Voltage Drop Compendium

Discussion in 'Landscape Lighting' started by INTEGRA Bespoke Lighting, Dec 11, 2007.

  1. INTEGRA Bespoke Lighting

    INTEGRA Bespoke Lighting LawnSite Platinum Member
    Messages: 4,102

    Ok Everyone.... Lets see them....

    I would like to build a compendium of VOLTAGE DROP FORMULAS.

    So, get out your handy slide rule, or look up the info in your favourite manufactueres tech manuals, or call your local cable rep, and lets post up as many different Voltage Drop Formulas as we can find.

    No Bickering, lets keep it clean!

    Then once we have a few different versions here, I will purchase a brand new, calibrated, TRUE RMS Meter from either Fluke or Greenlee and set about to test each method, reporting back my findings.

    This should be fun!
  2. Chris J

    Chris J LawnSite Silver Member
    Messages: 2,843

    I just use the simple formula to get me very close. I always use the meter for final calculations and settings.
    Length x watts divided by cable constant, then multiply by 2.
    This generally puts me right where I need to be, but the meter has the final say.
  3. INTEGRA Bespoke Lighting

    INTEGRA Bespoke Lighting LawnSite Platinum Member
    Messages: 4,102

    Ahh I should have been more specific here! Can you please also include the "Cable Constant" figure you are using. Lets keep things simple and go with 12/2 as our base line cable.

  4. pete scalia

    pete scalia LawnSite Senior Member
    Messages: 960

    If you must rely upon referencing a mathematical formula every time you build a low voltage lighting circuit then you'd better find another line of work cause you'll never make it in this business. If you have experience and a voltage and amperage meter then you will never ever need to even know any voltage drop formulas.
  5. irrig8r

    irrig8r LawnSite Platinum Member
    Messages: 4,553

    Pete, please explain to me how you plan a job on paper with your multimeter. It simply ain't gonna happen my friend.
  6. Pro-Scapes

    Pro-Scapes LawnSite Platinum Member
    Messages: 4,180

    its rare i find myself determining voltage drop before a job. I can usually come REAL close by adding up the run what tap I need to be on then without a formula I check a lamp underload for the proper voltage and adjust if needed. Often times I find wire runs need to be routed another route or otherwise changed in lenght. There is no one formula that will compensate for real world testing and adjustment of each and every system

    Its also a rarity I need a tap over 14v too. I think I recall 1 time needed a trans that would go over 15v and that was on a retro/reinstall where client declined all new

    If I need to determine vd before a job I use chris's method

    As for planning on paper we use a sketch or site plans... determine wattages and ALWAYS assume more will be added. Additional lights are an easy upsell if they do not need to purchase a new trans. I do run calculations but I have no practice of determining VD before a Job. The cast wire calculator or unique wizard will do all of this for you
  7. JoeyD

    JoeyD LawnSite Silver Member
    Messages: 2,933

    AMPS x DISTANCE x 2 x .00162 (12/2 RESISTANCE)

    No need to run a test, I am telling you that this is the best and most accurate VD method. Trust me when I say we have run all the tests you plan to run and this method always is most accurate. Like Pete said a Volt Meter is the absolute best way to calculate and then to echo Billy, my Wireng Wizard does all of the thinking for you.

    But have at it, MAKE SURE YOU POST YOUR METHOD OF CHOICE JAMES!!!!! and let us know the criteria becuase for fun I will run the same tests. So what is our citeria

    200ft 12/2 run w/ 100watts? testing to the first point of connection?? Hub wireing or Daisy Chain, or T?
  8. irrig8r

    irrig8r LawnSite Platinum Member
    Messages: 4,553

    Nate Mullen's formula doesn't use a cable constant... as I showed in my example the other day, it's like this:

    (Distance x 2) x Amperage x Resistance​

    At first I thought that they might have a vested interest in raising voltage drop numbers to justify their multi-tap system.

    Then I checked it with the formula Vista uses. It is the same as Nightscaping's formula except that the wire length is multiplied by 2 (by some notion that the electricity is flowing both directions on the wire) and it gets results similar to but not quite as high as the Unique formula..

    (Distance x 2) x VA/ cable constant​

    (using the same cable constants as the NS formula)

    I've tested Vista's formula against my meter readings, and it comes the closest and is good enough to plan jobs with.

    BTW, I was talking with the Vista rep yesterday at my distributor about it. He said they sell multi-tap systems that go up to 22 V too (probably just to maintain market share) but he'd never run into a job where more than 15 V was needed.

    He said that in the long run the client pays more for wasted electricity and that as the cost of power goes up (who on earth thinks it won't?) it will outweigh the initial materials savings.

    On the other hand, we both agreed that LEDs will change everything.
  9. JoeyD

    JoeyD LawnSite Silver Member
    Messages: 2,933

    Your Vista rep needs to do a bit more research. why would having taps 16-22v cause wasted electricity.makes no sense?

    And yeah Vista makes a TF that is NON UL 1838 it is listed UL 506 and goes to 22v to COMPETE WITH UNIQUE!! obviously.

    We did not manipulate the formula to increase VD, unlike what most think we are not slimeballs looking to make an extra buck. Anyone who comes to our classes for the last 10 years knows we lead the industry in education and we always inform people based off of FACT and we prove this with volt meters and amp probes. And no we do not calibrate our own meters.

    unbelieveable. Never been on a job that needed more than 15V HAHAHAHAHAH

    Must not of ever pulled a 12/2 run more than 200 ft with 80 watts on it???? Sounds like the rep has never been on a big job. Oh wait let me guess, he uses 10/2 for those long runs.......sounds real cost effective.
  10. Lite4

    Lite4 LawnSite Gold Member
    Messages: 3,187

    "Vd" voltage drop = "A" Amps in wire x "R" Resistance in wire x "L" Length of the cable to the hub x 2

    Resistance for 12/2 wire = .00162

    Always verify with Voltmeter

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