When adding a new circuit, does the nuetral wire have to be attached to the neutral bar, or can it be tied in with another neutral wire from another circuit? My old service box is used as a junction box for the new service panel, so can the neutrals just be connected here with one another?
The neutral wire must tie into the neutral bar in the same panel as the hot wire connects to the breaker. The neutral wire can not tie in with a neutral wire from a different circuit unless the hot wire also ties into that circuit. The shared neutral from the two circuit could carry the sum of the current from both circuits.