I am installing five 12 volt 50 watt downlights in my bedroom ceiling.I am useing one single 250 watt transformer.Do I wire each lamp individually back to the transformer or can they be wired in parallel one after the other.What guage wire should be used and will there be a voltage drop.
The total load is 20A approx and 3% volt drop is permitted in appendix 12 of BS7671:2008 so working from that.
1) Min cable size is 2.5mm
2) 2.5mm has a resistance of 0.018 ohms per meter.
So for 2.5mm cable to supply all lights max length is 1 meter.
6mm cable has a resistance of 0.0073 ohms giving 2.3 meters max length.
Over 6mm unlikely to get the cable into terminals.
Even with a single lamp at 50W max length of 6mm cable = 11 meters. And this does not allow for the volt drop in supply although with an inverter (That's an electronic unit that replaces a transformer) the volt drop on supply to inverter will not really matter.
The 12 volt extra low voltage lamps have and advantage over the 230 volt low voltage GU10 lamps because of the voltage control to the lamps. So unless the lamps are grouped very close together one should use individual inverters (transformers) for each lamp. Otherwise may as well use the much cheaper GU10 lamps.
Only where the supply has voltage peaks which reduce the life of GU10 lamps or where regulations require SELV would I consider using an inverter and the little spot lights.
Spot lights are to light a spot and I also have two in my bedroom on the wall as reading lamps aimed to light a book when reading in bed. However to light the room totally useless. Makes it look like a planetarium. Mind you some people like sleeping under the stars!
So there is no need to use separate cables to each lamp but in most cases that would be sensible approach.
Since I don't know lengths of cable can't give a real answer. So it all close to light a picture on the wall for example than maybe they could all share same cable. No one would I hope try and light whole room with 50mm spots so you need to say why the spots are being used and how far away from each other etc.