1. The problem statement, all variables and given/known data The question is find R Thevenien using voltage test source across terminals a and b of the following circuit: http://img255.imageshack.us/img255/8660/rthevenen.png [Broken] 2. Relevant equations N/A 3. The attempt at a solution I did it by adding a 20 test voltage source and used mesh to get the following 4 equations of the 4 meshes ( I deactivated the 480 V independent source to be able to use test voltage method) M1:6(x-t) + 40(x-y)+4x=0 M2: 40(y-x)+20=0 M3: 80(z-t) - 20q +2z-20=0 M4:4t + 8(t-z) +6(t-x)=0 q=x-t I R thevenen to be 11.55 but actually the correct answer is 12 ohms. Where is my mistake.