Another troubleshooting challenge

Discussion in 'Landscape Lighting' started by steveparrott, Aug 25, 2010.

  1. steveparrott

    steveparrott Sponsor
    Posts: 1,199

    Have you ever wondered how changes in transformer input voltage affect the total amperage draw? This is good to know so you can evaluate changes in amperage readings.

    Say, for example, you install a system and make the following intial measurements:

    Voltage at the GFCI (under full load): 115V

    Primary transformer amperage: 8.0 amps​

    You return to the site a year later and take new measurements:

    Voltage at the GFCI (under full load): 126V (about 10% higher)

    Primary transformer amperage: ???? amps​

    What would you predict (if only the voltage has changed)? Would the amperage be higher, lower, or stay the same? If it does change, by how much?

    Tips: Amps=Watts/Volts but this will not give the correct answer, unless you consider what's happening with the lamps.
  2. Pro-Scapes

    Pro-Scapes LawnSite Platinum Member
    Posts: 4,181

    you say only the voltage has changed. Why has it changed. Was there an AC unit on when you took it the first time and now there isnt ? Some other draw ?

    I had a situation at a recent job where I had 103v coming in. As soon as the AC cut off it jumped to 123. Cause... corrosion due to a lousy job at the sub panel.

    Did you check the system for proper lamps ? Perhaps it was lamped with smaller wattages reducing the draw and increasing the voltage. More info is needed for a proper diagnosis
  3. emby

    emby LawnSite Senior Member
    from Ontario
    Posts: 380

    Faraday's Law states that the EMF generated is proportional to the rate of change of the magnetic flux.
    Our 12 volt transformers are a 10 to 1 ratio in windings. 120 windings of copper on the primary and 12 windings on the secondary.
    If the primary voltage is 115 volts then the secondary voltage will be 11.5 volts at the 12 volt tap.
    Using OHM's law we know that the combined wattage at initial start up was 920 watts. (115v x 8 amps)
    If the primary voltage increases to 126 volts then our 12 volt tap would now be 12.6 volts.
    The first question I have is why did the primary jump up 11%? Less load or resistance on the circuit? Some lamps are burned out, if so then we started with 920 watts and we have lost 11% of the load. 920 x 11%=101.2
    920-101.2 = 818.8 watts. This is now the total watts. Using OHM's law we know the voltage is 126v and we now know the wattage so the ampacity will be P/V=I 818.8/126= 6.5A
    Just my guess if that is the reason the voltage went up. If the primary voltage is taken without a load on it and it is still 126v at the GFCI then the ampacity would be 7.3 roughly.
    Just thinking here but good question.
    The nutty professor :)
  4. emby

    emby LawnSite Senior Member
    from Ontario
    Posts: 380

    Just another guess. But here it is. The ampacity will be 7.12 if you have 11% difference in primary voltage.
    If the voltage is increased then the current is decreased by the same factor (11%).
    I have to stop drinking coffee after 11pm. :)
  5. steveparrott

    steveparrott Sponsor
    Posts: 1,199

    Let's step back a bit. I stated in this case scenario that the only thing that's changed is the primary voltage - everything else about the system has stayed the same. Let's assume that, for some unknown reason, the house voltage has increased - maybe a seasonal difference, or time of day, air conditioners turned off, or something else.

    I should say that this is not the actual project troubleshooting scenario. Rather, it came from a contractor's question about why he's seeing a change in the primary amperage draw when nothing in the system has changed. I got to thinking about what kind of amperage change you would see if the primary voltage increased.

    I did some calculations and came up with an answer, then set up a lab experiment and got a different result. I did some more digging into technical data sheets and found out why. It was surprising, interesting, and potentially useful so I created this simplified scenario to challenge our group.
  6. steveparrott

    steveparrott Sponsor
    Posts: 1,199

    This was essentially what I predicted would happen, but my lab test proved it wrong. I saw my mistake as I ramped up voltage and saw the lamp brightness increase. Aha, lamp wattage increases also! That changes the equation.
  7. Pro-Scapes

    Pro-Scapes LawnSite Platinum Member
    Posts: 4,181

    well of course lamp wattage increases. A lamp is esentially a variable resistance isnt it ? The harder you drive it the more it will consume.

    You would think amperage would drop yet it increased due to as you said the wattage increasing. My quick guess is your now drawing closer to 8.75 amps because your driving the system harder.

    This is another reason you need to check primary and seconday amperages and voltages and record them at the time of installation. I want to find a mobile laminating device so I can make a simple card with all my specs on it and laminate it and stick it in the transformers. Including colorcodes...amperages...voltages and date the system was initialized.
  8. steveparrott

    steveparrott Sponsor
    Posts: 1,199

    Your guess is on the right track, but. . .

    Let me ask you this, if voltage increases by 10% and wattage increases by 10%, wouldn't the amperage stay the same? (Amps = Watts/Volts x 0.1/0.1)

    The answer is no - amperage doesn't stay the same. Why not?
  9. steveparrott

    steveparrott Sponsor
    Posts: 1,199

    Correction: the above equation should be: Amps = Watts/Volts x 1.1/1.1
  10. emby

    emby LawnSite Senior Member
    from Ontario
    Posts: 380

    Ok wow I was out to lunch on that one.

    I am a little confused..ok well maybe really confused on this one. We are talking OHM's law correct?
    If you increase the voltage on the primary and the resistance remains the same on the secondary than your ampacity will rise porportionally. If you raise the voltage up by 10% than your ampacity will raise 10% only if the resistance remains the same.
    Now lets talk about the bulbs here. If you have installed a 20 watt light bulb are you telling me that by placing 12.6 volts to that bulb it will use more than 20 watts of power? The filament is resistance measured in ohms and that is a constant piece of tungsten so the resistance would not change for that particular bulb.
    I am way confused on this one lets here the answer.....

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