Power consumption and bulb life 12v vs 120v exterior lighting

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williamsmithaud

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I have a client ask what the difference in power consumption was between the 12 volt 5 light (250 watt total) system I will install, versus a 120 volt 5 five light (250 watt total) system.

Also is there any difference in the on/off stess to light bulbs between the two systems?

What is the bulb life expectancy between say a 12v MR16 and a 120v 50 watt halogen.

I hope someone can help with these questions. If there is a difference in power consumption between the two systems, and someone knows how to calculate it I would greatly appreciate knowing that formula.

Bill
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William (Bill) Smith
 
Assuming the same wattage the low voltage system will always cost more to operate as a result of the transformer losses.
Don
 
Is your customer trying to get illumination or little decorative points of light.

If he is trying to get little points of light, then the 12 Volt will probably be less expensive to install. The bulbs will probably last longer because they have much thicker filaments. He will probably not get as many lumens per watt because they usually operate at lower temperatures.

If he is trying to get the most illumination per kilowatt hour, then 120 Volts is the way to go and consider halogen lamps or even fluorescent lights. I recently installed a 65 Watt flourescent to replace an outside floodlight that gives about 3 times as much light as the 150 Watt flood that it replaced.
 
Eletricity is sold by watts, not by volts or amps. So all things being equal, 250 watts is 250 watts and that is what you pay for. As another poster stated, with the 12-volt system there will be some power "lost" through the transformer that converts the 120-volt power to 12-volts. This is the power that is seen as heat from the transformer. You will also need to be much more aware of voltage drop in the 12-volt system. The amperage for a give wattage of light bulb will be higher in a 12-volt system so with higher amperage comes increased voltage drop. In a 12-volt system you cannot afford to lose much voltage due to voltage drop. 3% of 120-volts is 3.6 volts; 3% of 12-volts is only .36 volts. I believe that most 12-volt systems specify that no fixture may be more than 50 feet from the transformer. So with transformer losses and increased voltage drop, I would have to say that the 120-volt system would be more economical to operate. This operational economy may be offset by increased installation costs for the 120-volt system. It all comes down to ease of installation and if the fixture that the customer wants is available in the voltage that is selected. You may also want to investigate the possibility that some sort of fluorescent fixture may be available that accomplishes what the customer wants. If so, you get many more lumens per watt consumed than with incandescent lamps.
 
Seperate but I think related question. What is the benefit of using a recess fixture that has an internal stepdown transformer. I have heard that the LV bulbs produce a better light. I have installed them but I don't see any visual difference between a 120v halogen bulb and a 12v halogen bulb of the same wattage.
 
mkgrady said:
Seperate but I think related question. What is the benefit of using a recess fixture that has an internal stepdown transformer. I have heard that the LV bulbs produce a better light. I have installed them but I don't see any visual difference between a 120v halogen bulb and a 12v halogen bulb of the same wattage.
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You need to look at the light output measured in lumens or candlepower not the wattage.

For example a Sylvania ER-50 is a 50 watt elliptical reflector lamp that produces about 450 lumens. A comparable PAR 30 50 watt lamp produces about 600 lumens. That a 33% increase in light output for the same wattage lamp.
 
infinity said:
You need to look at the light output measured in lumens or candlepower not the wattage.

For example a Sylvania ER-50 is a 50 watt elliptical reflector lamp that produces about 450 lumens. A comparable PAR 30 50 watt lamp produces about 600 lumens. That a 33% increase in light output for the same wattage lamp.
Click to expand...
So what is the point of having a stepdown transformer? Somebody must think it is better than line voltage or why else would they be made?
 
There was mention of lamp life in the original post. A quick and easy way to increase lamp life for incandescent fixtures is to put them on a dimmer. If the lamps are dimmed to 90% of full brightness, the lamp life is approximately doubled. This can be extremely valuable when doing high ceilings.

Hope this helps.
 
Look at the "Center Beam Candle Power" figures for a 50W 120V Par 20 and a 20W 12V MR16. The 20W MR16 is almost as high as the 50W Par20. Why? THey are both halogen, so life is roughly the same (though there are 10,000 hour MR-16's from Ushio). The smaller voltage lets you have a more compact filament. The closer your filament is to a "point source" (a dot), the more efficient your reflector is. In other words, more light gets thrown out of the fixure. You also have less spill light, and a more controleld beam of light, without weird lines around the edges and glare. This is why we use low voltage lighting.

note that when dealing with reflector lamps, you must look at the beam spread and CBCP, not lumens. (1 footcandle = 1 lumen spread over 1 square foot).

As far as integral versus remote transformers: All that matters is that you have a full 12V at the lamp. Most cheap built in transformers will only give the lamp 11V or so, even less if you are going through a dimmer, which drops the voltage some, even in 100% on. High end cans have a "boost" tap to make up the voltage drop from the dimmer. Measure your voltage on the lamp (under load) on your next can install, you'll be surprised. Remote transformers and the secondary wiring can be designed to deliver a full 12V to every fixure, using boost taps and large wires. With copper prices today, you may be better of delivering 120V to your fixure and stepping down there, or at least grouping the transformer near the lights. With integral transformers, you won't need to do much engineering, and they will all have uniform brightness. (Think of the PoCo, they distribute at 12kV and step down at the load to 240v, you are doing the same thing).

Mike
 
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