[LazyPup]

With some rare exceptions the T&P valves are the same. The plumbing code states they must regulate pressure below 150psi and temperature below 210 degF.

The only major consideration is the discharge diameter of the valve. In most residential applications the discharge diameter will be 3/4" and the discharge line must be equal to or greater than the discharge diameter of the valve. Absolutely do not be tempted to save money by reducing a T&P discharge line. Reducing a 3/4 line to 1/2" cuts its volume in 1/2 and thereby negates its ability to protect your home.

line capacity is determined by cross sectional area rather than diameter , therefore to compare line capacity you must use the formula Large diamter squared divided by small diameter squared.

thus 3/4" compares to 1/2" as,,,,

3/4 x 3/4) / (1/2 x 1/2) =
(.75 x . 75) / ( .5 x .5)=
0.5625 / .25 = 2.25

which means a 3/4" line has the same capacity as 2.25 lines of 1/2"