Circuit Design

As electrical energy passes through a wire it looses a little bit of voltage due to the resistance of the wire. This reduction is called voltage drop. Considering that it is thought of as the most important thing to understand about low voltage wiring, you would think that by now there was total agreement on how to calculate it. That’s not the case. There are several ways to calculate voltage drop and they often result in significantly different answers. That’s the bad news. The good news is that in most cases close enough is good enough. Although voltage drop is calculated with precision (two or three decimal places), the assumptions that the calculations are based on are not that precise. In addition, most people will not notice the difference in fairly significant differences of measured illumination.

Wire Size	Constant Value	Wire Size	Constant Value
#18 Wire	1380	#12 Wire	7500
#16 Wire	2200	#10 Wire	11,920
#14 Wire	3500	#8 Wire	18,960

We have 7 fixtures on a circuit, all of which use 20 watt MR16 halogen lamps (total of 140 watts). The length of the run is 90 feet and we want to use #12 wire.

This is too much because 12 volts – 1.68 volts is 10.32 volts, well under the 10.8 volts that we want to operate halogen lamps. There are several ways to fix this problem as will be discussed below; however if we want to fix it by changing the wire we need to redo the calculation. Let’s try #10 wire:

This will work because 12 volts – 1.06 volts is 10.94 volts which is more than the 10.8 volt minimum. If we used this deign, the voltage should be measured at the lamp with a digital volt meter when the fixture is installed to confirm that there is adequate voltage.

Use the table below as a starting point for determining the wire size for various lighting loads.

Wire size	#12	#10	#8
Feet	Maximum Wattage*
50	225	300	300
100	100	175	225
150	75	100	150
200	50	75	110
250	40	60	90
300	35	45	75

* These values assume an equal spacing of the load. If the load is at or near the end of the run, the maximum wattage may have to be reduced. If there are no halogen lamps in the circuit then you may increase the maximum wattage by 10%. Always confirm the actual voltage with a digital volt meter.

By looking at the voltage drop formula we can see the factors that affect voltage drop: the load, the length of the run, the voltage at the transformer, and the size of the wire. Therefore if we need to reduce the voltage drop we must change one of these factors.

The load in watts can be reduced by changing the lamps to ones with a lower wattage. You can also eliminate a fixture or two or move them to another circuit. Obviously this will have an impact on the intensity of the light and or size of the area that will be lit.

The length of the run can be changed several ways. It may be possible to use multiple transformers so that they may be moved closer to the fixtures. By looping back to the transformer from the last fixture you effectively cut the length of the run in half. When using this technique polarity of the cables must be maintained. (SP wire has a rib on one of the conductors so that it is easy to identify. Make sure both ribbed ends are under the same terminal at the transformer.)

The size of the wire can be changed by using a larger gauge of wire or by doubling the same size wire. You may also use a large wire such as #8 to get to the area where the fixtures are located and then switch to a smaller gauge for the connection to the fixtures.

The first thing to do with the transformer is determine the actual output voltage. Remember the transformer simply reduces the primary voltage by a fixed factor of 10 to produce the output or secondary voltage. If the primary voltage is 130 volts, the output will be 13 volts and voltage drop will be desirable.

If the output voltage is 12 volts or less and the length of the run results in excessive voltage drop, you may want to use a multi-tap transformer that has outputs of more than 12 volts. These transformers have terminals that are marked with the higher (and lower) voltages or have a switch used to select the alternative voltages. The type with several terminals allow you to have shorter runs that are not affected by voltage drop receive only 12 volts while the longer runs can be connected to the higher voltage terminals.

Voltage drop can work in your favor as well. Operating the lamp at a lower voltage reduces light output, but it also increases lamp life, particularly with non-halogen lamps. This can be used to your advantage when there are fixtures in the system that are located in difficult to service places such as in tall trees. By selecting a lamp with a higher wattage than required and reducing the voltage to 10.5, the lamp could last 5 times longer than it would if operated at 12 volts.

Once the circuits have been designed you will have the information needed to select the type and number of transformers and their controls. The first step is to add the wattages of all the fixtures that are going to be connected to the transformer. It is recommended that this load be no more than 80% of the maximum output of the transformer. For example if the total load is 341 watts, you divide that by .8 which equals 426 watts. The closest transformer that will handle that load is a 600 watt model. And although 600 watts is almost 75% more that the actual load it is a good selection because there may be the need to add additional lighting after seeing how the design actually works. Also changes in the landscape as the plants mature or the use of the property may change which may require additional lighting later on. We would have to use a combination of smaller transformers to get closer to the actual calculated load. This would cost more than the single larger transformer.

If there are no voltage drop problems and no need to operate the system at a lower voltage for lamp life then there is no need to incur the expense of a multi-tap transformer. Also if the line voltage circuit is controlled by a timer or X10 system there is no need for a timer in the transformer. In that case it may still be advisable to have the transformer controlled by a photocell to keep it from operating too early during the summer when the sun is shinning late into the evening.

The location of the transformer is also important in selection. If it is to be located inside it must be approved for indoor use. It must also have the right type of punch outs to accept conduit which is required by code to run the wires through the wall from outside. If a photocell is required then a remote model with a 10’ – 15’ connecting cable will be needed so that it can be in the open daylight.

Transformers which will be subject to harsh environments should be constructed of materials that hold up to the elements. Stainless steel cabinets are available at reasonable prices and will resist corrosion better than painted steel. Of course transformers that are to be installed below grade must be designed for direct burial.

Your scaled drawing will also be a good starting point for calculating the amount of wire that you’ll need for the installation. Remember, the plan will only help you with the lengths along the ground. You must add the vertical runs like up columns for deck lighting, getting to tree mounted down lights, and up to the transformer. Allow an extra 2 feet of cable for splices and connections. For area and directional lighting add a few feet to make a loop so that the fixture can be moved if necessary. Planning ahead for moves will prevent the necessity of adding a couple of splices later on.

You must also go through the circuits in your mind to determine how many and what kind of connectors that will be needed. Most fixtures are shipped with some type of quick connector. For above grade connections they will probably work fine for many years. For below grade connections it may pay to use a sealable type connector. It does not take long for the dampness to cause corrosion which will cause a failure. It frequently takes a long time to find the bad connection. Using a sealable connector when the system is installed will more than pay for the extra cost of trying to find a bad connection later on.