I am upgrading the service to the residents from a 100 amp service to a 200 amp service. I am planning to install a 200 Amp Load Center at the service entrance in the house. Would I be within code specs if I make a 60 foot run of 1-1/2" PVC schedule. 40 conduit holding three #3 THHN conductors and #4 TNNN conductor, going from a 100 Amp circuit breaker to the original 100 Amp Load center. Also from a second 100 amp circuit breaker 160 feet to a 100 amp load center in a garage. The garage run is (USE 1/0) underground cable with #4 copper ground.

First, are you planning on upgrading the service entrance cable and meter socket, or just changing the panel? If you are planning on just changing the panel, you have a major problem in that you double the size of the load capacity of the panel, but not the service entrance cable into the house. The panel main breaker is there to protect the service into the house from overload.

Why would you be using the existing panel if you are installing a new one? Is it in a different location? Are you moving the service to another part of the house? Also, how do you plan to isolate the ground from the neutral in the 100-Amp existing panel? Does it have separate bussbars for ground and neutral? Can you remove a bonding jumper between two bussbars, or do you have to install a ground buss? Then, don't forget, you have to move all of the ground wires from the neutral buss to the ground buss in this panel, and move the bonding jumper or screw from the neutral buss to the ground buss. There are other considerations also - one of which is removing the main breaker and installing lugs.

Quite frankly, I don't believe this to be the best plan I've ever heard. I think that you need to give it some serious engineering thought before you even start. Even better, hire a qualified electrical contractor who will do the job correctly and safely - not to mention, to Code.