230V AC to 24V AC Transformer Wiring (Black, Red, Brown, Brown)
Which wires should connect to live and neutral on the 220VAC supply
and which are live and neutral on the 24V output?


Black and red are 230 volts
Brown and brown are 24 volts


My best guess is that the black wire is used on both the input and
output as a common neutral

No, the two transformer windings are isolated from each other.

Since I have 220V AC, I asked the electrical store for a 220VAC to
24VAC transformer

Well, nearly right - they shop supplied you with a 230 volts AC rated transformer so, on a supply of 220 volts, you can expect the 24 volts AC output to be around 23 volts RMS.