I want to use 10, 12 volt low voltage lights 50watt each with its own transformer. The mains voltage is 240 volts.
Will the total power consumption be the same as a 500watt incandescent lamp or will it be 12/240 x 500 = 25 watts ?
Electric Watts = Volts X Amps X Power factor correction which means with AC the Watts is slightly higher than VA.
So the VA ratting of the transformers needs to be a bit higher than the Watts of the item being used.
Transformers and Inverters (like transformer but lighter and corrects voltage) will absorb some power and there will be a slight power factor adjustment but in real terms the units should be able to handle this and for most practical purposes one can think of Watts and VA being the same.
As the voltage is changed the amps will also change to deliver the same power but power is measured in Watts so that remains static.
So a 50W 12V lamp uses 50/12 = 4.17A and a 50W 230V lamp uses 50/230 = 0.22A. Using low voltage lamps (230v) means the cables are lighter and they do not suffer too much from volt drop.
In the normal domestic using the GU10 lamp is cheaper and safer as the GZ10 lamp will not fit in a GU10 lamp fitting plus they can use cold cathode lamps.
However where the voltage has spikes the life of the GU10 lamp can be very much shorter than that of the separated extra low voltage (SELV) system as the inverter will correct the voltage and because the voltage is more stable the lamps last longer. However you can't use cold cathode lamps and the G5.3 base used with SELV does not stop the use of Dichromic lamps which can overheat the area behind the lamp.
The problem with the little 2 inch spot lamps is they are spot lights and great for lighting up a picture on the wall but useless at lighting a room. The extra low voltage lamps have a greater range of angles of light emitted and can be selected to illuminate items from a verity of distances but the low voltage cold cathode lamps tend to have a greater spread than most of the tungsten types which for a spot light is of course bad but when not used as a spot light can be far better.
However with 10 lamps in one room as well as making the room look like a planetarium it will also increase the heat in the room. Although this may be great in the winter it is not such a good idea in the summer. Plus it will use a large chunk of the 6A used for lighting circuits. Since ceiling roses both hang lamps and are junction boxes which are only rated at 5 to 6 Amp in the main that's the largest size of fuse/MCB that can be used.
Although you can use LED lamps to reduce the power used this can be a problem with inverters which have both a max and a min wattage so in some cases you can't fit LED lamps into a SELV system.
It's not all plane sailing with low voltage either. Many of the cold cathode and LED lamps are longer than the tungsten lamp and stick out of the fitting in an unsightly manor.
With the way the governments are withdrawing from sale tungsten lamps I would ensure what ever fitting you use is future proof and will take cold cathode lamps. Although LED lamps are improving they are very expensive for high output types. I would also consider lighting levels and where quartz tungsten is used one should not really dim the lamps as it reduces the lamp life. So spiting into 1/3 and 2/3 of lamps on a pair of switches gives three levels of light without need to dim any lamps.
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