wire load, rec vs max

Discussion in 'Landscape Lighting' started by Mike M, Jul 6, 2008.

  1. Mike M

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


    I always played it safe, using CAST's sensible recommendations and guidelines, such as 10 gauge cable for hubs beyond 100 feet, etc.

    Unique and Cast specify max lamp wattage load, and I was wondering if I shouldn't be getting more out of my wire by pushing more toward the max of #12 cable.

    Using Unique's quick-reference-slider-card, I can see the benefit of higher (17 plus) taps.

    I assume this is okay, but is this why Unique pushes the fuses at hubs?

    The only downside I can see for higher taps and pushing toward the max load, is for possible future add-ons at the hub, or if I need to up the wattage from 20 to 35 on a couple fixtures.

    I'm not trying to be cheap or hazardous, just efficient.

    I suppose I should do the math for maximum load at my distance (per code), with a margin for a possible add-on or two, or some possible bulb-upgrades. In other words, keep the lamp load below max (192w/#12)(or 16 amps), even with the possible add-ons (e.g. 40 watts). This would mean 152w on #12 might work for a specific application which I feel confident I won't go beyond 40 more watts.
  2. The Lighting Geek

    The Lighting Geek LawnSite Senior Member
    Messages: 886

    the fuse at the hub is so that only that home run is out if the problem is between the fixture and the hub. Otherwise the whole system goes down.

    I would try to plan your home runs under 100-150'. Depending on the load, I always leave at least 35-40 watts available on every home run.

    Remember, when you use the higher taps, it is a trade off for capacity. in other words, you will not be able to fit as many fixtures on a transformer when using the higher taps (17-20v).
  3. Mike M

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


    I have been installing transformers based on lamp wattage to not exceed 80% of the rated capacity. Are you saying the higher taps/long runs/12 gauge cable will decrease the lamp wattage I can use?

    I have an amp probe, but is there a chart somewhere as a reference for this?

  4. Chris J

    Chris J LawnSite Silver Member
    Messages: 2,843

    I think what he's trying to explain is this: With longer home runs, the resistance in the wire generates the equivalent of more wattage. If you use any of the voltage drop calculator software, you will see this measurement built into the calculations. As a desk estimate, you can use ohms law. For instance, lets assume you have 100w on 100' of 12ga home run. Watts / volts = amps, so 100 / 12 = 8.3 amps. Now, take the amps and mutiply by the correct voltage tap your in to get true (realized) watts. In this case you would be in the 14v tap, so: 14v x 8.3amps = 116.2 watts, not 100.
    This is what's called a desk estimate, so it is not "exactly" what you might see with your amp meter. But it's close.
  5. Mike M

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

    Thanks, Chris.

    This is why they say not to use the full rating. I will play with the calculations to see how much I should be able to load up on the cable when using higher taps.

    I will also check in the field with the amp probe.
  6. The Lighting Geek

    The Lighting Geek LawnSite Senior Member
    Messages: 886

    Chris explained it better than I did.

    The reason they say to use 80% is because NEC says so. If you amp probe all your home runs and they are under 16 amps (provided that you are using 12/2), your fine. Your transformer also has an amp rating as well. Amp probe the primary and make sure you are not exceeding that rating. That's really it. Try to keep it simple or you will over think it.
  7. David Gretzmier

    David Gretzmier LawnSite Gold Member
    Messages: 3,646

    I think different transformers play into the equation of what you can do on a 12 guage wire as well. I know that with FX trans, i can easily exceed every chart I have found on voltage drop, and it just amazes me. I know what it starts with and what I end with, and it does not make sense, but it works. by the same token, I am often shocked at how much voltage drop I get on Nightscaping trans. A load of 20 watts at 40 feet should not drop a new trans by 1/2 volt. yet it happens. Also, older trans seem to drop volts over time, with higher loaded trans tending to drop more voltage.

    So I feel confident in recommending not to push things. In fact, it seems I've gotta go the other way- the first systems I put in 10-15 years ago with 8 guage wire that were ridiculously overbuilt and over "transed" have held up very well. When I sense I am close to 75-85% max on 12 guage, I go to 10 or 2 12's. when I am bidding systems I always seem to jump to a higher trans when I get close to 60-70% capacity. This has always paid dividends when I need to add lights later, and Ultimately I feel the systems that are overbuilt are the ones still working in 20-30 years.
  8. The Lighting Geek

    The Lighting Geek LawnSite Senior Member
    Messages: 886

    well said David
  9. Mike M

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


    What I am trying to do is go with more 12 than 10, and learning where the cut off margin should be.

    Nate advocates this, and I'm not looking for info on trans load, but on realistic wire load numbers. NEC states "max" load, not "recommended."

    There is a huge gap between the "recommended" load on 12 gauge as published in the CAST installation guide, and the "Max Load."

    I was wondering where others tend to be, and I assume with the frequent use of #12, that Unique installers know how to get a lot from their wire while still staying under the max load.

    The CAST manual says if you lamp over 100 watts, you should use #10. This is a nice, safe number, but way below the max.
    Also, they say if your distance is over 100, you should also use #10, without mention of watts.
  10. NightScenes

    NightScenes LawnSite Silver Member
    Male, from Kingsland, Texas
    Messages: 2,214

    Mike, remember that the NEC says max load on 12/2 is 20 amps but it also says that you can only load that 12/2 to 80% of that max.

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