Low Voltage Basics Part II

This Part II – of a two-part series – explains the reasons and ramifications of using Low Voltage (LV) for outdoor garden lighting. (Part I, deals with basic electric theory).

Lighting by way of Low Voltage (LV) is like driving a Sherman tank. It’s safe, but costly to move around.

Why is LV safe? Voltage is electromotive pressure and 12 volts is not enough to force an electron flow felt by humans. The 110-120 volts running your home is a different story. It easily produces a painful shock that’s potentially fatal.

Why does it cost more to move Low Voltage energy? Think of  copper wire as highway. Electrons travel up one lane, through a light fixture and back again. Electrons circulate, thus the name, “circuit”.  To deliver a given wattage, lowering the voltage means increasing the current flow by the same proportion. The size of the highway, the copper, is dictated by the current flow, the electron traffic. Here’s another view.

With less pressure (voltage) forcing electrons through a light fixture, the bulb is constructed to have less opposition to current flow. The electrical of a 12V bulb is 1% of its 120V counterpart. The overall bulb resistances lowers with each lamp added to the project. Problems surface when that resistance approaches the vicinity of the actual highway, the copper. That is, the highway itself appears as an unintended component, impeding the designed flow of electron traffic while robbing voltage intended for the bulbs.

Low voltage is perfect for lengths such as that found in an automobile. Moving a few hundred watts down the driveway is another matter.

Later we’ll find out how the LED’s adjusts its own resistance automatically to deal with resistance changes imposed by varying lengths of copper wire.

Wiring Outside

There are two methods of lighting used around landscaping:

  1. Intense lighting. More likely found around an office building than a home, is the 110-120 volt power. No doubt installed by a professional, wire from a surface lamp first goes straight down almost two feet before going horizontal. Typical wattage: 150 to 1000 watt fixtures.
  2. Low level landscape lighting.|Fixtures are small and there’s more of them. Averting the expense of 110 wiring is accomplished by way of a voltage reduction. This is where “Low Voltage” shines. By eliminating the risk of shock, wires can be run safely at ground level (by anyone) and connections can snapped on, even on a “hot” circuit. Typical wattage: 2 to 50 watt.

Low Voltage (LV) is made possible by way of a transformer, 110v goes in on one pair of wires and 12v comes out on another. In addition to reducing voltage, the transformer provides isolation. Between in and out there exist no conductive path – only magnetic. Thus, electrons cannot get from earth [ground], through you, over to that nasty 110V (which itself is connected to earth). Regardless of what spin might be found on the net, using 12 volts from a transformer, instead of direct 110V, is ALL about safety.

Cost of Electricity and Brightness

A light bulb is designed to work at some particular voltage. A 25-watt, 12 volt bulb costs just as much to operate as its 25-watt, 110v counterpart and, assuming the same technological construction, the two burn at the same brightness, the same color. Don’t believe anything else. If someone claims, “low voltage uses less energy” it’s only true because the application demands less energy.

Advantage Summary

The 12 volt approach looks good, we don’t need:

  • An electrician
  • Expensive mounting
  • Wires buried deep
  • To worry about shock

So far, except for the need of a transformer, 12V is a winner.

Disadvantages of Low Voltage

Scroll down and read the summary if the physics is not for you.

The power used by a electrical device is measured in watts. The watt is the product of the voltage (VOLT) pressure multiplied by the current (AMP) flow. For instance, a 120-watt bulb could be in the form of (120Vx1A) OR (12Vx10A).

Here’s the catch, it takes a lot less copper getting the 120 watts to the 120Vx1A bulb than to 12Vx10A. Why? Picture amperage (electron flow) as cars going through a tunnel. The copper wire is the tunnel and the traffic, electrons. Of our two bulbs, both 120 watts, the 12Vx10A has ten times the [electron] traffic.

The price that is paid for moving electron traffic is in the form of a voltage loss (or voltage drop). That is, from the beginning of the copper to the end, one could measure the voltage that was once intended to do work (i.e. light landscape bulbs). Thus, the 120 watt, 120Vx1A bulb has 10% the voltage loss as its 120 watt, 12 volt/10 amp, paternal twin.

Here’s the killer and it’s a two-edged sword. One can afford to pay the copper 6 volts in a 120 volt circuit (you still deliver 114 volts to the destination). Losing 6 volts in that which was only 12 to begin with, is a show-stopper. So, while Low Voltage lighting incurs 10 times the voltage loss, it is the Low Voltage system that can lest afford it.

Fighting this problem:

  1. More expensive, bulky, wire that has more copper
  2. Creative wiring techniques that, while using more wire, spreads the consequences more evenly throughout the project.
  3. Use LED and forget that this problem ever existed

There’s no doubt that countless homeowners experienced a rude awaking following their back-breaking effort burying wire, only to discover half their project barely lit.

The effects of voltage change is squared. A 50% reduction in voltage across a bulb means the wattage consumed, and the corresponding brightness, drops to 25%.  (See our Long Run article to discover how the LED system makes the transformer pay for the loss).

Hold on, it gets a little uglier.

Transformer manufacturer’s rate a given voltage output based on the unit running at full capacity. Meaning, running under capacity, the transformer puts out a higher voltage forcing a bulb to run hotter then designed. Don’t forget, the effects of voltage change is squared.

So, a well advised sale of landscaping lights included the recommendation to match (or balance) the transformer to the load (the sum of the lamp wattage) to avoid the premature bulb failure due to this higher voltage. Until LED lamps, one would typically be advised to purchase a transformer based on the projected total wattage or compelled to match lamp selections based on the available transformer. Either way, the system is not scalable, you can’t add lamps later without upgrading the transformer. Virtually, all these issues vanish using our LED lamps!

Summary

Think of voltage loss in terms of the commuting cost associated with driving a Sherman tank. The commuting cost associated with our low voltage circuit diverts voltage intended for a lamps to that of the delivery wire. The longer the distance, the greater the loss. The primary offset is using thicker wire. There comes a point, however, where the heaviest landscape wire, 8 gauge, can’t get the job done [in a single run].

Even for the experts, the problem imposed by low voltage/high current is not fun, getting it right can be as expensive as is it daunting.

Good News

The good news that the LED lamp is not only tens times as efficient but each lamp has its own regulator. You can purchase a transformer based on potential growth while connecting as few light as you wish, it does not matter to our LED’s. For even more detail see this article.

Written by Joe Greene
LEDthisWay.com

Resistance is a materials opposition to electron flow. Copper wire is a great conductor, but it has resistance. 500 feet (1000 linear feet) of 18 gauge lamp cord has roughly the same resistance as a 25 watt, 12 volt bulb.
The LED driver powering a 1-Watt LED
LED and Driver7VAC-16VAC comes in on the orange wires and the DRIVER converts it, sending the precise DC power required by the LED down through black and red wires.