage Landscape Lighting Installation

Simple Ways to Reduce Energy Consumption in Landscaped Lighting Systems
Author: David Beausoleil

"Replacing 120-volt outdoor lighting with 12-volt systems is a growing trend that results in significant energy savings. Further savings can be realized through an intelligent selection of low voltage materials and methods."

We live in an age where energy conservation is paramount. Every landscape lighting installer has a responsibility to make conscious choices to best conserve energy.

This effort is in the best interest of the homeowner and society as a whole. If we are to be good steards of the planet, then we must employ best practices during design and installation. The following article focuses on critical lighting system components and installation methods with a view to recognize where energy is lost and where it can be preserved.

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.

Example: A 25-fixture system with an average wire length of 80 ft. per wire run.

Initial System:

  • EI-type 1200w transformers
  • All #12/2 wire
  • 5 wire runs @ 80 ft. each
  • All 35w lamps
  • All fixtures on a single transformer

Step 1: Transformers
A low voltage lighting transformer steps down a 120v current to 12v. This is accomplioshed 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 can create 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 had 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).

Enery Consumption Related to Transformer Type
Transformer Amps on Primary Actual System Watts Energy Cost* (per year @ 8hrs.per day)
EI-type (1200w capacity) 85% Efficient 11.42A 1,370w $600.06
CAST Toroidal (1200w capacity) 95% Efficient 10.21A 1,226w $536.99
 Energy cost savings per year using Toroidal Transformer  $63.07
     After 5 years  $315.36
     After 10 years  $630.72
  Cost to upgradefrom EI-type model to Toroidal $38.00

 System with First Improvement: Total Cost Savings - $63.07 per year

  • Toroidal 1200w transformer (savings - $63.07)
  • All #12/2 wire
  • 5 wire runs @ 80 ft. each
  • All 35w lamps
  • All fixtures on single transformer

Step 2: Wire Sizing
Another source of energy loss is wire. When AC current passes through wire, electrons collide randomly and thransfer 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 loss) 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 guage 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 savings 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:

Energy Consumption Related to Wire Gauge
Wire Gauge Avg. Voltage Drop on Wire Run Total Watts (Lamps & Wire) Energy Cost* (per year @ 8 hrs. per day)
#12/2 3.78v 1165 $537.13
#10/2 2.52v 1095 $504.85
Energy cost/year saved after using #10/2 $32.28
    After 5 years $161.39
    After 10 years $322.80
Difference in contractor's price between 500 ft. of 10/2 and 12/2 No-Ox® Wire $45.00

 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:

Quick Wire Sizing Guide

More than 100 watts lamps load on run

Run is longer than 100 ft.

Use #10/2
Less than 100 watts lamp load on run
Run is shorter than 100 ft.
Use #12/2

You will also want to pay attention to the recommended and NEC guidelines that specify maximum load on each wire type:

Recommended and Mazimum Wire Amp Ratings
Wire Gauge Recommended Maximum
#12/2 100w/8.3A 192w/16A
#10/2 140w/12.0A 288w/24A

System with Two Improvements: Cost savings – $95.35 per year

  • Toroidal 1200w transformer (savings – $63.07 per year)
  • All #10/2 wire (savings – $32.28 per year)
  • 5 wire runs @ 80 ft. each
  • All 35w lamps
  • All fixtures on single transformer

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.

Energy Consumption Related to Wiring Configuration (Load per Wire Run)
Wire Run Configuration Total Length of Wire Total Watts (Lamps & Wire) Energy Cost* (per year @ 8 hrs. per day)
Fixtures distributed among 5 wire runs 400 ft. (Five 80 ft. runs) 1095 $504.85
Fixtures distributed among 7 wire runs 460 ft. (Five 80 ft. runs & Two 30 ft. runs) 1003 $462.44
Energy cost saving per year by distributing fixtures over more wire runs $42.41
    After 5 years $212.07
    After 10 years $424.10
  Additional wire cost at installation $24.60

System with Three Improvements: Cost savings - $137.76 per year

  • Toroidal 1200w transformer (savings - $63.07 per year)
  • All #10/2 wire (savings - $32.28 per year)
  • 5 wire runs @ 80 ft. each and 2 wire runs @ 30 ft. each (savings $42.41)
  • All 35w lamps
  • All fixtures on single transformer

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:

Energy Consumption Related to Lamp Wattage
Lamping Configuration Total Lamp Wattage Total Watts (Lamps & Wire) Energy Cost* (per year @ 8 hrs. per day)
All 35 watt lamps (25 x 35w) 875 1026 $462.44
Half the lamps replaced with 20 watt lamps (13 x 20w) & (12 x 35w) 680 761 $350.86
Cost savings per year by replacing half the lamps with 20w $111.58
    After 5 years $557.90
    After 10 years $1,115.80
  Additional cost to replace 35w lamps with 20w lamps $0.00

System with Four Improvements: Cost savings $249.34 per year

  • Toroidal 1200w transformer (savings - $63.07)
  • All #10/2 wire (savings - $32.28 per year)
  • 5 wire runs @ 80 ft. each and 2 wire runs @ 30 ft. each (savings - $42.41 per year)
  • About half the lamps at 20w instead of 35w (savings $111.58 per year)
  • All fixtures on single transformer

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.

System with Five Improvements: Cost savings - $249.34

  • Toroidal 1200w transformer (savings - $63.07)
  • All #10/2 wire (savings - $32.28 per year)
  • 5 wire runs @ 80 ft. each and 2 wire runs @ 30 ft. each (savings - $42.41 per year)
  • About half the lamps at 20w instead of 35w (savings - $111.58 per year)    
  • Fixtures functioning mainly for safety and security on separate transformer from fixtures used solely for decorative purpose; homeowner instructed in operation of timer

Summary of Recommendations

  1. Transformers - Use Toroidal, not EI-type
  2. Wire Sizing - Either use all 10/2 wire, or size wire according to suggested limits
  3. Wiring Configuration - Instead of carrying heavy loads on single wire runs, distribute load of many runs
  4. Lamp Wattage - Use the lowest possible wattage for all fixtures
  5. Operation Time - Educate the homeowner in the proper use of time clocks and use separate transformers for safety/security zones whenever possible

To bring our example to a close, let's look at this final table that illustrates the difference between an installed (let's call him an 'Energy Hog') that does everything wrong compared to an installer (an 'Energy King') who follows these energy conservation guidelines.

Energy Consumption Related to Type of Installer  
Type of Lighting Installer Energy Cost* (per year, per 25-fixture installation)
Energy Hog: Uses EI-type transformers, all #12/2 wire, all 35w lamps and assigns heavy loads on wire runs. $600.06
Energy King: Uses toroidal transformers, primarily #10/2 wire, at least half of the lamps are 20w and distributes fixtures across many wire runs. $350.86
Energy cost savings per year (per installation) when the Energy Hog changes his ways and becomes an Energy King $249.20 (1,661 kWh)
30 installations (reasonable yearly estimate per installer)

$7,476 (49,840 kWh)

 The Bottom Line: implementing these energy conservation methods will greatly reduce the homeowners' energy bills and save this nation millions of kilowatt hours per year.

It's time all landscape lighting installers rise to the challenge and prove to the people of this country that they are responsible stewards for the planet.

Notes: Energy cost is calculated at $0.15 per kWh with fixtures powered 8 hours/day 365 days/yr. Energy calcualations in these examples incorporate the transformer efficiencies; if you are checking these calculation 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.