Voltage Drop Equation For Landscape Lighting
Use this voltage drop equation to figure out how many volts your wire will lose over a distance:
Total watts on low voltage wire * length of wire run in feet / wire size constant listed in chart below = VOLTAGE DROP
WATTS * WIRE RUN LENGTH / CONSTANT = VOLTAGE DROP
| WIRE SIZE | CONSTANT |
| #16/2 | 2200 |
| #12/2 | 7490 |
| #10/2 | 11920 |
| #8/2 | 18960 |
(note: the smaller the number the bigger the wire. 8/2 low voltage wire is the largest size)
Example:
100 watts of lights at the end of a 200ft wire run on a 10/2 wire
100 * 200 = 20,000 / 11,920 = 1.68 volts
So if you have 12 volts coming out of the transformer the voltage drop equation tells us the voltage will be reduced by 1.68 volts to 10.32 volts.
10.32 volts will produce a slightly yellowish light.
We recommend using a multi-tap transformer so you can increase the voltage to get each landscape light above 10.8 volts.
Increase voltage on far wire runs with our multi-tap transformers.
Click Here to —-> Learn More About Voltage Drop.
Southwire Voltage Drop Calculator —-> Southwire has a great voltage drop equation calculator and even a mobile app for iPad, iPhone & Android devices. We do not stock Southwire low voltage cable we stock and sell Colman Cable low voltage wire. If you are interested in Southwire products we can order them.
Cast Lighting Voltage drop Calculator —-> Cast lighting provides the most in depth tools for landscape lighting professionals. We sell Cast Lighting but the products are not loaded on our website. If you are interested in any of their landscape lighting products please contact us for a quote.
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whats the constant for 14/2?
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I think this will be influenced by the wall thickness of the cable won’t it? What wall thickness were you using. I looked up Southwire 10/2 55272 30V cable is 30 mil and 10/2 552135 150V cable is 45 mil.
Can I mitigate the voltage drop by using a bigger transformer ( higher wattage)? I am currently using a 200 W transformer and the wiring that I laid out has caused a significant voltage drop. I like to know if instead of rewiring, I can buy a bigger transformer?
Thanks for your advice, in advance.
in your voltage calculation equation, is that equation based on a series run? or, for a T
connection? I also have a run about 300 ft.
thx mike
How can i do a 250ft run?
I am only using 10ea. 3 watt led fixtures on the run. So total of 80 watts. The fixtures are evenly spaced over the 250 feet of straight driveway.
I already have a 12volt 300 watt transformer but im not sure it will work over the long distance.
I will run 12-2 cable.
Any suggestions??
Help is greatly appreciated!!
Good information concerning voltage drops referencing a light-scape installation project. Probably spend the extra money upfront and install 10/2 cable as this is the majority of labor involved. If you install a smaller size, might be concern of too much voltage drop that can be overcome by a larger transformer tap output.
Very, very helpful
Thank you
I believe the correct equation should be:
volt amps total (all lights on the wire zone) x the wire length or run total x 2
divided by the wire constant
ie 10 LEDS on a zone @ 11VA = 110 VA x
total length of wire from transformer to last light 150 ft = 16500
x 2 = 33000
divided by the constant for 12/2 wire 7490 = 4.4 volt drop
assuming you are using a 15 volt tap 15VA – 4.4VA = 10.6VA
this is below the minimum 11VA for landscape LED
you will have to install in a T configuration.
from the transformer the first light is 60 ft and then another 90 ft to the last light
you have a total run of 150 ft.
11 va x 10 LEDS = 110 VA
O _____60 f to first LED__I ___I___I___I___I___I___I ___I___I___I
transformer wire 12/2 90 ft of LED run total 150 ft
If you attach the wire run from the transformer to the wire between the 5th and 6th
light your wire run will be 60 + 45 = 105 ft
you will now have 5 lights on each side of the T = 55VA (you still have 10 light @ 11va)
but you have 1/2 the load because of where the wire from the transformer is connected.
attach wire to middle of run so there is 45 ft on either side
0 _____60 ft + ________I___I___I___I___I___I___I___I___I___I
transformer 45 ft____________________T_____45ft_______ = 105 ft
45 ft to first LED that is now between the 5th and 6th LED ______T
new equation is 55VA x 105 ft x2 / 7490 (12/2 constant) = 1.5 VA drop
assuming we are still using a 15 VA tap from the transformer 15-1.5 = 13.5
this is above the minimum 11 VA required for LEDs so we an use a 14VA tap and leave
the 15 open for another light zone.
Hi, this is a question in David’s comment above.
Thanks for your explanation, very helpful. I wanted to make sure I understand it correctly to design my lighting scenario. One question I have in following along; would the new run length still be 150’?
Am I understanding the scenario correctly: light 1 is 60’ from the transformer and the remainder of lights go away from the transformer/down the line to light 10, at 150’ from the transformer. If a T is installed at 105’, 45’ of wire would be installed back towards the light stations 1-5. The second leg of the T would include lights 6-10 for a 45’ run to the original end of the run for light 10 at 150’ from the transformer.
In drawing the total distance or run for lights 1-5, the run would be 105’ (60’+45’) to the T + 45’ for the light leg. Similar run calculation for the light 6-10 leg (150’).
I think another’s way to say it would be it now takes a 105’ run to get to the T between light 5&6, plus a 45’ run for each leg?
Voltage drop would then be 2.2V. Would be great if someone could confirm. Thanks!