I have a lighting circuit with no earth, what size earth cable shall I put in please?

Thank you

Match it too the fuse or breaker amp setting.

Running a earth ground is not a good idea esp if you do not follow the code and run that ground back to your panel and also have it bonded.

If the wiring has no ground and you need one... install the correct type wire (Romex 14/2 or MC 14/2) between your fixture and the panel. Or if it's being feed from another device(outlet/switch) or box run it back to that point.

Before you do anything, you need to verify a few things:

Is this a newer or older home? What sort of wiring is present... Cotton Braid, AC/BX, MC, Romex?

Does the switch box for this fixture have a ground?

Just because the you don't see a ground wire... the box might still be grounded if the wiring is AC/BX.

Have you tested the ceiling box yet? AC/BX has a steel wire that is wrapped around the outer jacket before it's attached to the box. So just because you don't see a ground... doesn't mean it's not there.

Joe

JoeDunn, you are asking a lot of follow up questions to the OP. Thing is, they made this one and only post almost a year ago and I get the feeling they have not been back since.