plumbing in a electric shower

[steveo]

I understand the cold feed has to be [for a electric shower] from the cold supply from the mains, can i tap into this, from a branch, which is coming off the cold pipe has it enters the house, the exact tap in point would be 7 inches along the branch off the cold supply ? i.e the mains come in the house and then theres a branch which feeds a sink i intend to tap into the sink branch some 7 inches above were it joins the cold mains pipe .

 

[MANDATE]

You can tap in into the cold suppy anywhere you want.
I have tapped in by fitting a tee junction in the cold pipe going to the bath tap, and last month I tapped in to the pipe going to the toilet cistern.
In both these cases it was the main supply, in some cases these would come from the water tank in the loft which you must avoid.
Best thing to do is trace the cold supply and find the nearest conveinient point so you have less pipe to fit.
The main supply always goes to the kitchen tap, but you will find even if it does not feed any other taps or toilets there will be a cold feed going up to your water tank so you could use that if you so wish