I installed a new load-center and sub-panel a couple years back as well as a 30A generator feed and the load meters listed above. My panel is GE and I was able to install a mechanical interlock similar to the ones shown above. My main and sub panels are properly grounded and bonded per code. The nice thing about my setup is that it allows me to power anything circuit I please (although not all at once), without having to decide when wiring the panels.
Regarding bonding the neutral and ground in sub-panels...
Frequently Asked Question: Why do the grounds and neutrals need to be separated in a subpanel? What happens if they aren't?
Answer: Though the neutral doesn't have significant voltage, it does carry current. Remember, it's current that kills, not voltage. In a 2-wire circuit, the neutral carries just as much current as the hot conductor. If the neutral and ground are connected in a subpanel, that current will travel on other paths, such as bare ground wires, equipment enclosures, and metal piping systems, on its way back to the service panel. One problem created by this condition is possible shock hazards, the severity of which depends on the locations of the equipment and the person touching the enclosure or piping system. Another problem is magnetic fields that do not cancel themselves out. Since the return current has multiple paths, the current remaining in the neutral will not counterbalance the current in the hot wire. The resulting imbalance creates a magnetic field that can interfere with sensitive electronic equipment. In a metal conduit system, the imbalance will induce current into the conduit, which could cause the conduit to overheat.
Q: Given that this is a fairly common condition, why don't we hear about problems, and why do some electricians not understand the problem?
A: For the feeder from one panel to another, the neutral only carries the imbalanced load between the two hot legs. Most of the time, the amount of current on the neutral is very low. However, in a situation where a single 120-volt appliance is in use, and there are few other loads in operation, the current on the equipment grounding conductors and other paths could be quite high, resulting in the problems noted above. Other conditions could cause the load imbalance to be quite high, if for example, all the lights in use at a given time just happened to be originating from the same phase conductor.
The National Electrical Code (NEC) has prohibited re-grounding the neutral after the service since the 1923 edition. Exceptions have been made for dryers and ranges that use the neutral as a grounding means, and for separate buildings. However, those loopholes have been effectively closed in the 1996 and 1999 editions of the NEC. The references in the 1999 edition of the NEC are found in 250-24(a)(5), 250-142(b), and 384-20.
