8X50=400watts, so dimmer must be rated for that.
the voltage won't alter the power which is rated in watts so changing for LV lights won't alter power used, in fact it will use more due to losses in the transformer.
why not just change the existing lamps for 20 or 35watt ones?
8X20=160W or 8X35=280W.
Much cheaper than replacing fittings for LV ones which require 230v-12v transformers , possible larger cables etc. as 400w @ 230v = 1.7Amps but 400w @ 12v = 33Amps on the LV wiring.
Either way the dimmer needs to be the correct one for the type of lamp in use.
As sparx says dimmer switch must match lamps but the whole idea of a quartz lamp is the quartz gets that hot the tungsten will not be deposited on the quartz so will instead return to the tungsten element in the end the thickness of the element will vary so it will blow but keeping the whole lamp at the correct temperature is important to stop them blowing premature.
The whole idea of extra low voltage lamps is to control the voltage better so extending their life. To do this the inverter (often referred to as transformer) needs to be close to the lamp and of course dimmers should not be used.
If you do want a dimmer you must expect the lamps to blow more often and there is no point in using extra low voltage and the low voltage (230v is low voltage) type would be better.
Because many of the low energy lamps either LED or Cold Cathode will not dim very well in Europe the lamps are often arranged in a 1/3 to 2/3 split so you control the amount of light by how many lamps are on rather than the amount of light from each bulb. This also means the colour stays correct.
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