My hvac contractor installed a 3 ton unit which was to small for the house. I since than obtained two bids with a load cal. which he did not do. we are 1.5ton short.  i have to add more ground loop.  i plan on coming off the header  supply and return lines(1.25")with another 1200ft'(3/4"). the supplies that they are going to use is a  tee manifold with 3 3/4" outlets reducing my supply line for a short distance to the existing loop will this be a problem.. causing to much head pressure.

sa3,
i'm not sure where to start.
when you say 1.5 tons short what did your calc end up at (in btu's)?
What is the total existing loop system?
What flow center do you have?
How long have you had the system?
What makes you think it's too small?
J