Basically DC Bus 1, section 1 powers FCC A and DC Bus 2, section 1 powers FCC B (See full list of NG bus services
here).
However, there are 6 other power inputs required for services to the FCC (MCP, Mach trim, stab trim, etc), the failure of any of these would give problems with the FCC or render it u/s.
S&L
Edit: link changed.