Just to pick up on the "(after transformer if low voltage )" part of your reply, This could be construed (by the untrained)as wiring all the downlighters in parallel on the transformer output, all well if the transformer is hefty enough to take the load, what should be included is that there is the wiring option that the mains wiring is paralleled to individual transformers supplying the downlighters seperately (again assuming that the downlighters were "low voltage")
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