Converting 3" THD CI to 2 1/2" THD Copper

[Batman84]

A friend of mine is trying to run a new supply line to his water holding tank, and the line coming off the bottom of the holding tank is 3" threaded cast iron, and the new line being put in is 2 1/2" threaded copper.

now for my question; what is the best method for making this conversion, can i use a galvanized malleable reducing coupling (i know this raises the issue of oxidization) or would the better and more expensive option be to reduce the 3" down to 2 1/2 and use a dielectric reducer?

thanks for giving this a read, and hopefully i provided enough info for you to provide an answer, if not, feel free to ask another question and ill answer it best i can.

 

[doitall]

One needs to get the terminology correct.

For starters I doubt it is cast iron, perhaps galvanized steel, in which case you have answered the question.

A DZR reducing socket should be ok if it's a dead leg.

Why are you using copper and not plastic.