How To Figure Voltage Drop On A High Power TF

Discussion in 'Landscape Lighting' started by MEXANDME, Feb 6, 2013.


    MEXANDME LawnSite Senior Member
    Messages: 396

    I have an existing 1200W HP transformer (taps up to 22V) on a lighting rearrangement job (reusing the existing lights and transformers but replacing all of the wire) with some really long leads, i.e., one lead is 475 feet with a 140W load.

    How does a 140W load tied to a 22V tap effect the overall load on the transformer versus a 140W load tied to a 12V tap?

    Is there a way (formula) to calculate various senarios?


  2. S&MLL

    S&MLL LawnSite Senior Member
    Messages: 751

    The one thing you failed to mention is your gauge wire.

    I suggest using 18 gauge wire and at 475ft with a 140watt load using the 14volt tap will net you 11.8 volts at fixture.

    Next time i will have to charge you for this vd calculation
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  3. Lite4

    Lite4 LawnSite Gold Member
    Messages: 3,187

    I think you meant #8 wire
  4. S&MLL

    S&MLL LawnSite Senior Member
    Messages: 751

    Even on 8gauge your vd is 8volts. So i guess ill answer correctly for you.

    If you use 8gauge wire use the 20volt tap
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    MEXANDME LawnSite Senior Member
    Messages: 396

    I understand how to figure voltage drop, i.e., 475 x 2 = 950 x 140w =133000 / 18960 (#8 wire) =7.02 vd. Then 7.02 + 10.5 (minimum voltage at light) = 17.52, so use the 19v tap.

    My question was how does using a 22v tap for a 140w load effect the TF versus using the 12v tap for a 140w load.

    I was told that on the 22v tap you can have 180w max, the 19v tap 200w max, the 17v tap 220w max, etc.

    I was told that if you were to put 240w on the 22v tap, the coil would get extremely hot and burn up the TF in a few weeks.

    If this is true, is there a formula/equation to figure the various senarios?


  6. Lite4

    Lite4 LawnSite Gold Member
    Messages: 3,187

    OOPS, I guess I didn't read the joke in it the first time. My bad :laugh:
  7. S&MLL

    S&MLL LawnSite Senior Member
    Messages: 751

    Done here
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  8. Terradek

    Terradek LawnSite Member
    Messages: 104

    What you are dealing with is the difference between Nominal and Effective wattage. The way to solve for this is as follows: wattage/12v = amps (OHMS law) in your case, 140w(nominal watts) /12v = 11.67amps. You then take the amps (11.67) x the voltage tap you intend to use (22v) to calculate Effective wattage; 11.67 x 22v = 256.67w.

    As you can see the "Effective" wattage that you are asking the transformer to accommodate is actually 256.67w not 140w. 256.67w/12v = 21.39amps (OHMs Law). Since 8 gauge wire is rated to 40 amps you should be OK with your system.

    Actual Amperage will be different than what is calculated above because the Ohm Law formula assumes that the system is operating at exactly 12v when more than likely it will be something different...but this will give you a good approximation. Also remember that 300w circuits are the equivalent of 25amps...NEC recommends not using more than 80% of the circuit which is 20amps. Based on the calculations above you would be only slightly more than the 20amp limit. So it will be critical to take a field amp reading so as to verify that you are within limits. This is what is termed "Dynamic Wattage" as it is the actual wattage requirement of the system as it takes all 7 causes of voltage drop into account not just distance, load, and wire gauge.

    As you can see there is no "Rule of Thumb" for this our business it is always about the amperage of a system that drives what we do and how we do it.

    The above calculation is part of the AOLP Test for Certified Low Voltage Lighting Technician (CLVLT). Good luck to all taking the test in Atlanta next week.

    MEXANDME LawnSite Senior Member
    Messages: 396


    Your reply is exactly what I was trying to determine.

    Thank you very much for taking your time to provide such a detailed answer!

    Thanks again,

  10. steveparrott

    steveparrott LawnSite Bronze Member
    Messages: 1,276

    There's a few things going on here.

    All transformer common taps have a 25 amp limit (per UL listed restriction). At 12 volts, this translates to 300 watts (12 x 25 = 300).

    Voltage taps, on the other hand, have no limit imposed by UL - except that they are limited based on restrictions of the ratings of the terminal block itself and the wiring that connects the tap to other transformer components.

    Some manufacturers spec these components so the full load of the transformer can be carried on any one voltage tap - others restrict this load.

    So, it's likely that you need not be concerned about the 22-volt tap (since it's likely the manufacturer would rate its load fairly high - but check to make sure) but instead you need to focus on the common tap for that wire pair.

    As I said above, there's a 25 amp limit on the common - 300 watts at 12 volts. You might think that the equation changes for the 22 volt tap, but it doesn't. As long as you have 12 volts at the fixture, it doesn't matter what voltage tap you use - the equation is the same.

    The big concern here is the wattage represented by voltage loss in your wire. If you are starting at 22 volts and ending up with 12 volts at the fixture, then you are losing 10 volts - or consuming nearly double the wattage. That would bring you to about 260 watts on the common. (NEC limits you to 80% max. load on a circuit - 240 watts).

    The forumula you site is correct, you just need to calculate the wattage contribution of the voltage loss to ensure you don't overload the common. You can work that into your existing formula or just use our calculator to get the total watts (lamps plus wire). Here's the link to the calculator.

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