Low Voltage Transformer Trouble

Discussion in 'Landscape Lighting' started by jimmy1111, Jan 19, 2009.

  1. jimmy1111

    jimmy1111 LawnSite Member
    Messages: 56

    Opened a VISTA STT 900 and found the secondary wires from the trans to the 3 breakers in pretty bad shape - corrosion, melted insulation, one wire completely melted.

    The wires are 12AWG rated to 600V - I'm assuming low voltage wires. Can I replace them with regular 10AWG also reated for 600 V that I got from home depot as high voltage wires ?? isn't better wire since it is rated for high voltage power versus what vista seem to have installed low voltage wires knowing that on the secondary it is low voltage anyways ???

    The other question, what could have caused this mess ?? had the Transformer for 4 years with 270 W at each circuit without any problems. I moved to a new place and and went to 290W for each of the 3 circuits....I know I should have stayed 10% of max capacity...but could this extra 20W on each circuit created this nightmare eventhough i'm still below the 300W threshold and this is a vista not a cheap transformer ??

  2. INTEGRA Bespoke Lighting

    INTEGRA Bespoke Lighting LawnSite Platinum Member
    Messages: 4,102

    Please tell me that you had more then one 12awg wire to each of the 3 circuits. It sounds as if you loaded 290W onto each of three 12 ga. wires. If this is the case, you have greatly over loaded the wire.

    Jimmy... are you a contractor or a DIY'er?
  3. TXNSLighting

    TXNSLighting LawnSite Fanatic
    from DFW, TX
    Messages: 6,464

    Sounds a little fishy, definately need more info and possibly pictures...
  4. LightYourNight

    LightYourNight LawnSite Member
    Messages: 101

    i would call vista a cheap transformer... I replace them all day.

    post some pics
  5. Mike M

    Mike M LawnSite Bronze Member
    from usa
    Messages: 1,988

    Does sound fishy.

    Anywho, you measure the amps with a probe, don't worry about the watts, sounds like the load on the trans, and each wire, is over amped.

    No mention of voltage taps. Bad grammar. It might be the bad grammar.
  6. David Gretzmier

    David Gretzmier LawnSite Gold Member
    Messages: 3,645

    when wires melt, something is over amped, and probably the wires. 600v is rated for high voltage, but wires are rated by the amps they can hold at any voltage within thier range. 290 watts at 12 volts is actually 24 amps, and you need 8 guage to be safe from the trans to a close hub of at least 3 12 guage wires for EACH 290 watt run. most folks just run multiple 10-12 guage wire, loaded properly, into the trans and put multiple wire on the same tap.

    I could go into what load those 12 guage lines could hold, but we've discussed it tons of times and I won't rehash it here, other than to say that a single 12 guage won't handle a 270 watt 12 volt load any day of the week.
  7. jimmy1111

    jimmy1111 LawnSite Member
    Messages: 56

    well I didn't want to write a page in my first question so here is more details....2 of the three circuits are ran with 12 AWG Loops. so it is 2 12 AWG at each tap for these two circuits. The third circuit got also 2 12AWG wires running split in two sepearte directions. The STT 900 is a Multitap Transformer for 11, 12 & 13 Volts that VISTA stop making. The 13 V tap is the one that melted than i switched it to another circuit and that one started to get corroded and melting.

    Is the rated amps of a wire labeled on the wires ? vista installed like 3 inch or so of these wires inside the transformer and I can't see the full label. I saw the 600 V. Is the low voltage wire capable of holding more amps than the same AWG of a High Voltage wire?

    The other question i had was corrossion ? is it when the wires melt they get corroded or is it water / humidity creating the corrosion?...I can't see how the water goes in a VISTA stanless steel transformer.
  8. INTEGRA Bespoke Lighting

    INTEGRA Bespoke Lighting LawnSite Platinum Member
    Messages: 4,102

    Dude, you have overloaded the 12 gauge wire, pure and simple. I would suggest hiring a professional or at the very least taking a LV lighting installation seminar.

  9. jimmy1111

    jimmy1111 LawnSite Member
    Messages: 56

    well...how much Amps a 12 AWG cable can hold ? isn't supposed to be written on the cable because I don't see it. Also; would having loops of 12 AWG is actually doubling the 7500 Cable Constant for the 12AWG cable, or in other words is it equivalent to say that I have the same load on 2 12AWG Wires? This is one part of the issue.

    The second part which i don't seem to receive comments on but Dave got close is: Can a High Voltage 10 AWG wire be a substitute of a 12 AWG Low Voltage Wire that's inside the Transformer secondary side which is from the coil to the 25 Amp Breakers and than from the 3 Breakers to each Taps? If I overloaded why the 25 Amp breakers didn't trip ?
  10. TXNSLighting

    TXNSLighting LawnSite Fanatic
    from DFW, TX
    Messages: 6,464

    Man, you really need to hire a professional to fix this mess.

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