Low Voltage Layout Guide

Consider the threat of voltage drop when designing the layout of landscape lighting. This occurs in low voltage systems when the electricity loses its voltage as it travels down a cable. The amount of drop depends on the length of the run, the number of fixtures on the run, and the cable gauge thickness.

Here is the Voltage Drop Formula:

TOTAL WATTS ON CABLE X LENGTH OF RUN (IN FEET) / CABLE SIZE CONSTANT (FROM CHART BELOW) = VOLTAGE DROP

CABLE SIZE                  CONSTANT

#16                                  2200

#12                                  7490

#10                                  11920

#8                                    18960

Example:  a 100' run with 150 watts on it using 12 AWG wire will have:  150 w x 100 ft / 7490 cable constant = 2 voltage drop.  Accordingly the run should be connected to the 13 or 14 volt tap so that after it loses 2 volts, the fixtures will still receive between 11-12 volts.

The best way to avoid voltage drop is to create an efficient layout. You can start by grouping fixtures into distance zones. Group lighting fixtures with short distances of 0'-25' in the same zone, medium distance fixtures from 25'-50' in the same zone, and lighting fixtures with long distances 75'-100' in a zone. A landscape lighting fixture that is 15 feet away from the transformer, and a fixture that is 100 feet away from the transformer should not be on the same run.

Centralize your voltage load. Use a junction box/hub to connect lighting fixtures to the cable run. Run one cable to the center of the zone and branch out cables to reach each fixture within that zone. This way, the voltage load will be more evenly disbursed. This avoids daisy chaining, a method that strings each fixture to one cable in a straight line and increases voltage drop. If fixtures are daisy chained, each fixture will drop the voltage, so as you move progressively to farther fixtures they will have less and less voltage.  If you do daisy chain, try and keep 2-3 fixtures at most in a daisy chain line.

Finally, using a thick cable will reduce the voltage drop constant and make it easier for voltage to flow. Each lighting fixture should have between 10.75 and 12 volts going to it, and a maximum of one volt difference between surrounding fixtures. 

Note:  Voltages under 10.5 reduce lamp life and result in dim, yellowish outputs.  Voltages over 12 significantly reduce lamp life as a result of over volting.  For example 13 w cuts lamp life in 1/2, 14 w and lamp life will be 1/4!

Layout TIPS for perfect installation and results:

TIP #1

Lower loads per run means lower voltage drop.  Break your layout up into multiple cable runs of ~ 100-150 watts per run.  If you have more than 150 watts in an area -- break it up into 2 runs.

TIP#2

Break your layout into Distance Zones.   For example put  fixtures 15-30 ft away from the transformer on one run, fixtures 25-40 ft on another run, fixtures 30-50 ft on another run, and so on.  The goal is have all the fixtures on a run be roughly the same distance from the transformer so they have similar amounts of voltage drop.  That way when you adjust for voltage drop by increasing the voltage for that one run, the lights are not over-volted or under-volted.

For example, never have a fixture 20 ft from the transformer on the same run as a fixture 80 ft from the transformer.  Why?  Because if you use the 12v tap, the close fixture will have the correct voltage (about 11 volts) but the far fixture will only have about 8 volts and be dim (because of voltage drop).  However if you use a higher voltage tap to make the far fixture brighter, the close fixture will be over-volted (dangerous, fire risk, lights will be uneven, lamp lifespan drastically shortened).  Anything over 12 volts dramatically reduces lamp life--13 volts cuts the lamp life in half!).

TIP #3

Do not Daisy Chain the fixtures.   In other words do not connect fixture after fixture in-line to the same cable.  You can do a couple fixtures in series, but we don’t recommend exceeding more than 3 fixtures or 75 watts in series.  Instead form T or spider splices so that there is never more than 2 fixtures between any one fixture and the transformer.  Example you might have 6 fixtures on a run but the cable layout is like a T with 3 fixtures on the upper left part of the T and 3 fixtures on the upper right part of the T.  Even the end fixture only has 2 fixtures between it and the transformer.

