Why is my water heater leaking from the relief valve on the top?
I think the pressure on your house plumbing should be about 80 psi or less right after you run a tap and then shut it off. Do you have an expansion tank to prevent pressure increase? If you use a lot of hot water, then don't use any water, the pressure in your system can increase as the cold water in the tank water heater expands on being heated.

If this is what is happening, if your TP valve is dripping, then it should stop if you momentarily open a cold water tap (or if you open a hot water tap but then more cold water would flow into the tank causing more expansion after a while).

See pressure relief tank.

EDIT
Informative readings here from Watts.