Would the following be the correct approach? I am a bit concerned with node C as it introduces another unknown. Do I need to write a constraint for this problem?
$$A: ((A-Ref)/4000) + ((A-B)/2000) - .002A = 0$$
$$B: ((B-A)/2000) + ((B-Ref)/2000) + ((B-C)/2000) = 0$$
$$C: ((C-B)/2000) - I(source) + .002A = 0$$