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Looking for ideas or technical answers to your landscape lighting questions? 

Landscape Lighting Transfomer How To

East West | April 28, 2018

Landscape lighting transformers are used to reduce 120-volt house power to a lower output voltage, typically 12 volts.  Low voltage landscape lighting primarily uses 12-volt ac. Many contractor-grade transformers come in above or below grade options and have multiple voltage taps allowing for more precise adjustment of voltages supplied to each fixture run and most will accept control accessories like a wifi-controller or photocell.


Add up all the lamp wattages you expect to connect to each individual wire run and then include about 20% for additions to the system and another 20% to accommodate LED inrush. (It’s advisable to use the maximum allowable wattage of each fixture so a system is not accidentally undersized, but this is left to the contractor’s discretion.)  This will give you a minimum starting point for estimating the transformer size needed for the job.


Compare your total lamp wattage to the available transformer sizes (typically 75, 150, 300, 600, 900 and 1200W).  If your lighting load adds up to 45 watts, then use a 1500-watt transformer, the price is very close.  By code, transformers should be de-rated to 80% of their capacities so a 900W unit should be de-rated to 720W max (900 x .8).


Estimate the expected wire run length from transformer location to approximate fixture locations.  If you have very long runs or heavily loaded runs, this will increase voltage drop.  Many transformers have multiple voltage taps (12V – 15V; 12V – 18V, 12V – 22V, etc.).  Just make sure that the unit you select gives you sufficient capacity to compensate for whatever your voltage drops per run are calculated to be.



For example, a 12V – 15V unit provides a maximum of three (3) extra volts so if your calculations show a voltage drop of four or more, then you will need to adjust some aspect of your design:  reduce the load, use thicker wire, move the transformer closer, specify a unit with higher voltage taps, etc.


Using Ohms Law (Watts = Amps x Volts), divide total expected watts by the voltage and you will get the maximum amps.  Cross check this number against the max amp load label on the transformer.  Do not exceed this number or the transformer will be overloaded.  This can be a tripping nuisance and a fire hazard.


300W/120V = 2.5A        600W/120V = 5.0A        900W/120V = 7.5A        1200W/120V = 10.0A


Long wire feeds and heavy loads force the use of higher voltage taps and thicker wire to compensate.  Another option is to run 120V power to a more centralized location within the landscape to accomidate the farthest areas needing illumination.



If there are any particularly long or heavily loaded wire runs, then it is best to run a second calculation called the “voltage drop calculation.”  This will allow desk calculations to be run so that transformer(s) and wire are not accidentally undersized thus compromising the job.


Total Length of Wire Run x Distance to Point of 1st Connection    =    Voltage Drop


Do not over-load transformers beyond their rated wattage or amperage capacities – this can cause over-heating and become a fire hazard.


The NEC (National Electric Code) requires that appliances which operate more than three (3) hours per day be de-rated to 80% of their capacity, so 300W x .8 = 240W max.


Make sure all copper wire strands are contained and tightly held inside each terminal block tap.  You do not want any frayed wire strands or arcing to occur.


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