We all must start somewhere. For those of you just exploring landscape lighting or for those needing to teach their crews, this article reviews some electrical terminology and calculations needed in the field.
Voltage is a measure of pressure in an electrical circuit, amps is a measure of the current flowing, and watts is a measurement of power or the rate at which energy is used.
Alternating current is a type of current that can be found in residential and commercial spaces. This current fluctuates from north pole to south pole every 1/60 of a second. The low voltage current coming out of most landscape lighting power supplies is AC voltage, hence the abbreviation VAC. When connecting cable to light fixtures, it is not necessary to maintain polarity. However, when working with VAC, it is.
Direct current is the type of current that flows from a battery or an LED driver. When using DC current, one must maintain polarity and keep separate positive and negative cables. Most LED low voltage lamps and integrated fixtures will work on both AC or DC low-voltage systems. Be sure to check all manufacturer specifications before use.
The watt unit is named after James Watt, who made improvements to the steam engine. One watt is defined as the energy consumption rate. One watt is also defined as the current flow of one ampere with voltage of one volt. Lamps and integrated fixtures are labeled with the wattage value.
Volt-amps is the amount of energy in an AC circuit that is consumed but does not contribute to light output.
This is important when designing and for a low-voltage lighting system. Because of VA, LED lamps and integrated fixtures may draw more power on the transformer than what is listed on the box. Manufacturers often have VA information listed on their website or in literature.
If VA info is not available, an easy rule of thumb is to take the wattage of the light source listed on the lamp or fixture and multiply stated wattage by 1.67. This figure will be a little higher than most specs and will provide a slight buffer when selecting a transformer. When in doubt, round up your figures. DC systems will normally not require a VA calculation.
Back in the halogen and incandescent days of landscape lighting, it was a common practice by many contractors to install lamps while the system was on. This is known as a “hot plug-in”. This practice has been widely used, especially if a contractor was performing maintenance on a lighting system.
As we have evolved into the era of LED light sources, the practice of hot plugging should be avoided. A hot plug-in can cause electrical over stress on many parts of the internal circuitry of an LED lamp or integrated fixture. EOS can simply be described as an electrical component that is operated beyond its maximum rated electrical limit, accidentally or deliberately, according to its rating on the specification sheet. In landscape lighting, EOS can occur with a hot plug-in, a lightning strike or a poorly made connection. Some common signs of EOS are that one or more diodes is out, a pungent, burnt smell emitting from the lamp or a burn hole at the back of the lamp.
Many new lighting contractors do not calculate voltage drop. However, just as you size your pipe for friction loss on an irrigation system, voltage drop needs to be figured out on a lighting system. The Association of Outdoor Lighting Professionals recommends the calculation:
Total Watts on the Cable x Length of the run ×2 / Cable Constant = Voltage Drop
To complete that calculation, here are the cable constants for each gauge of cable:
| Cable Size Constants | |
| #18 | 1380 |
| #16 | 2200 |
| #14 | 3500 |
| #12 | 7500 |
| #10 | 11920 |
| #08 | 18960 |
| #06 | 30150 |
Stay tuned for the next part of this article in a later column on irrigationandlighting.org.
