Identifying the switches live

[Steveohim]

I’m changing our rocker switch for a dimmer switch and need to identify the switched live wire. I have an earth, red and black. I’m guessing it’s the black but want to be sure. This is the dimmer switch I’ll be wiring

[wine~o]

Almost always the black... Buuut if a diyer has had a hand in it at some point. If the wiring at the light is easily accessible then just check that all reds are connected together. if so deffo the black.

[OnlyMe]

I don't think that it matters.

One cable into L1 and the other into the squiggly terminal.

[wine~o]

I don't think that it matters.

One cable into L1 and the other into the squiggly terminal.

Concur buuut was answering the question "Identifying the switched live"