Why?  Well as an analogy, if you had a hose and it had 6 equal sized holes, the first hole would squirt a lot of water but the last hole would only be dribbling water due to lack of pressure (i.e. voltage drop in electricity).  However if you T off the hose, that is connect the water supply to the middle of that hose (between holes 3 and 4) the holes would squirt a more equal amount from each hole.   You want to do the same with your low voltage cable layout—equalize the voltage to each fixture.  Make sense?

TIP #4

For each run, bring the wire from the transformer to the middle of that zone, and then branch off more cable from there to reach different areas.  This can be done with a junction box/hub or by simply splicing in more cable to form a T layout.  This T layout somewhat incorporates objectives we discussed above in TIPS 2 & 3, getting all fixtures with roughly the same length of cable to reach the transformer (TIP #2) and avoiding daisy chaining (TIP #3). 

Layout Example:

For example if you have 4 path lights along a driveway each 10 ft apart, run the cable from the transformer to the area between fixtures 2 & 3 (even if it means passing fixtures 1 & 2 without connecting them).  Then splice in a T with cable going back to fixtures 1 and 2 and cable going out to fixtures 3 and 4, then connect the fixtures.  This way electricity has to travel the same distance to reach the closest fixture (fixture #1) as it does to reach the farthest fixture (fixture #4).  Additionally you only have 2 fixtures daisy chained in a row.  If the farthest fixture is dim you can use a higher voltage tap on the transformer without over volting the closest fixture.  All your lights in that distance zone/on that run have the same amount of cable to reach the transformer and accordingly will all have the same voltage and all be bright and even.  It may seem odd to pass by a fixture with cable and then run more cable back to it, but what you are really doing is adding more cable distance to your close fixtures so that they equal the cable used in far fixtures.

Junction Box/Hub Layouts

Junction box/Hubs are an easy, efficient way to insure that you have done your layout properly.  They virtually guarantee perfect results, bright even lighting, prevents over/under volting.  Additionally all your connections are in one place (so they are easy to trouble shoot/service).  Lastly, with VoltPro exclusive above ground Junction Box, all your connections are above ground in a dry, weather tight junction box, not in the ground exposed to constant moisture and harsh electrical conditions like traditional connections or even competing company's that sell "hub systems".

Place the junction box in the middle of a distance zone; run a cable from the transformer to the junction box.  Inside the junction box connect all the fixtures to the end of the cable.   The end result is a home run with several fixtures branching off the end of it like a flower stem (home run) with multiple flower pedals (the fixture wires).  Electricity has to travel exactly equal distances from the transformer to each fixture, plus there are no daisy chained fixtures, perfect layout.  Lastly measure the voltage in any one fixture (it will be the same for all of them on that hub).  If the voltage is below 12 volts, move that run up to the appropriate voltage tap on the transformer until the fixtures have between 11-12 volts.  For hub Systems you can use more fixtures per run, up to 200 watts worth of fixtures.

To hub or not to Hub, that is the question:

You do not have to hub every fixture.  T-layouts are perfectly acceptable for a couple fixtures.  A junction box is just more convenient if you have several fixtures all in a same area.  Hubs can also make a great junction box for spider splicing several main cables together (10-2 or 12-2 cables).  Many direct burial connectors cannot handle several thick gauge cables connecting together, hubs make for a convenient, dry, above ground place to make these connections.  When doing more than a T connection but a spider splice with several cables in one place, junction boxes are great.  

Realistically most installs call for a combination of layout methods, most fixtures junction box/hub/spider spliced/or whatever you want to call a group of fixtures connected at one location.   Other fixtures will have the cable run T off to reach them with a couple daisy chained in line off the T.  Some T will be connected with a regular direct burial splice because there are only 3 wires in a T.  Another run may have 4 or 5 cables coming off the home run more like a flower, and thus have the home run hooked to a hub.  No layout or property is the same, and you will have to use a variety of techniques.  The end goal is to be a champ and get all you fixtures to output between 11.25-11.75 volts for perfection!

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