Question on voltage drop in landscape lighting???

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jango

jango

Senior Member
Here's a scenario:

Wire used: 8-2 low voltage direct burial landscape lighting wire
Load: 150VA (qty. 3 MR16 bulbs @ 50W each)
Distance from Transformer to Load: approx. 50-75 ft.
Transformer specs: 120V input, 12.5V output

Can I get some input on a scenario like this and if my bulbs should be at there optimum level based on these specifics? Or should I be using transformers with multi-secondary taps. i.e. 13.5V, 14.5V etc.

Thanks!
 
H

hardworkingstiff

Senior Member
Location
Wilmington, NC
The question is will you get enough light at approximately 11-volts instead of 12-volts. If the lumen output is sufficient, the lamps will last longer at the reduced voltage.

The problem with a transformer with a higher voltage is when lamps start burning out, the remaining lamps start seeing a voltage higher than nominal and their life span will be shortened (where they were lengthened in the under nominal voltage scenario).

IMO, if you are getting enough light, it's a good install.
 
W

WinZip

Senior Member
jango said:
Here's a scenario:

Wire used: 8-2 low voltage direct burial landscape lighting wire
Load: 150VA (qty. 3 MR16 bulbs @ 50W each)
Distance from Transformer to Load: approx. 50-75 ft.
Transformer specs: 120V input, 12.5V output

Can I get some input on a scenario like this and if my bulbs should be at there optimum level based on these specifics? Or should I be using transformers with multi-secondary taps. i.e. 13.5V, 14.5V etc.

Thanks!
Click to expand...

This calculator will get you close.
http://www.residential-landscape-lighting-design.com/calculator.asp
 
S

sparrott4

Member
jango said:
Here's a scenario:

Wire used: 8-2 low voltage direct burial landscape lighting wire
Load: 150VA (qty. 3 MR16 bulbs @ 50W each)
Distance from Transformer to Load: approx. 50-75 ft.
Transformer specs: 120V input, 12.5V output

Can I get some input on a scenario like this and if my bulbs should be at there optimum level based on these specifics? Or should I be using transformers with multi-secondary taps. i.e. 13.5V, 14.5V etc.

Thanks!
Click to expand...

Based on your specs, your voltage loss will be between 0.8V (50 ft.) and 1.2V (75ft.). Your ideal voltage (for optimal lamp life) at the fixture (under load) is between 10.8V and 11.3V. This means that only a 12V tap is required; although, it makes sense to purchase a multi-tap transformer (12 - 15V) to allow for future expansion of the system.

To make the calculations yourself, go to http://www.cast-lighting.com/support-installers/system-calculator-installers/.

I would like to add that 50W MR-16's are overkill in most landscape lighting situations. More typical is the use of 20W or 35W. Of course, there are times when 50W is needed.

I should also add that the use of #8/2 is also overkill. That's an expensive wire and hard to work with. Most would use #10/2 or #12/2.
 
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S

sparrott4

Member
hardworkingstiff said:
The problem with a transformer with a higher voltage is when lamps start burning out, the remaining lamps start seeing a voltage higher than nominal and their life span will be shortened (where they were lengthened in the under nominal voltage scenario).
Click to expand...

This is an important point. It's good to recognize when a system design presents a higher risk of premature subsequent lamp burnout. Any wire run with a high voltage loss (especially when there are only a few lighting fixtures on that run) presents a high risk of burning out multiple lamps in quick succession.

Here's the formula to calculate the risk:

Voltage loss / number of fixtures = voltage jump when one lamp burns out

Any voltage jump of more than 1.0V presents a high risk of successive burnouts.

For more on this: http://www.cast-lighting.com/search/2/display-document/47
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
With 8/2 wire and 150 watts you should be able to go 125' without any problems. At 75' you should be plenty good.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
don_resqcapt19 said:
Dennis,
I show that as a 17.5% drop at 125'. Is that an acceptable drop for 12 volt landscape lamps.
Click to expand...

I am going by Kim lightings Chart. Last time I did this I read it wrong by this time I am reading it correctly. It states 150 watt with 8/2 has a max. circuit distance of 125'

ry%3D400
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
At 17% drop that leaves almost 10v at the end of the line. I can only assume that this is an okay drop. You have to be careful using those trivolt units as putting more than 12v on the circuit may hurt the bulbs at the beginning of the run.

At 100 feet and 100 watts we are looking at 10.87v and 50 watts at 50 feet you would have 11.72v.

If the op goes to a 13.5 tap then at the 50 feet he will have about 13.2volts. This may not be good for the lamps. I think I did this correctly
 
W

WinZip

Senior Member
sparrott4 said:
Based on your specs, your voltage loss will be between 0.8V (50 ft.) and 1.2V (75ft.). Your ideal voltage (for optimal lamp life) at the fixture (under load) is between 10.8V and 11.3V. This means that only a 12V tap is required; although, it makes sense to purchase a multi-tap transformer (12 - 15V) to allow for future expansion of the system.

To make the calculations yourself, go to http://www.cast-lighting.com/support-installers/system-calculator-installers/.

I would like to add that 50W MR-16's are overkill in most landscape lighting situations. More typical is the use of 20W or 35W. Of course, there are times when 50W is needed.

I should also add that the use of #8/2 is also overkill. That's an expensive wire and hard to work with. Most would use #10/2 or #12/2.
Click to expand...

We just finished one using existing 900 watt tranny with one common terminal left 130 ft run 200 watts , used 8-2 to a junction location that handled 10 - 20 watt fixtures had to use the 13v tap to get 11.2 v to junction.
 
W

WinZip

Senior Member
Also we only use 50 watt lamps for whats called moon lighting from 25-35 ft up in trees aimed down through large tree limbs to the ground areas, really hot looking to.
 
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