On Mon, 09 Mar 2015 12:40:14 -0400, Adam Kubias
wrote:

On 2015-03-09 12:32 PM, dpb wrote:
On 03/09/2015 9:30 AM, trader_4 wrote:
On Monday, March 9, 2015 at 10:05:22 AM UTC-4, dpb wrote:
On 03/09/2015 2:53 AM, gregz wrote:
wrote:
I got a bathroom. I'm adding a second downstairs right below. ...
...
Run from hot water tank 20-25 feet, and cold tap. Still going to use
the
1/2 feed from regulator and hot water tank.
...

I think you'll be sorely dissatisfied by the result if there's any
possibility whatsoever the two are being used together. That line needs
to go to 3/4"...

--

A big unstated factor is if the new bathroom is a full one, ie with
shower or tub, or just a half bath. Half bath, 1/2" would be fine.

I'd say "marginal"...

--

Agreed. Usually the purpose for a second bathroom is if the first one is
in use. This negates that purpose.
Unless they are on separate floors. You use the one that's closest
and requires the fewest stairs to be climbed.