I have a 12 volt AC 300 watt plug in transformer used for outdoor landscape lighting. I would like to control individual lights with switches, however the transformer manufacturer insists no load side switching is allowed. They indicated to control lights, each group has to be on its own transformer with switches on the input side. Why would this be? Certainly a 120 volt AC switch should be able to handle 12 volts AC. Is there something unique about the transformer that makes it unsafe? Any insight is appreciated.
If you’re using LEDs then there’s a possibility of inductive spikes damaging the LEDs when you switch the load. If you’re using filament lamps that’s unlikely to be a problem but you are likely to see a change in brightness of any lights that stay in-circuit when you switch others on and off. If you can give the manufacturer and part number of the transformer then it may be possible to give a more definitive answer.
They indicated to control lights, each group has to be on its own transformer with switches on the input side. Why would this be?
It sells more transformers.
You could put a TVS transient suppressor on the output, as Tony says.
Note 12VAC LED bulbs will normally include a rectifier since they internally run on DC, so you can run them on DC from a 12-15V switching power supply. If you want to use a controller that runs on 12VDC, or use MOSFETs to switch them, that could be convenient.